Cyclopedia of Telephony and Telegraphy, Vol. 2
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Title: Cyclopedia of Telephony and Telegraphy, Vol. 2
Author: American School of Correspondence
Release date: August 15, 2010 [eBook #33437]
Most recently updated: January 6, 2021
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK CYCLOPEDIA OF TELEPHONY AND TELEGRAPHY, VOL. 2 ***
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[Illustration: THOMAS A. EDISON Pioneer Electrical Investigator and
Inventor of Numerous Telegraph, Telephone, Lighting, and Other
Electrical Devices.]
Cyclopedia
of
Telephony and Telegraphy
_A General Reference Work on_
TELEPHONY, SUBSTATIONS, PARTY LINE SYSTEMS, PROTECTION, MANUAL
SWITCHBOARDS, AUTOMATIC SYSTEMS, POWER PLANTS, SPECIAL
SERVICE FEATURES, CONSTRUCTION, ENGINEERING,
OPERATION, MAINTENANCE, TELEGRAPHY, WIRELESS
TELEGRAPHY AND TELEPHONY, ETC.
_Prepared by a Corps of_
TELEPHONE AND TELEGRAPH EXPERTS, AND ELECTRICAL ENGINEERS OF
THE HIGHEST PROFESSIONAL STANDING
_Illustrated with over Two Thousand Engravings_
FOUR VOLUMES
CHICAGO
AMERICAN SCHOOL OF CORRESPONDENCE
1919
COPYRIGHT, 1911, 1912,
BY
AMERICAN SCHOOL OF CORRESPONDENCE
COPYRIGHT, 1911, 1912
BY
AMERICAN TECHNICAL SOCIETY
Entered at Stationers' Hall, London
All Rights Reserved
Authors and Collaborators
* * * * *
KEMPSTER B. MILLER, M.E.
Consulting Engineer and Telephone Expert Of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago
American Institute of Electrical Engineers
Western Society of Engineers
* * * * *
GEORGE W. PATTERSON, S.B., Ph.D.
Head, Department of Electrical Engineering, University of Michigan
* * * * *
CHARLES THOM
Chief of Quadruplex Department, Western Union Main Office, New York City
* * * * *
ROBERT ANDREWS MILLIKAN, Ph.D.
Associate Professor of Physics, University of Chicago
Member, Executive Council, American Physical Society
* * * * *
SAMUEL G. McMEEN
Consulting Engineer and Telephone Expert Of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago
American Institute of Electrical Engineers
Western Society of Engineers
* * * * *
LAWRENCE K. SAGER, S.B., M.P.L.
Patent Attorney and Electrical Expert
Formerly Assistant Examiner, U.S. Patent Office
* * * * *
GLENN M. HOBBS, Ph.D.
Secretary, American School of Correspondence
Formerly Instructor in Physics, University of Chicago
American Physical Society
* * * * *
CHARLES G. ASHLEY
Electrical Engineer and Expert in Wireless Telegraphy and Telephony
* * * * *
A. FREDERICK COLLINS
Editor, _Collins Wireless Bulletin_
Author of "Wireless Telegraphy, Its History, Theory, and Practice"
* * * * *
FRANCIS B. CROCKER, E.M., Ph.D.
Head, Department of Electrical Engineering, Columbia University
Past-President, American Institute of Electrical Engineers
* * * * *
MORTON ARENDT, E.E.
Instructor in Electrical Engineering, Columbia University, New York
* * * * *
EDWARD B. WAITE
Head, Instruction Department, American School of Correspondence
American Society of Mechanical Engineers
Western Society of Engineers
* * * * *
DAVID P. MORETON, B.S., E.E.
Associate Professor of Electrical Engineering, Armour Institute of
Technology
American Institute of Electrical Engineers
* * * * *
LEIGH S. KEITH, B.S.
Managing Engineer with McMeen and Miller, Electrical Engineers and
Patent Experts Chicago
Associate Member, American Institute of Electrical Engineers
* * * * *
JESSIE M. SHEPHERD, A.B.
Associate Editor, Textbook Department, American School of Correspondence
* * * * *
ERNEST L. WALLACE, B.S.
Assistant Examiner, United States Patent Office, Washington, D. C.
* * * * *
GEORGE R. METCALFE, M.E.
Editor, _American Institute of Electrical Engineers_
Formerly Head of Publication Department, Westinghouse Elec. & Mfg. Co.
* * * * *
J. P. SCHROETER
Graduate, Munich Technical School
Instructor in Electrical Engineering, American School of Correspondence
* * * * *
JAMES DIXON, E.E.
American Institute of Electrical Engineers
* * * * *
HARRIS C. TROW, S.B., _Managing Editor_
Editor-in-Chief, Textbook Department, American School of Correspondence
Authorities Consulted
The editors have freely consulted the standard technical literature of
America and Europe in the preparation of these volumes. They desire to
express their indebtedness particularly to the following eminent
authorities, whose well-known works should be in the library of every
telephone and telegraph engineer.
Grateful acknowledgment is here made also for the invaluable
co-operation of the foremost engineering firms and manufacturers in
making these volumes thoroughly representative of the very best and
latest practice in the transmission of intelligence, also for the
valuable drawings, data, suggestions, criticisms, and other courtesies.
* * * * *
ARTHUR E. KENNELY, D.Sc.
Professor of Electrical Engineering, Harvard University.
Joint Author of "The Electric Telephone," "The Electric Telegraph,"
"Alternating Currents," "Arc Lighting," "Electric Heating," "Electric
Motors," "Electric Railways," "Incandescent Lighting," etc.
* * * * *
HENRY SMITH CARHART, A.M., LL.D.
Professor of Physics and Director of the Physical Laboratory, University
of Michigan.
Author of "Primary Batteries," "Elements of Physics," "University
Physics," "Electrical Measurements," "High School Physics," etc.
* * * * *
FRANCIS B. CROCKER, M.E., Ph.D.
Head of Department of Electrical Engineering, Columbia University, New
York; Past-President, American Institute of Electrical Engineers.
Author of "Electric Lighting;" Joint Author of "Management of Electrical
Machinery."
* * * * *
HORATIO A. FOSTER
Consulting Engineer; Member of American Institute of Electrical
Engineers; Member of American Society of Mechanical Engineers.
Author of "Electrical Engineer's Pocket-Book."
* * * * *
WILLIAM S. FRANKLIN, M.S., D.Sc.
Professor of Physics, Lehigh University.
Joint Author of "The Elements of Electrical Engineering," "The Elements
of Alternating Currents."
* * * * *
LAMAR LYNDON, B.E., M.E.
Consulting Electrical Engineer; Associate Member of American Institute
of Electrical Engineers; Member, American Electro-Chemical Society.
Author of "Storage Battery Engineering."
* * * * *
ROBERT ANDREWS MILLIKAN, Ph.D.
Professor of Physics, University of Chicago.
Joint Author of "A First Course in Physics," "Electricity, Sound and
Light," etc.
* * * * *
KEMPSTER B. MILLER, M.E.
Consulting Engineer and Telephone Expert; of the Firm of McMeen and
Miller, Electrical Engineers and Patent Experts, Chicago.
Author of "American Telephone Practice."
* * * * *
WILLIAM H. PREECE
Chief of the British Postal Telegraph.
Joint Author of "Telegraphy," "A Manual of Telephony," etc.
* * * * *
LOUIS BELL, Ph.D.
Consulting Electrical Engineer; Lecturer on Power Transmission,
Massachusetts Institute of Technology.
Author of "Electric Power Transmission," "Power Distribution for
Electric Railways," "The Art of Illumination," "Wireless Telephony,"
etc.
* * * * *
OLIVER HEAVISIDE, F.R.S.
Author of "Electro-Magnetic Theory," "Electrical Papers," etc.
* * * * *
SILVANUS P. THOMPSON, D.Sc., B.A., F.R.S., F.R.A.S.
Principal and Professor of Physics in the City and Guilds of London
Technical College.
Author of "Electricity and Magnetism," "Dynamo-Electric Machinery,"
"Polyphase Electric Currents and Alternate-Current Motors," "The
Electromagnet," etc.
* * * * *
ANDREW GRAY, M.A., F.R.S.E.
Author of "Absolute Measurements in Electricity and Magnetism."
* * * * *
ALBERT CUSHING CREHORE, A.B., Ph.D.
Electrical Engineer; Assistant Professor of Physics, Dartmouth College;
Formerly Instructor in Physics, Cornell University.
Author of "Synchronous and Other Multiple Telegraphs;" Joint Author of
"Alternating Currents."
* * * * *
J. J. THOMSON, D.Sc., LL.D., Ph.D., F.R.S.
Fellow of Trinity College, Cambridge University; Cavendish Professor of
Experimental Physics, Cambridge University.
Author of "The Conduction of Electricity through Gases," "Electricity
and Matter."
* * * * *
FREDERICK BEDELL, Ph.D.
Professor of Applied Electricity, Cornell University.
Author of "The Principles of the Transformer;" Joint Author of
"Alternating Currents."
* * * * *
DUGALD C. JACKSON, C.E.
Head of Department of Electrical Engineering, Massachusetts Institute of
Technology; Member, American Institute of Electrical Engineers, etc.
Author of "A Textbook on Electromagnetism and the Construction of
Dynamos;" Joint Author of "Alternating Currents and Alternating-Current
Machinery."
* * * * *
MICHAEL IDVORSKY PUPIN, A.B., Sc.D., Ph.D.
Professor of Electro-Mechanics, Columbia University, New York.
Author of "Propagation of Long Electric Waves," and "Wave-Transmission
over Non-Uniform Cables and Long-Distance Air Lines."
* * * * *
FRANK BALDWIN JEWETT, A.B., Ph.D.
Transmission and Protection Engineer, with American Telephone &
Telegraph Co.
Author of "Modern Telephone Cable," "Effect of Pressure on Insulation
Resistance."
* * * * *
ARTHUR CROTCH
Formerly Lecturer on Telegraphy and Telephony at the Municipal Technical
Schools, Norwich, Eng.
Author of "Telegraphy and Telephony."
* * * * *
JAMES ERSKINE-MURRAY, D.Sc.
Fellow of the Royal Society of Edinburgh; Member of the Institution of
Electrical Engineers.
Author of "A Handbook of Wireless Telegraphy."
* * * * *
A. H. McMILLAN, A.B., LL.B.
Author of "Telephone Law, A Manual on the Organization and Operation of
Telephone Companies."
* * * * *
WILLIAM ESTY, S.B., M.A.
Head of Department of Electrical Engineering, Lehigh University.
Joint Author of "The Elements of Electrical Engineering."
* * * * *
GEORGE W. WILDER, Ph.D.
Formerly Professor of Telephone Engineering, Armour Institute of
Technology.
Author of "Telephone Principles and Practice," "Simultaneous Telegraphy
and Telephony," etc.
* * * * *
WILLIAM L. HOOPER, Ph.D.
Head of Department of Electrical Engineering, Tufts College.
Joint Author of "Electrical Problems for Engineering Students."
* * * * *
DAVID S. HULFISH
Technical Editor, _The Nickelodeon_; Telephone and Motion-Picture
Expert; Solicitor of Patents.
Author of "How to Read Telephone Circuit Diagrams."
* * * * *
J. A. FLEMING, M.A., D.Sc. (Lond.), F.R.S.
Professor of Electrical Engineering in University College, London; Late
Fellow and Scholar of St. John's College, Cambridge; Fellow of
University College, London.
Author of "The Alternate-Current Transformer," "Radiotelegraphy and
Radiotelephony," "Principles of Electric Wave Telegraphy," "Cantor
Lectures on Electrical Oscillations and Electric Waves," "Hertzian Wave
Wireless Telegraphy," etc.
* * * * *
F. A. C. PERRINE, A.M., D.Sc.
Consulting Engineer; Formerly President, Stanley Electric Manufacturing
Company; Formerly Professor of Electrical Engineering, Leland Stanford,
Jr. University.
Author of "Conductors for Electrical Distribution."
* * * * *
A. FREDERICK COLLINS
Editor, _College Wireless Bulletin_.
Author of "Wireless Telegraphy, Its History, Theory and Practice,"
"Manual of Wireless Telegraphy," "Design and Construction of Induction
Coils," etc.
* * * * *
SCHUYLER S. WHEELER, D.Sc.
President, Crocker-Wheeler Co.; Past-President, American Institute of
Electrical Engineers.
Joint Author of "Management of Electrical Machinery."
* * * * *
CHARLES PROTEUS STEINMETZ
Consulting Engineer, with the General Electric Co.; Professor of
Electrical Engineering, Union College.
Author of "The Theory and Calculation of Alternating-Current Phenomena,"
"Theoretical Elements of Electrical Engineering," etc.
* * * * *
GEORGE W. PATTERSON, S.B., Ph.D.
Head of Department of Electrical Engineering, University of Michigan.
Joint Author of "Electrical Measurements."
* * * * *
WILLIAM MAVER, Jr.
Ex-Electrician Baltimore and Ohio Telegraph Company; Member of the
American Institute of Electrical Engineers.
Author of "American Telegraphy and Encyclopedia of the Telegraph,"
"Wireless Telegraphy."
* * * * *
JOHN PRICE JACKSON, M.E.
Professor of Electrical Engineering, Pennsylvania State College.
Joint Author of "Alternating Currents and Alternating-Current
Machinery."
* * * * *
AUGUSTUS TREADWELL, Jr., E.E.
Associate Member, American Institute of Electrical Engineers.
Author of "The Storage Battery, A Practical Treatise on Secondary
Batteries."
* * * * *
EDWIN J. HOUSTON, Ph.D.
Professor of Physics, Franklin Institute, Pennsylvania; Joint Inventor
of Thomson-Houston System of Arc Lighting; Electrical Expert and
Consulting Engineer.
Joint Author of "The Electric Telephone," "The Electric Telegraph,"
"Alternating Currents," "Arc Lighting," "Electric Heating," "Electric
Motors," "Electric Railways," "Incandescent Lighting," etc.
* * * * *
WILLIAM J. HOPKINS
Professor of Physics in the Drexel Institute of Art, Science, and
Industry, Philadelphia.
Author of "Telephone Lines and their Properties."
[Illustration: GROSSE POINT EXCHANGE RACK Detroit Home Telephone
Company, Detroit, Mich. _The Dean Electric Co._]
[Illustration: LINE SIDE OF LARGE MAIN DISTRIBUTING FRAME]
Foreword
The present day development of the "talking wire" has annihilated both
time and space, and has enabled men thousands of miles apart to get into
almost instant communication. The user of the telephone and the
telegraph forgets the tremendousness of the feat in the simplicity of
its accomplishment; but the man who has made the feat possible knows
that its very simplicity is due to the complexity of the principles and
appliances involved; and he realizes his need of a practical, working
understanding of each principle and its application. The Cyclopedia of
Telephony and Telegraphy presents a comprehensive and authoritative
treatment of the whole art of the electrical transmission of
intelligence.
The communication engineer--if so he may be called--requires a knowledge
both of the mechanism of his instruments and of the vagaries of the
current that makes them talk. He requires as well a knowledge of plants
and buildings, of office equipment, of poles and wires and conduits, of
office system and time-saving methods, for the transmission of
intelligence is a business as well as an art. And to each of these
subjects, and to all others pertinent, the Cyclopedia gives proper space
and treatment.
The sections on Telephony cover the installation, maintenance, and
operation of all standard types of telephone systems; they present
without prejudice the respective merits of manual and automatic
exchanges; and they give special attention to the prevention and
handling of operating "troubles." The sections on Telegraphy cover both
commercial service and train dispatching. Practical methods of wireless
communication--both by telephone and by telegraph--are thoroughly
treated.
The drawings, diagrams, and photographs incorporated into the Cyclopedia
have been prepared especially for this work; and their instructive value
is as great as that of the text itself. They have been used to
illustrate and illuminate the text, and not as a medium around which to
build the text. Both drawings and diagrams have been simplified so far
as is compatible with their correctness, with the result that they tell
their own story and always in the same language.
The Cyclopedia is a compilation of many of the most valuable Instruction
Papers of the American School of Correspondence, and the method adopted
in its preparation is that which this School has developed and employed
so successfully for many years. This method is not an experiment, but
has stood the severest of all tests--that of practical use--which has
demonstrated it to be the best yet devised for the education of the
busy, practical man.
In conclusion, grateful acknowledgment is due to the staff of authors
and collaborators, without whose hearty co-operation this work would
have been impossible.
Table of Contents
VOLUME II
MANUAL SWITCHBOARDS _By K. B. Miller and S. G. McMeen_[A] Page[B] 11
Common-Battery Switchboards--Line Signals--Cord
Circuit--Lamps--Mechanical Signals--Relays--Jacks--Switchboard
Assembly--Transfer Switchboard--Transfer Lines--Handling
Transfers--Multiple Switchboard--Busy Test--Influence of
Traffic--Magneto-Multiple Switchboard--Multiple Boards: Series,
Branch-Terminal, Modern Magneto, Common-Battery--Western Electric No. 1
Relay Board--Western Electric No. 10 Board--Types of Multiple
Boards--Apparatus--Trunking--Western Electric and Kellogg Trunk Circuits
AUTOMATIC SYSTEMS _By K. B. Miller and S. G. McMeen_ Page 135
Automatic vs. Manual--Operation--Selecting Switch--Line Switch--Trunking
Systems--Two- and Three-Wire Systems--Subscriber's Station
Apparatus--First and Second Selector Operation--Connector--Release after
Conversation--Multi-Office System--Automatic Sub-Offices--Rotary
Connector--Party Lines--Two-Wire Automatic System--Lorimer
System--Central-Office Apparatus--Operation--Automanual
System--Operation--Subscriber's Apparatus--Operator's
Equipment--Switching Equipment--Distribution of Calls--Connection--Speed
POWER PLANTS AND BUILDINGS _By K. B. Miller and S. G. McMeen_ Page 227
Currents Employed--Types--Operator's Transmitter Supply--Ringing-Current
Supply--Auxiliary Signaling Current--Primary Sources--Duplicate
Apparatus--Storage Batteries--Power
Switchboards--Circuits--Central-Office Building--Arrangement of
Apparatus--Manual Offices--Automatic Offices
SPECIAL SERVICE FEATURES _By K. B. Miller and S. G. McMeen_ Page 271
Private-Branch Exchanges--Switchboards--Supervision--With Automatic
Offices--Battery Supply--Ringing Current--Inter-Communicating
Systems--Magneto System--Common-Battery Systems--Types--Long-Distance
Switching--Operator's Orders--Trunking--Way Stations--Traffic--Measured
Service--Charging--Rates--Toll Service--Local Service
TELEGRAPH AND RAILWAY WORK _By K. B. Miller and S. G. McMeen_ Page 321
Phantom, Simplex, and Composite Circuits--Ringing--Railway
Composite--Telephone Train Dispatching--Railroad
Conditions--Transmitting Orders--Apparatus--Telephone Equipment--Types
of Circuits--Test Boards--Blocking Sets--Dispatching on Electric
Railways
REVIEW QUESTIONS Page 359
INDEX Page 373
[Footnote A: For professional standing of authors, see list of Authors
and Collaborators at front of volume.]
[Footnote B: For page numbers, see foot of pages.]
[Illustration: PORTION OF TERMINAL ROOM OF LARGE COMMON-BATTERY OFFICE
Prospect Office, New York Telephone Co.]
CHAPTER XXII
THE SIMPLE COMMON-BATTERY SWITCHBOARD
=Advantages of Common-Battery Operation.= The advantages of the
common-battery system of operation, alluded to in Chapter XIII, may be
briefly summarized here. The main gain in the common-battery system of
supply is the simplification of the subscribers' instruments, doing away
with the local batteries and the magneto generators, and the
concentration of all these many sources of current into one single
source at the central office. A considerable saving is thus effected
from the standpoint of maintenance, since the simpler common-battery
instrument is not so likely to get out of order and, therefore, does not
have to be visited so often for repairs, and the absence of local
batteries, of course, makes the renewal of the battery parts by members
of the maintenance department, unnecessary. Another decided advantage in
the common-battery system is the fact that the centralized battery
stands ready always to send current over the line when the subscriber
completes the circuit of the line at his station by removing his
receiver from its hook. The common-battery system, therefore, lends
itself naturally to the purposes of automatic signaling, since it is
only necessary to place at the central office a device in the circuit of
each line that will be responsive to the current which flows from the
central battery when the subscriber removes his receiver from its hook.
It is thus that the subscriber is enabled automatically to signal the
central office when he desires a connection; and as will be shown, it is
by the same sort of means, associated with the cord circuits used in
connecting his line with some other line, that the operator is
automatically notified when a disconnection is desired, the cessation of
current through the subscriber's line when he hangs up his receiver
being made to actuate certain responsive devices which are associated
with the cord at that time connected with his line, and which convey the
proper disconnect signal to the operator.
Concentration of sources of energy into a single large unit, the
simplification of the subscriber's station equipment, and the ready
adaptability to automatic signaling from the subscriber to the central
office are, therefore, the reasons for the existence of the
common-battery system.
=Common Battery vs. Magneto.= It must not be supposed, however, that the
common-battery system always has advantages over the magneto system, and
that it is superior to the magneto or local-battery system for all
purposes. It is the outward attractiveness of the common-battery system
and the arguments in its favor, so readily made by over-zealous
salesmen, that has led, in many cases, to the adoption of this system
when the magneto system would better have served the purpose of utility
and economy.
To say the least, the telephone transmission to be had from
common-battery systems is no better than that to be had from
local-battery systems, and as a rule, assuming equality in other
respects, it is not as good. It is perhaps true, however, that under
average conditions common-battery transmission is somewhat better,
because whereas the local batteries at the subscribers' stations in the
local-battery system are not likely to be in uniformly first-class
condition, the battery in a common-battery system will be kept up to its
full voltage except under the grossest neglect.
The places in which the magneto, or local-battery, system is to be
preferred to the common-battery system, in the opinion of the writers,
are to be found in the small rural communities where the lines have a
rather great average length; where a good many subscribers are likely to
be found on some of the lines; where the sources of electrical power
available for charging storage batteries are likely either not to exist,
or to be of a very uncertain nature; and where it is not commercially
feasible to employ a high-grade class of attendants, or, in fact, any
attendant at all other than the operator at the central office.
In large or medium-sized exchanges it is always possible to procure
suitable current for charging the storage batteries required in
common-battery systems, and it is frequently economical, on account of
the considerable quantity of energy that is thus used, to establish a
generating plant in connection with the central office for developing
the necessary electrical energy. In very small rural places there are
frequently no available sources of electrical energy, and the expense of
establishing a power plant for the purpose cannot be justified. But even
if there is an electric light or railway system in the small town, so
that the problem of available current supply does not exist, the
establishment of a common-battery system with its storage battery and
the necessary charging machinery requires the daily attendance at the
central office of some one to watch and care for this battery, and this,
on account of the small gross revenue that may be derived from a small
telephone system, often involves a serious financial burden.
There is no royal road to a proper decision in the matter, and no sharp
line of demarcation may be drawn between the places where common-battery
systems are superior to magneto and _vice versa_. It may be said,
however, that in the building of all new telephone plants having over
about 500 local subscribers, the common-battery system is undoubtedly
superior to the magneto. If the plant is an old one, however, and is to
be re-equipped, the continuance of magneto apparatus might be justified
for considerably larger exchanges than those having 500 subscribers.
Telephone operating companies who have changed over the equipment of old
plants from magneto to common battery have sometimes been led into
rather serious difficulty, owing to the fact that their lines, while
serving tolerably well for magneto work, were found inadequate to meet
the more exacting demands of common-battery work. Again in an old plant
the change from magneto to common-battery equipment involves not only
the change of switchboards, but also the change of subscribers'
instruments that are otherwise good, and this consideration alone often,
in our opinion, justifies the replacing of an old magneto board with a
new magneto board, even if the exchange is of such size as to demand a
small multiple board.
Where the plant to be established is of such size as to leave doubt as
to whether a magneto or a common-battery switchboard should be employed,
the questions of availability of the proper kind of power for charging
the batteries, the proper kind of help for maintaining the batteries and
the more elaborate central-office equipment, the demands and previous
education of the public to be served, all are factors which must be
considered in reaching the decision.
It is not proper to say that anything like all exchanges having fewer
than 500 local lines, should be equipped with magneto service. Where all
the lines are short, where suitable power is available, and where a good
grade of attendants is available--as, for instance, in the case of
private telephone exchanges that serve some business establishment or
other institution located in one building or a group of buildings--the
common-battery system is to be recommended and is largely used, even
though it may have but a dozen or so subscribers' lines. It is for such
uses, and for use in those regular public-service exchange systems where
the conditions are such as to warrant the common-battery system, and yet
where the number of lines and the traffic are small enough to be handled
by such a small group of operators that any one of them may reach over
the entire face of the board, that the simple non-multiple
common-battery system finds its proper field of usefulness.
=Line Signals.= The principles and means by which the subscriber is
enabled to call the central-office operator in a common-battery system
have been referred to briefly in Chapter III. We will review these at
this point and also consider briefly the way in which the line signals
are associated with the connective devices in the subscribers' lines.
_Direct-Line Lamp._ The simplest possible way is to put the line signal
directly in the circuit of the line in series with the central-office
battery, and so to arrange the jack of the corresponding line that the
circuit through the line signal will be open when the operator inserts a
plug into that jack. This arrangement is shown in Fig. 307 where the
subscriber's station at the left is indicated in the simplest of its
forms. It is well to repeat here that in all common-battery manual
systems, the subscriber's station equipment, regardless of the
arrangement or type of its talking and signaling apparatus, must have
these features: First, that the line shall be normally open to direct
currents at the subscriber's station; second, that the line shall be
closed to direct currents when the subscriber removes his receiver from
its hook in making or in answering a call; third, that the line
normally, although open to direct currents, shall afford a proper path
for alternating or varying currents through the signal receiving device
at the sub-station. The subscriber's station arrangement shown in Fig.
307, and those immediately following, is the simplest arrangement that
possesses these three necessary features for common-battery service.
[Illustration: Fig. 307. Direct-Line Lamp]
Considering the arrangement at the central office, Fig. 307, the two
limbs of the line are permanently connected to the tip and sleeve
contacts of the jack. These two main contacts of the jack normally
engage two anvils so connected that the tip of the jack is ordinarily
connected through its anvil to ground, while the sleeve of the jack is
normally connected through its anvil to a circuit leading through the
line signal--in this case a lamp--and the common battery, and thence to
ground. The operation is obvious. Normally no current may flow from the
common battery through the signal because the line is open at the
subscriber's station. The removal of the subscriber's receiver from its
hook closes the circuit of the line and allows the current to flow
through the lamp, causing it to glow. When the operator inserts the plug
into the jack, in response to the call, the circuit through the lamp is
cut off at the jack and the lamp goes out.
This arrangement, termed the direct-line lamp arrangement, is largely
used in small common-battery telephone systems where the lines are very
short, such as those found in factories or other places where the
confines of the exchange are those of a building or a group of
neighboring buildings. Many of the so-called private-branch exchanges,
which will be considered more in detail in a later chapter, employ this
direct-line lamp arrangement.
[Illustration: Fig. 308. Direct-Line Lamp with Ballast]
_Direct-Line Lamp with Ballast._ Obviously, however, this direct-line
lamp arrangement is not a good one where the lines vary widely in length
and resistance. An incandescent lamp, as is well known, must not be
subjected to too great a variation in current. If the current that is
just right in amount to bring it to its intended degree of illumination
is increased by a comparatively small amount, the life of the lamp will
be greatly shortened, and too great an increase will result in the
lamp's burning out immediately. On the other hand, a current that is too
small will not result in the proper illumination of the lamp, and a
current of one-half the proper normal value will just suffice to bring
the lamp to a dull red glow. With lines that are not approximately
uniform in length and resistance the shorter lines would afford too
great a flow of current to the lamps and the longer lines too little,
and there is always the danger present, unless means are taken to
prevent it, that if a line becomes short-circuited or grounded near the
central office, the lamp will be subjected to practically the full
battery potential and, therefore, to such a current as will burn it out.
One of the very ingenious and, we believe, promising methods that has
been proposed to overcome this difficulty is that of the iron-wire
ballast, alluded to in Chapter III. This, it will be remembered,
consists of an iron-wire resistance enclosed in a vacuum chamber and so
proportioned with respect to the flow of current that it will be
subjected to a considerable heating effect by the amount of current that
is proper to illuminate the lamp. As has already been pointed out,
carbon has a negative temperature coefficient, that is, its resistance
decreases when heated. Iron, on the other hand, has a positive
temperature coefficient, its resistance increasing when heated. When
such an iron-wire ballast is put in series with the incandescent lamp
forming the line signal, as shown in Fig. 308, it is seen that the
resistance of the carbon in the lamp filament and of the iron in the
ballast will act in opposite ways when the current increases or
decreases. An increase of current will tend to heat up the iron wire of
the ballast and, therefore, increase its resistance, and the ballast is
so proportioned that it will hold the current that may flow through the
lamp within the proper maximum and minimum limits, regardless of the
resistance of the line in which the lamp is used. This arrangement has
not gone into wide use up to the present time.
_Line Lamp with Relay._ By far the most common method of associating the
line lamp with the line is to employ a relay, of which the actuating
coil is in the line circuit, this relay serving to control a local
circuit containing the battery and the lamp. This arrangement and the
way in which these parts are associated with the jack are clearly
indicated in Fig. 309. Here the relay may receive any amount of current,
from the smallest which will cause it to pull up its armature, to the
largest which will not injure its winding by overheat. Relays may be
made which will attract their armatures at a certain minimum current and
which will not burn out when energized by currents about ten times as
large, and it is thus seen that a very large range of current through
the relay winding is permissible, and that, therefore, a very great
latitude as to line resistance is secured. On the other hand, it is
obvious that the lamp circuit, being entirely local, is of uniform
resistance, the lamp always being subjected, in the arrangement shown,
to practically the full battery potential, the lamp being selected to
operate on that potential.
[Illustration: Fig. 309. Line Lamp with Relay]
_Pilot Signals._ In the circuits of Figs. 307, 308, and 309, but a
single line and its associated apparatus is shown, and it may not be
altogether clear to the uninitiated how it is that the battery shown in
those figures may serve, without interference of any function, a larger
number of lines than one. It is to be remembered that this battery is
the one which serves not only to operate the line signals, but also to
supply talking current to the subscribers and to supply current for the
operation of the cord-circuit signals after the cord circuits are
connected with the lines.
In Fig. 310 this matter is made clear with respect to the association of
this common battery with the lines for operating the line signals, and
also another important feature of common-battery work is brought out,
viz, the pilot lamp and its association with a group of line lamps.
Three subscribers' lines only are shown, but this serves clearly to
illustrate the association of any larger number of lines with the common
battery. Ignoring at first the pilot relay and the pilot lamp, it will
be seen that each of the tip-spring anvils of the jacks is connected to
a common wire _1_ which is grounded. Each of the sleeve-contact anvils
is connected through the coil of the line relay to another common wire
_2_, which connects with the live side of the common battery. Obviously,
therefore, this arrangement corresponds with that of Fig. 309, since the
battery may furnish current to energize any one of the line relays upon
the closure of the circuit of the corresponding line. Each of the relay
armatures in Fig. 310 is connected to ground.
Here we wish to bring out an important thing about telephone circuit
diagrams which is sometimes confusing to the beginner, but which really,
when understood, tends to prevent confusion. The showing of a separate
ground for each of the line-relay armatures does not mean that literally
each one of these armatures is connected by a separate wire to earth,
and it is to be understood that the three separate grounds shown in
connection with these relay armatures is meant to indicate just such a
set of affairs as is shown in connection with the tip-spring anvils of
the jacks, all of which are connected to a common wire which, in turn,
is grounded. Obviously, the result is the same, but in the case of this
particular diagram it is seen that a great deal of crossing of lines is
prevented by showing a separate ground at each one of the relay
armatures. The same practice is followed in connection with the common
battery. Sometimes it is very inconvenient in a complicated diagram to
run all of the wires that are supposed to connect with one terminal of
the battery across the diagram to represent this connection. It is
permissible, therefore, and in fact desirable, that separate battery
symbols be shown wherever by so doing the diagram will be simplified,
the understanding being, in the absence of other information or of other
indications, that the same battery is referred to, just as the same
ground is referred to in connection with the relay armatures in the
figure under discussion.
Each line lamp in Fig. 310 is shown connected on one hand to its
corresponding line relay contact and on the other hand to a common wire
which leads through the winding of the pilot relay to the live side of
the battery. It is obvious here that whenever any one of the line relays
attracts its armature the local circuit containing the corresponding
lamp and the common battery will be closed and the lamp illuminated.
Whenever any line relay operates, the current, which is supplied to its
lamp, must come through the pilot-relay winding, and if a number of line
relays are energized, then the current flow of the corresponding lamps
must flow through this relay winding. Therefore, this relay winding must
be of low resistance, so that the drop through its winding may not be
sufficient to interfere with the proper burning of the lamps, even
though a large number of lamps be fed simultaneously through it. The
pilot relay must be so sensitive that the current, even through one
lamp, will cause it to attract its armature. When it does attract its
armature it causes illumination of the pilot lamp in the same way that
the line relays cause the illumination of the line lamps.
The pilot lamp, which is commonly associated with a group of line lamps
that are placed on any one operator's position of the switchboard, is
located in a conspicuous place in the switchboard cabinet and is
provided with a larger lens so as to make a more striking signal. As a
result, whenever any line lamp on a given position lights, the pilot
lamp does also and serves to attract the attention, even of those
located in distant portions of the room, to the fact that a call exists
on that position of the board, the line lamp itself, which is
simultaneously lighted, pointing out the particular line on which the
call exists.
Pilot lamps, in effect, perform similar service to the night alarm in
magneto boards, but, of course, they are silent and do not attract
attention unless within the range of vision of the operator. They are
used not only in connection with line lamps, but also in connection with
the cord-circuit lamps or signals, as will be pointed out.
[Illustration: Fig. 311. Battery Supply Through Impedance Coils]
[Illustration: Fig. 312. Battery Supply through Repeating Coils]
[Illustration: Fig. 313. Battery Supply with Impedance Coils and
Condensers]
=Cord Circuit.= _Battery Supply._ Were it not for the necessity of
providing for cord-circuit signals in common-battery switchboards, the
common-battery cord circuit would be scarcely more complex than that for
magneto working. Stripped of all details, such as signals, ringing and
listening keys, and operator's equipment, cord circuits of three
different types are shown in Figs. 311, 312, and 313. These merely
illustrate the way in which the battery is associated with the cord
circuits and through them with the line circuits for supplying current
for talking purposes to the subscribers. It is thought that this matter
will be clear in view of the discussion of the methods by which current
is supplied to the subscribers' transmitters in common-battery systems
as discussed in Chapter XIII. While the arrangements in this respect of
Figs. 311, 312, and 313 illustrate only three of the methods, these
three are the ones that have been most widely and successfully used.
_Supervisory Signals._ The signals that are associated with the cord
circuits are termed supervisory signals because of the fact that by
their means the operator is enabled to supervise the condition of the
lines during times when they are connected for conversation. The
operation of these supervisory signals may be best understood by
considering the complete circuits of a simple switchboard and must be
studied in conjunction with the circuits of the lines as well as those
of the cords.
[Illustration: Fig. 314. Simple Common-Battery Switchboard]
_Complete Circuit._ Such complete circuits are shown in Fig. 314. The
particular arrangement indicated is that employed by the Kellogg
Company, and except for minor details may be considered as typical of
other makes also. Two subscribers' lines are shown extending from
Station A and Station B, respectively, to the central office. The line
wires are shown terminating in jacks in the same manner as indicated in
Figs. 307, 308, and 309, and their circuits are normally continued from
these jacks to the ground on one side and to the line relay and battery
on the other. The jack in this case has three contacts adapted to
register with three corresponding contacts in each of the plugs. The
thimble of the jack in this case forms no part of the talking circuit
and is distinct from the two jack springs which form the line terminals.
It and the auxiliary contact _1_ in each of the plugs with which it
registers, are solely for the purpose of co-operating in the control of
the supervisory signals.
The tip and sleeve strands of the cord are continuous from one plug to
the other except for the condensers. The two batteries indicated in
connection with the cord circuit are separate batteries, a
characteristic of the Kellogg system. One of these batteries serves to
supply current to the tip and sleeve strand of the cord circuit through
the two windings _3_ and _4_, respectively, of the supervisory relay
connected with the answering side of the cord circuit, while the other
battery similarly supplies current through the windings _5_ and _6_ of
the supervisory relay associated with the calling side of the cord
circuit. The windings of these relays, therefore, act as impedance coils
and the arrangement by which battery current is supplied to the cord
circuits and, therefore, to the lines of the connected subscribers, is
seen to be the combined impedance coil and condenser arrangement
discussed in Chapter XIII.
As soon as a plug is inserted into the jack of a line, the line relay
will be removed from the control of the line, and since the two strands
of the cord circuit now form continuations of the two line conductors,
the supervisory relay will be substituted for the line relay and will be
under control of the line. Since all of the current which passes to the
line after a plug is inserted must pass through the cord-circuit
connection and through the relay windings, and since current can only
flow through the line when the subscriber's receiver is off its hook, it
follows that the supervisory relays will only be energized after the
corresponding plug has been inserted into a jack of the line and after
the subscriber has removed his receiver. Unlike the line relays, the
supervisory relays open their contacts to break the local circuits of
the supervisory lamps _7_ and _8_ when the relay coils are energized,
and to close them when de-energized; but the armatures of the
supervisory relays do alone control the circuits of the supervisory
lamps. These circuits are normally held open in another place, that is,
between the plug contacts _1_ and the jack thimbles. It is only,
therefore, when a plug is inserted into a jack and when the supervisory
relay is de-energized, that the supervisory lamp may be lighted. When a
plug is inserted into a jack and when the corresponding supervisory
relay is de-energized, the circuit may be traced from ground at the
cord-circuit batteries through the left-hand battery, for instance,
through lamp _7_, thence through the contacts of the supervisory relay
to the contact _1_ of the plug, thence through the thimble of the jack
to ground. When a plug is inserted into the jack, therefore, the
necessary arrangements are completed for the supervisory lamp to be
under the control of the subscriber. Under this condition, whenever the
subscriber's receiver is on its hook, the circuit of the line will be
broken, the supervisory relay will be de-energized, and the supervisory
lamp will be lighted. When, on the other hand, the subscriber's receiver
is off its hook, the circuit of the line will be complete, the
supervisory relay will be energized, and the supervisory lamp will be
extinguished.
_Salient Features of Supervisory Operation._ It will facilitate the
student's understanding of the requirements and mode of operation of
common-battery supervisory signals in manual systems, whether simple or
multiple, if he will firmly fix the following facts in his mind. In
order that the supervisory signal may become operative at all, some act
must be performed by the operator--this being usually the act of
plugging into a jack--and then, until the connection is taken down, the
supervisory signal is under the control of the subscriber, and it is
displayed only when the subscriber's receiver is placed on its hook.
_Cycle of Operations._ We may now trace through the complete cycle of
operations of the simple common-battery switchboard, the circuits of
which are shown in Fig. 314. Assume all apparatus in its normal
condition, and then assume that the subscriber at Station A removes his
receiver from its hook. This pulls up the line relay and lights the line
lamp, the pilot relay also pulling up and lighting the common pilot lamp
which is not shown. In response to this call, the operator inserts the
answering plug and throws her listening key _L.K._ The operator's
talking set is thus bridged across the cord circuit and she is enabled
to converse with the calling subscriber. The answering supervisory lamp
_7_ did not light when the operator inserted the answering plug into the
jack, because, although the contacts in the lamp circuit were closed by
the plug contact _1_ engaging the thimble of the jack, the lamp circuit
was held open by the attraction of the supervisory relay armature, the
subscriber's receiver being off its hook. Learning that the called-for
subscriber is the one at Station B, the operator inserts the calling
plug into the jack at that station and presses the ringing key _R.K._,
in order to ring the bell. The act of plugging in, it will be
remembered, cuts off the line-signaling apparatus from connection with
that line. As the subscriber at Station B was not at his telephone when
called and his receiver was, therefore, on its hook, the insertion of
the calling plug did not energize the supervisory relay coils _5_ and
_6_, and, therefore, that relay did not attract its armature. The
supervisory lamp _8_ was thus lighted, the circuit being from ground
through the right-hand cord-circuit battery, lamp _8_, back contacts of
the supervisory relay, third strand of the cord to contact _1_ of the
calling plug, and thence to ground through the thimble of the jack. The
lighting of this lamp is continued until the party at Station B responds
by removing his receiver from its hook, which completes the line
circuit, energizes relay windings _5_ and _6_, causes that relay to
attract its armature, and thus break the circuit of the lamp _8_. Both
supervisory lamps remain out as long as the two subscribers are
conversing, but when either one of them hangs up his receiver the
corresponding supervisory relay becomes de-energized and the
corresponding lamp lights. When both of the lamps become illuminated,
the operator knows that both subscribers are through talking and she
takes down the connection.
Countless variations have been worked in the arrangement of the line and
cord circuits, but the general mode of operation of this particular
circuit chosen for illustration is standard and should be thoroughly
mastered. The operation of other arrangements will be readily understood
from an inspection of the circuits, once the fundamental mode of
operation that is common to all of them is well in mind.
=Lamps.= The incandescent lamps used in connection with line and
supervisory signals are specially manufactured, but differ in no sense
from the larger lamps employed for general lighting purposes, save in
the details of size, form, and method of mounting. Usually these lamps
are rated at about one-third candle-power, although they have a somewhat
larger candle-power as a rule. They are manufactured to operate on
various voltages, the most usual operating pressures being 12, 24, and
48 volts. The 24-volt lamp consumes about one-tenth of an ampere when
fully illuminated, the lamp thus consuming about 2.4 watts. The 12- and
48-volt lamps consume about the same amount of energy and corresponding
amounts of current.
[Illustration: Fig. 315. Switchboard Lamp]
_Lamp Mounting._ The usual form of screw-threaded mounting employed in
lamps for commercial lighting was at first applied to the miniature
lamps used for switchboard work, but this was found unsatisfactory and
these lamps are now practically always provided with two contact strips,
one on each side of the glass bulb, these strips forming respectively
the terminals for the two ends of the filament within. Such a
construction of a common form of lamp is shown in Fig. 315, where these
terminals are indicated by the numerals _1_ and _2_, _3_ being a dry
wooden block arranged between the terminals at one end for securing
greater rigidity between them.
[Illustration: Fig. 316. Line Lamp Mounting]
The method of mounting these lamps is subject to a good deal of
variation in detail, but the arrangement is always such that the lamp is
slid in between two metallic contacts forming terminals of the circuit
in which the lamp is to operate. Such an arrangement of springs and the
co-operating mounting forming a sort of socket for the reception of
switchboard lamps is referred to as a _lamp jack_. These are sometimes
individually mounted and sometimes mounted in strips in much the same
way that jacks are mounted in strips. A strip of lamp jacks as
manufactured by the Kellogg Company is shown in Fig. 316. The
opalescent lens is adapted to be fitted in front of the lamp after it
has been inserted into the jack. Fig. 317 gives an excellent view of an
individually-mounted lamp jack with its lamp and lens, this also being
of Kellogg manufacture. This figure shows a section of the plug shelf
which is bored to receive a lamp. In order to protect the lamps and
lenses from breakage, due to the striking of the plugs against them, a
metal shield is placed over the lens, as shown in this figure, this
being so cut away as to allow sufficient openings for the light to shine
through. Sometimes instead of employing lenses in front of the lamps, a
flat piece of translucent material is used to cover the openings of the
lamp, this being protected by suitable perforated strips of metal. A
strip of lamp jacks employing this feature is shown in Fig. 318, this
being of Dean manufacture. An advantage of this for certain types of
work is that the flat translucent plate in front of the lamp may readily
carry designating marks, such as the number of the line or something to
indicate the character of the line, which marks may be readily changed
as required.
[Illustration: Fig. 317. Supervisory Lamp Mounting]
[Illustration: Fig. 318. Line Lamp Mounting]
[Illustration: Fig. 319. Individual Lamp Jacks]
In the types made by some manufacturers the only difference between the
pilot lamp and the line lamp is in the size of the lens in front of it,
the jack and the lamp itself being the same for each, while others use a
larger lamp for the pilot. In Fig. 319 are shown two individual lamp
jacks, the one at the top being for supervisory lamps and the one at the
bottom being provided with a large lens for serving as a pilot lamp.
[Illustration: TERMINAL ROOM APPARATUS IN PROCESS OF INSTALLATION
Installed by Dean Electric Company at Detroit, Mich.]
=Mechanical Signals.= As has been stated the so-called mechanical
signals are sometimes used in small common-battery switchboards instead
of lamps. Where this is done the coil of the signal, if it is a line
signal, is substituted in the line circuit in place of the relay coil.
If the signals are used in connection with cord circuits for supervisory
signals, their coils are put in the circuit in place of the supervisory
relay coils. (These signals are referred to in Chapter III in connection
with Fig. 23.) They are so arranged that the attraction of the armature
lifts a target on the end of a lever, and this causes a display of color
or form. The release of the armature allows this target to drop back,
thus obliterating the display. Such signals, often called _visual
signals_ and _electromagnet signals_, should be distinguished from the
drops considered in connection with magneto switchboards in which the
attraction of the armature causes the display of the signal by the
falling of a drop, the signal remaining displayed until restored by some
other means, the restoration depending in no wise on when the armature
is released.
_Western Electric._ The mechanical signal of the Western Electric
Company, shown in Fig. 320, has a target similar to that shown in Fig.
254 but without a latch. It is turned to show a different color by the
attraction of the armature and allowed to resume its normal position
when the armature is released.
[Illustration: Fig. 320. Mechanical Signal]
_Kellogg._ Fig. 321 gives a good idea of a strip of mechanical signals
as manufactured by the Kellogg Company. This is known as the _gridiron_
signal on account of the cross-bar striping of its target. The white
bars on the target normally lie just behind the cross-bars on the shield
in front, but a slight raising of the target--about one-eighth of an
inch--exposes these white bars to view, opposite the rectangular
openings in the front shield.
[Illustration: Fig. 321. Strip of Gridiron Signals]
_Monarch._ In Fig. 322 is shown the visual signal manufactured by the
Monarch Telephone Company.
[Illustration: Fig. 322. Mechanical Signal]
=Relays.= The line relays for common-battery switchboards likewise
assume a great variety of forms. The well-known type of relay employed
in telegraphy would answer the purpose well but for the amount of room
that it occupies, as it is sometimes necessary to group a large number
of relays in a very small space. Nearly all present-day relays are of
the single-coil type, and in nearly all cases the movement of the
armature causes the movement of one or more switching springs, which are
thus made to engage or disengage their associated spring or springs. One
of the most widely used forms of relays has an L-shaped armature hung
across the front of a forwardly projecting arm of iron, on the
knife-edge corner of which it rocks as moved by the attraction of the
magnet. The general form of this relay was illustrated in Fig. 95.
Sometimes this relay is made up in single units and frequently a large
number of such single units are mounted on a single mounting plate. This
matter will be dealt with more in detail in the discussion of
common-battery multiple switchboards. In other cases these relays are
built _en bloc_, a rectangular strip of soft iron long enough to afford
space for ten relays side by side being bored out with ten cylindrical
holes to receive the electromagnets. The iron of the block affords a
return path for the lines of force. The L-shaped armatures are hung over
the front edge of this block, so that their free ends lie opposite the
magnet cores within the block. This arrangement as employed by the
Kellogg Company is shown in two views in Figs. 323 and 324.
[Illustration: Fig. 323. Strip of Relays]
[Illustration: Fig. 324. Strip of Relays]
A bank of line relays especially adapted for small common-battery
switchboards as made by the Dean Company, is shown in Fig. 325.
[Illustration: Fig. 325. Bank of Relays]
=Jacks.= The jacks in common-battery switchboards are almost always
mounted in groups of ten or twenty, the arrangement being similar to
that discussed in connection with lamp strips. Ordinarily in
common-battery work the jack is provided with two inner contacts so as
to cut off both sides of the signaling circuit when the operator plugs
in. A strip of such jacks is shown in Fig. 326.
[Illustration: Fig. 326. Strip of Cut-Off Jacks]
Ringing and listening keys for simple common-battery switchboards differ
in no essential respect from those employed in magneto boards.
[Illustration: Fig. 327. Details of Lamp, Plug, and Key Mounting]
=Switchboard Assembly.= The general assembly of the parts of a simple
common-battery switchboard deserves some attention. The form of the
switchboard need not differ essentially from that employed in magneto
work, but ordinarily the cabinet is somewhat smaller on account of the
smaller amount of room required by its lamps and jacks. An excellent
idea of the line jacks and lamps, plugs, keys, and supervisory signals
may be obtained from Fig. 327, which is a detail view taken from a
Kellogg board. In the vertical panel of the board above the plug shelf
are arranged the line jacks and the lamps in rows of twenty each, each
lamp being immediately beneath its corresponding jack. Such jacks are
ordinarily mounted on 1/2-inch centers both vertically and horizontally,
so that a group of one hundred lamps and line jacks will occupy a space
only slightly over 10 by 5 inches. Such economy of space is not required
in the simple magneto board, because the space might easily be made
larger without in any way taxing the reach of the operator. The reason
for this comparatively close mounting is a result, not of the
requirements of the simple non-multiple common-battery board itself, but
of the fact that the jack strips and lamp strips, which are required in
very large numbers in multiple boards, have to be mounted extremely
close together, and as the same lamp strips and jack strips are often
available for simple switchboards, an economy in manufacture is effected
by adherence to the same general dimensions.
[Illustration: Fig. 328. Simple Common-Battery Switchboard with
Removable Relay Panel]
A rear view of a common form of switchboard cabinet, known as the
_upright type_ and manufactured by the Dean Company, is shown in Fig.
328. In this all the relays are mounted on a hinged rack, which, when
opened out as indicated, exposes the wiring to view for inspection or
repairs. Access to both sides of the relays is thus given to the
repairman who may do all his work from the rear of the board without
disturbing the operator.
Fig. 329 shows a three-position cabinet of Kellogg manufacture, this
being about the limit in size of boards that could properly be called
simple. Obviously, where a switchboard cabinet must be made of greater
length than this, _i. e._, than is required to accommodate three
operators, it becomes too long for the operators to reach all over it
without undue effort or without moving from their seats. The so-called
_transfer board_ and the _multiple board_ (to be considered in
subsequent chapters), constitute methods of relief from such a condition
in larger exchanges.
[Illustration: Fig. 329. Three-Position Lamp Board]
CHAPTER XXIII
TRANSFER SWITCHBOARD
When the traffic originating in a switchboard becomes so great as to
require so many operators that the board must be made so long that any
one of the operators cannot reach over its entire face, the simple
switchboard does not suffice. Either some form of transfer switchboard
or of multiple switchboard must be used. In this chapter the transfer
switchboard will be briefly discussed.
The transfer switchboard is so named because its arrangement is such
that some of the connections through it are handled by means of two
operators, the operator who answers the call transferring it to another
operator who completes the connection desired.
=Limitations of Simple Switchboard.= Conceive a number of simple magneto
switchboards, or a number of common-battery switchboards, arranged side
by side, their number being so great as to form, by their combination, a
board too long for the ordinary cords and plugs to reach between its
extremities. On each of these simple switchboards, which we will say are
each of the one-position type, there terminates a group of subscribers'
lines so great in number, considering the traffic on them, that the
efforts of one operator will just about be taxed to properly attend to
their calls during the busiest hours of the day. If, now, these
subscribers would be sufficiently accommodating to call for no other
subscribers than those whose lines terminate on the same switchboard
section or on one of the immediately adjacent switchboard sections, all
would be well, but subscribers will not be so restricted. They demand
universal service; that is, they demand the privilege of having their
own lines connected with the line of any other person in the exchange.
Obviously, in the arrangement just conceived, any operator may answer
any call originating at her own board and complete the connection with
the desired subscriber if that subscriber's jack terminates on her own
section or on one of the adjacent ones. Beyond that she is powerless
unless other means are provided.
=Transfer Lines.= In the transfer board these other means consist in the
provision of groups of local trunk lines or transfer lines extending
from each switchboard position to each other non-adjacent switchboard
position. When an operator receives a call for some line on a
non-adjacent position, having answered this call with her answering
plug, she inserts the calling plug into the jack of one of these
transfer lines that leads to the proper other section. The operator at
that section is notified either verbally or by signal, and she completes
the connection between the other end of the transfer line and the line
of the called subscriber; the connection between the two subscribers
thus being effected through the cords of the two operators in question
linked together by the transfer line. Such a transfer line as just
described, requiring the connection at each of its ends by one of the
plugs of the operator's cord pair, is termed a _jack-ended trunk_ or a
_jack-ended transfer line_ because each of its ends terminates in a
jack.
[Illustration: Fig. 330. Jack-Ended Transfer Circuit]
There is another method of accomplishing the same general result by the
employment of the so-called _plug-ended trunk_ or _plug-ended transfer
line_. In this the trunk or transfer line terminates at one end, the
answering end, in a jack as before, and the connection is made with it
by the answering operator by means of the calling plug of the pair with
which she answered the originating call. The other end of this trunk,
instead of terminating in a jack, ends in a plug and the second operator
involved in the connection, after being notified, picks up this plug and
inserts it in the jack of the called subscriber, thus completing the
connection without employing one of her regular cord pairs.
_Jack-Ended Trunk._ In Fig. 330 are shown the circuits of a commonly
employed jack-ended trunk for transfer boards. The talking circuit, as
usual, is shown in heavy lines and terminates in the tip and sleeve of
the transfer jacks at each end. The auxiliary contacts in these jacks
and the circuits connecting them are absolutely independent of the
talking circuit and are for the purpose of signaling only, the
arrangement of the jacks being such that when a plug is inserted, the
spring _1_ will break from spring _2_ and make with spring _3_.
Obviously, the insertion of a plug in either of the jacks will establish
such connections as to light both lamps, since the engagement of spring
_1_ with spring _3_ in either of the jacks will connect both of the
lamps in multiple across the battery, this connection including always
the contacts _1_ and _2_ of the other jack. From this it follows that
the insertion of a plug in the other end of the trunk will, by breaking
contact between springs _1_ and _2_, put out both the lamps. One plug
inserted will, therefore, light both lamps; two plugs inserted or two
plugs withdrawn will extinguish both lamps.
[Illustration: Fig. 331. Jack-Ended Transfer Circuit]
If an operator located at one end of this trunk answers a call and finds
that the called-for subscriber's line terminates within reach of the
operator near the other end of this trunk, she will insert a calling
plug, corresponding to the answering plug used in answering a call, into
the jack of this trunk and thus light the lamp at both its ends. The
operator at the other end upon seeing this transfer lamp illuminated
inserts one of her answering plugs into the jack, and by means of her
listening key ascertains the number of the subscriber desired, and
immediately inserts her calling plug into the jack of the subscriber
wanted and rings him in the usual manner. The act of this second
operator in inserting her answering plug into the jack extinguishes the
lamp at her own end and also at the end where the call originated, thus
notifying the answering operator that the call has been attended to. As
long as the lamps remain lighted, the operators know that there is an
unattended connection on that transfer line. Such a transfer line is
called a _two-way_ line or a _single-track_ line, because traffic over
it may be in either direction. In Fig. 331 is shown a trunk that
operates in a similar way except that the two lamps, instead of being
arranged in multiple, are arranged in series.
[Illustration: Fig. 332. Jack- and Plug-Ended Transfer Circuit]
_Plug-Ended Trunk._ In Fig. 332 is shown a plug-ended trunk, this
particular arrangement of circuits being employed by the Monarch Company
in its transfer boards. This is essentially a one-way trunk, and traffic
over it can pass only in the direction of the arrow. Traffic in the
opposite direction between any two operators is handled by another trunk
or group of trunks similar to this but "pointed" in the other direction.
For this reason such a system is referred to as a _double-track_ system.
The operation of signals is the same in this case as in Fig. 330, except
that the switching device at the left-hand end of the trunk instead of
being associated with the jack is associated with the plug seat, which
is a switch closely associated with the seat of a plug so as to be
operated whenever the plug is withdrawn from or replaced in its seat.
The operation of this arrangement is as follows: Whenever an operator at
the right-hand end of this trunk receives a call for a subscriber whose
line terminates within the reach of the operator at the left-hand end of
the trunk, she inserts the calling plug of the pair used in answering
the calling subscriber into the jack of the trunk, and thus lights both
of the trunk lamps. The operator at the other end of the trunk, seeing
the trunk lamp lighted, raises the plug from its seat and, having
learned the wishes of the calling subscriber, inserts this plug into the
jack of the called subscriber without using one of her regular pairs.
When she raised the trunk plug from its seat, she permitted the long
spring _1_ of the plug seat switch to rise, thus extinguishing both
lamps and giving the signal to the originating operator that the trunk
connection has received attention. On taking down the connection, the
withdrawal of the plug from the right hand of the trunk lights both
lamps, and the restoring of the trunk plug to its normal seat again
extinguishes both lamps.
=Plug-Seat Switch.= The plug-seat switch is a device that has received a
good deal of attention not only for use with transfer systems, but also
for use in a great variety of ways with other kinds of manual switching
systems. The placing of a plug in its seat or withdrawing it therefrom
offers a ready means of accomplishing some switching or signaling
operation automatically. The plug-seat switch has, however, in spite of
its possibilities, never come into wide use, and so far as we are aware
the Monarch Telephone Manufacturing Company is the only company of
prominence which incorporates it in its regular output. The Monarch
plug-switch mechanism is shown in Fig. 333, and its operation is
obvious. It may be stated at this point that one of the reasons why the
plug-seat switch has not been more widely adopted for use, is the
difficulty that has been experienced due to lint from the switchboard
cords collecting on or about the contact points. In the construction
given in the detailed cut, upper part, Fig. 333, is shown the means
adopted by the Monarch Company for obviating this difficulty. The
contact points are carried in the upper portion of an inverted cup
mounted on the under side of the switchboard shelf, and are thus
protected, in large measure, from the damaging influence of dust and
lint.
[Illustration: Fig. 333. Plug-Seat Switch]
[Illustration: Fig. 334. Order-Wire Arrangement]
=Methods of Handling Transfers.= One way of giving the number of the
called subscriber to the second operator in a transfer system is to
have that operator listen in on the circuit after it is continued to her
position and receive the number either from the first operator or from
the subscriber. Receiving it from the first operator has the
disadvantage of compelling the first operator to wait on the circuit
until the second operator responds; receiving it from the subscriber has
the disadvantage of sometimes being annoying to him. This, however, is
to be preferred to the loss of time on the part of the originating
operator that is entailed by the first method. A better way than either
of these is to provide between the various operators working in a
transfer system, a so-called _order-wire_ system. An order wire, as
ordinarily arranged, is a circuit terminating at one end permanently in
the head receiver of an operator, and terminating at the other end in a
push button which, when depressed, will connect the telephone set of the
operator at that end with the order wire. The operator at the
push-button end of the order wire may, therefore, at will, communicate
with the other operator in spite of anything that the other operator may
do. An order-wire system suitable for transfer switchboards consists in
an order wire leading from each operator's receiver to a push button at
each of the other operator's positions, so that every operator has it
within her power to depress a key or button and establish communication
with a corresponding operator. When, therefore, an operator in a
transfer system answers a call that must be completed through a transfer
circuit, she establishes connection with that transfer circuit and then
informs the operator at the other end of that circuit by order wire of
the number of the trunk and the number of the subscriber with which that
trunk is to be connected. Fig. 334 shows a system of order-wire buttons
by means of which each operator may connect her telephone set with that
of every other operator in the room, the number in this case being
confined to three. Assuming that each pair of wires leading from the
lower portion of this figure terminates respectively in the operator's
talking apparatus of the three respective operators, then it is obvious
that operator No. 1, by depressing button No. 2, will connect her
telephone set with that of operator No. 2; likewise that any operator
may communicate with any other operator by depressing the key bearing
the corresponding number.
=Limitations of Transfer System.= It may be stated that the transfer
system at present has a limited place in the art of telephony. The
multiple switchboard has outstripped it in the race for popular approval
and has demonstrated its superiority in practically all large manual
exchange work. This is not because of lack of effort on the part of
telephone engineers to make the transfer system a success in a broad
way. A great variety of different schemes, all embodying the fundamental
idea of having one operator answer the call and another operator
complete it through a trunk line, have been tried. In San Francisco, the
Sabin-Hampton system was in fairly successful service and served many
thousands of lines for a number of years. It was, however, afterwards
replaced by modern multiple switchboards.
_Examples of Obsolete Systems._ The Sabin-Hampton system was unique in
many respects and involved three operators in each connection. It was
one of the very first systems which employed automatic signaling
throughout and did away with the subscribers' generators. It did not,
however, dispense with the subscribers' local batteries.
Another large transfer system, used for years in an exchange serving at
a time as many as 5,000, was employed at Grand Rapids, Michigan. This
was later replaced by an automatic switchboard.
[Illustration: Fig. 335. Three-Position Transfer Switchboard]
=Field of Usefulness.= The real field of utility for the transfer system
today is to provide for the growth of simple switchboards that have
extended beyond their originally intended limits. By the adding of
additional sections to the simple switchboard and the establishment of a
comparatively cheap transfer system, the simple boards may be made to do
continued service without wasting the investment in them by discarding
them and establishing a completely new system. However, switchboards are
sometimes manufactured in which the transfer system is included as a
part of the original equipment. In Fig. 335 is shown a three-position
transfer switchboard, manufactured by the Monarch Telephone Company. At
first glance the switchboard appears to be exactly like those described
in Chapter XXI, but on close observation, the transfer jacks and signals
may be seen in the first and third positions, just below the line jacks
and signals. There is no transfer equipment in the second position of
this switchboard because the operator at that position is able to reach
the jacks of all the lines and, therefore, is able to complete all calls
originating on her position without the use of any transfer equipment.
Referring to Fig. 301, which illustrates a two-position simple
switchboard, it may readily be seen that if the demands for telephone
service in the locality in which this switchboard is installed should
increase so as to require the addition of more switchboard positions,
this switchboard could readily be converted to a transfer switchboard by
placing the necessary transfer jacks and signals in the vacant space
between the line jacks and clearing-out drops.
[Illustration: CABLE TURNING SECTIONS, BETWEEN A AND B BOARDS Cortlandt
Office, New York Telephone Co.]
CHAPTER XXIV
PRINCIPLES OF THE MULTIPLE SWITCHBOARD
=Field of Utility.= The multiple switchboard, unlike the transfer board,
provides means for each operator to complete, without assistance, a
connection with any subscriber's line terminating in the switchboard no
matter how great the number of lines may be. It is used only where the
simple switchboard will not suffice; that is, where the number of lines
and the consequent traffic is so great as to require so many operators
and, therefore, so great a length of board as to make it impossible for
any one operator to reach all over the face of the board without moving
from her position.
=The Multiple Feature.= The fundamental feature of the multiple
switchboard is the placing of a jack for every line served by the
switchboard within the reach of every operator. This idea underlying the
multiple switchboard may be best grasped by merely considering the
mechanical arrangement and grouping of parts without regard to their
details of operation. The idea is sometimes elusive, but it is really
very simple. If the student at the outset will not be frightened by the
very large number of parts that are sometimes involved in multiple
switchboards, and by the great complexity which is apparent in the
wiring and in the action of these parts; and will remember that this
apparent complexity results from the great number of repetitions of the
same comparatively simple group of apparatus and circuits, much will be
done toward a mastery of the subject.
The multiple switchboard is divided into sections, each section being
about the width and height that will permit an ordinary operator to
reach conveniently all over its face. The usual width of a section
brought about by this limitation is from five and one-half to six feet.
Such a section affords room for three operators to sit side by side
before it. Now each line, instead of having a single jack as in the
simple switchboard, is provided with a number of jacks and one of these
is placed on each of the sections, so that each one of the operators may
have within her reach a jack for each line. It is from the fact that
each line has a multiplicity of jacks, that the term multiple
switchboard arises.
_Number of Sections._ Since there is a jack for each line on each
section of the switchboard, it follows that on each section there are as
many jacks as there are lines; that is, if the board were serving 5,000
lines there would be 5,000 jacks. Let us see now what it is that
determines the number of sections in a multiple switchboard. In the
final analysis, it is the amount of traffic that arises in the busiest
period of the day. Assume that in a particular office serving 5,000
lines, the subscribers call at such a very low rate that even at the
busiest time of the day only enough calls are made to keep, say, three
operators busy. In this case there would be no need for the multiple
switchboard, for a single section would suffice. The three operators
seated before that section would be able to answer and complete the
connections for all of the calls that arose. But subscribers do not call
at this exceedingly low rate. A great many more calls would arise on
5,000 lines during the busiest hour than could be handled by three
operators and, therefore, a great many more operators would be required.
Space has to be provided for these operators to work in, and as each
section accommodates three operators the total number of sections must
be at least equal to the total number of required operators divided by
three.
Let us assume, for instance, that each operator can handle 200 calls
during the busy hour. Assume further that during the busy hour the
average number of calls made by each subscriber is two. One hundred
subscribers would, therefore, originate 200 calls within this busy hour
and this would be just sufficient to keep one operator busy. Since one
operator can handle only the calls of one hundred subscribers during the
busy hour, it follows that as many operators must be employed as there
are hundreds of subscribers whose lines are served in a switchboard, and
this means that in an exchange of 5,000 subscribers, 50 operators'
positions would be required, or 16-2/3 sections. Each of these sections
would be equipped with the full 5,000 jacks, so that each operator could
have a connection terminal for each line.
_The Multiple._ These groups of 5,000 jacks, repeated on each of the
sections are termed multiple jacks, and the entire equipment of these
multiple jacks and their wiring is referred to as the multiple. It will
be shown presently that the multiple jacks are only used for enabling
the operator to connect with the called subscriber. In other words these
jacks are for the purpose of enabling each operator to have within her
reach any line that may be called for regardless of what line originates
the call. We will now consider what arrangements are provided for
enabling the operator to receive the signal indicating a call and what
provisions are made for her to answer the call in response to such a
signal.
=Line Signals.= Obviously it is not necessary to have the line signals
repeated on each section of the board as are the multiple jacks. If a
line has one definite place on the switchboard where its signal may be
received and its call may be answered, that suffices. Each line,
therefore, in addition to having its multiple jacks distributed one on
each section of the switchboard, has a line signal and an individual
jack immediately associated with it, located on one only of the
sections. This signal usually is in the form of a lamp and is termed the
line signal, and this jack is termed the answering jack since it is by
means of it that the operator always answers a call in response to the
line signal.
_Distribution of Line Signals._ It is evident that it would not do to
have all of these line signals and answering jacks located at one
section of the board for then they would not be available to all of the
operators. They are, therefore, distributed along the board in such a
way that one group of them will be available to one operator, another
group to another operator, and so on; the number of answering jacks and
signals in any one group being so proportioned with respect to the
number of calls that come in over them during the busy hour that it will
afford just about enough calls to keep the operator at that position
busy.
We may summarize these conditions with respect to the jack and
line-signal equipment of the multiple switchboard by saying that each
line has a multiple jack on each section of the board and in addition to
this has on one section of the board an answering jack and a line
signal. These answering jacks and line signals are distributed in groups
along the face of the board so that each operator will receive her
proper quota of the originating calls which she will answer and, by
virtue of the multiple jack, be able to complete the connections with
the desired subscribers without moving from her position.
=Cord Circuits.= Each operator is also provided with a number of pairs
of cords and plugs with proper supervisory or clearing-out signals and
ringing and listening keys, the arrangement in this respect being
similar to that already described in connection with the simple
switchboard.
=Guarding against Double Connections.= From what has been said it is
seen that a call originating on a given line may be answered at one
place only, but an outgoing connection with that line may be made at any
position. This fact that a line may be connected with when called for at
any one of the sections of the switchboard makes necessary the provision
that two or more connections will not be made with the same line at the
same time. For instance, if a call came in over a line whose signal was
located on the first position of the switchboard for a connection with
line No. 1,000, the operator at the first position would connect this
calling line with No. 1,000 through the multiple jack on the first
section of the switchboard. Assume now that some line, whose signal was
located on the 39th position of the switchboard, should call also for
line No. 1,000 while that line was still connected with the first
calling subscriber. Obviously confusion would result if the operator at
the 39th position, not knowing that line No. 1,000 was already busy,
should connect this second line with it, thereby leaving both of the
calling subscribers connected with line No. 1,000, and as a result all
of these three subscribers connected together.
The provisions for suitable means for preventing the making of a
connection with a line that is already switched at some other section of
the switchboard, has offered one of the most fertile fields for
invention in the whole telephone art. The ways that have been proposed
for accomplishing this are legion. Fortunately common practice has
settled on one general plan of action and that is to so arrange the
circuits that whenever a line is switched at one section, such an
electrical condition will be established on the forward contacts of all
of its multiple jacks that any operator at any other section in
attempting to make a connection with that line will be notified of the
fact that it is already switched by an audible signal, which she will
receive in her head receiver. On the other hand the arrangement is such
that when a line is not busy, _i. e._, it is not switched at any of the
positions of the switchboard, the operator on attempting to make a
connection with such a line will receive no such guarding signal and
will, therefore, proceed with the connection.
We may liken a line in a multiple switchboard to a lane having a number
of gates giving access to it. One of these gates--the answering jack--is
for the exclusive use of the proprietor of that lane. All of the other
gates to the lane--the multiple jacks--are for affording means for the
public to enter. But whenever any person enters one of these gates, a
signal is automatically put up at all of the other gates forbidding any
other person to enter the lane as long as the first person is still
within.
[Illustration: Fig. 336. Principle of Multiple Switchboard]
=Diagram Showing Multiple Board Principle.= For those to whom the
foregoing description of the multiple board is not altogether clear, the
diagram of Fig. 336 may offer some assistance. Five subscribers' lines
are shown running through four sections of a switchboard. Each of these
lines is provided with a multiple jack on each section of the board.
Each line is also provided with an answering jack and a line signal on
one of the sections of the board. Thus the answering jacks and the line
signals of lines _1_ and _2_ are shown in Section I, that of line _4_ is
shown in Section II, that of line _3_ in Section III, and that of line
_5_ in Section IV. At Section I, line _1_ is shown in the condition of
having made a call and having had this call answered by the operator
inserting one of her plugs into its answering jack. In response to the
instructions given by the subscriber, the operator has inserted the
other plug of this same pair in the multiple jack of line _2_, thus
connecting these two lines for conversation. At Section III, line _3_ is
shown as having made a call, and the operator as having answered by
inserting one of her plugs into the answering jack. It happens that the
subscriber on line _3_ requests a connection with line _1_, and the
condition at Section III is that where the operator is about to apply
the tip of the calling plug to the jack of line _1_ to ascertain whether
or not that line is busy. As before stated, when the contact is made
between the tip of the calling plug and the forward contact of the
multiple jack, the operator will receive a click in the ear (by means
that will be more fully discussed in later chapters), this click
indicating to her that line _1_ is not available for connection because
it is already switched at some other section of the switchboard.
=Busy Test.= The busy signal, by which an operator in attempting to make
a connection is informed that the line is already busy, has assumed a
great variety of forms and has brought forth many inventions. It has
been proposed by some that the insertion of a plug into any one of the
jacks of a line would automatically close a little door in front of each
of the other jacks of the line, therefore making it impossible for any
other operator to insert a plug as long as the line is in use. It has
been proposed by others to ring bells or to operate buzzers whenever the
attempt was made by an operator to plug into a line that was already in
use. Still others have proposed to so arrange the circuits that the
operator would get an electric shock whenever she attempted to plug into
a busy line. The scheme that has met with universal adoption, however,
is that the operator shall, when the tip of her calling plug touches the
forward contact of the jack of a line that is already switched, receive
a click in her telephone which will forbid her to insert the plug. The
absence of this click, or silence in her telephone, informs her that she
may safely make the connection.
_Principle._ The means by which the operator receives or fails to
receive this click, according to whether the line is busy or idle, vary
widely, but so far as the writers are aware they all have one
fundamental feature in common. The tip of the calling plug and the test
contact of all of the multiple jacks of an idle line must be absolutely
at the same potential before the test, so that no current will flow
through the test circuit when the test is actually made. The test
thimbles of all the jacks of a busy line must be at a different
potential from the tip of the test plug so that a current will flow and
a click result when the test is made.
_Potential of Test Thimbles._ It has been found an easy matter to so
arrange the contacts in the jacks of a multiple switchboard that
whenever the line is idle the test thimbles of that line will be a
certain potential, the same as that of all the unused calling plug tips.
It has also been easy to so arrange these contacts that the insertion of
a plug into any one of the jacks will, by virtue of the contacts
established, change the potential of all the test thimbles of that line
so that they will be at a different potential from that of the tips of
the calling plugs. It has not been so easy, however, to provide that
these conditions shall exist under all conditions of practice. A great
many busy tests that looked well on paper have been found faulty in
practice. As is always the case in such instances, this has been true
because the people who considered the scheme on paper did not foresee
all of the conditions that would arise in practice. Many busy-test
systems will operate properly while everything connected with the
switchboard and the lines served by it remains in proper order. But no
such condition as this can be depended on in practice. Switchboards, no
matter how perfectly made and no matter with how great care they may be
installed and maintained, will get out of order. Telephone lines will
become grounded or short-circuited or crossed or opened. Such
conditions, in a faulty busy-test system, may result in a line that is
really idle presenting a busy test, and thus barring the subscriber on
that line from receiving calls from other lines just as completely as if
his line were broken. On the other hand, faulty conditions either in the
switchboard or in the line may make a line that is really busy, test
idle, and thus result in the confusion of having two or more subscribers
connected to the same line at the same time.
_Busy-Test Faults._ To show how elusive some of the faults of a busy
test may be, when considered on paper, it has come within the
observation of the writers that a new busy-test system was thought well
enough of by a group of experienced engineers to warrant its
installation in a group of very large multiple switchboards, the cost of
which amounted to hundreds of thousands of dollars, and yet when so
installed it developed that a single short-circuited cord in a position
would make the test inoperative on all the cords of that
position--obviously an intolerable condition. Luckily the remedy was
simple and easily applied.
In a well-designed busy-test system there should be complete silence
when the test is made of an idle line, and always a well-defined click
when the test is made of a busy line. The test on busy lines should
result in a uniform click regardless of length of lines or the condition
of the apparatus. It does not suffice to have a little click for an idle
line and a big click for a busy line, as practice has shown that this
results in frequent errors on the part of the operators.
Good operating requires that the tip of the calling plug be tapped
against the test thimble several times in order to make sure of the
state of the called line.
In some multiple switchboards the arrangement has been such that the
jacks of a line would test busy as soon as the subscriber on that line
removed his receiver from its hook to make a call, as well as while any
plug was in any jack of that line. The advocates of this added feature,
in connection with the busy test, have claimed that the receiver, when
removed from its hook in making a call, should make the line test busy
and that a line should not be connected with when the subscriber's
receiver was off its hook any more than it should be when it was already
connected with at some other section of the switchboard. While it is
true that a line may be properly termed busy when the subscriber has
removed his receiver in order to make a call, it is not true that there
is any real necessity for guarding against a connection with it while he
is waiting for the operator to answer. Leaving the line unguarded for
this brief period may result in the subscriber, who intended to make the
call, having to defer his call until he has conversed with the party who
is trying to reach him. This cannot be said to be a detriment to the
service, however, since the second party gets the connection he desires
much sooner than he otherwise would, and the first party may still make
his first intended call as soon as he has disposed of the party who
reached him while he was waiting for his own operator to answer. It may
be said, therefore, in connection with this matter of making the line
test busy as soon as a subscriber has removed his receiver from the
hook, that it is not considered an essential, and in case of those
switchboard systems which naturally work out that way it is not
considered a disadvantage.
=Field of Each Operator.= It was stated earlier in this chapter that as
each section accommodated three operators, the total number of sections
in a switchboard will be at least one-third the total number of required
operators. This thought needs further development, for to stop at that
statement is to arrive somewhat short of the truth. In order to do this
it is necessary to consider the field in the multiple, reached by each
operator. The section is of such size, or should be, that an operator
seated in the center position of it may, without undue effort, reach all
over the multiple. But the operator at the right-hand position cannot
reach the extreme left portion of the multiple of that section, nor can
the operator at the left reach the extreme right. How then may each
operator reach a jack for every line? Remembering that the multiple
jacks are arranged exactly the same in each section, each jack always
occupying the same relative position, it is easy to see that while the
operator at a right-hand position of a section cannot reach the
left-hand third of the multiple in her own section, she may reach the
left-hand third of the multiple in the section at her right, and this,
together with the center and right-hand thirds of her own section,
represents the entire number of lines. So it is with the left-hand
operator at any section, she reaches two-thirds of all the lines in the
multiple of her own section and one-third in that of the section at her
left.
_End Positions._ This makes it necessary to inquire about the operators
at the end positions of the entire board. To provide for these the
multiple is extended one-third of a section beyond them, so as to supply
at the ends of the switchboard jacks for those lines which the end
operators cannot reach on their own sections. Sometimes instead of
adding these end sections to the multiple for the end operators, the
same result is accomplished by using only the full and regular sections
of the multiple, and leaving the end positions without operators'
equipment, as well as without answering jacks, line signals, and cords
and plugs, so that in reality the end operator is at the middle position
of the end section. This, in our opinion, is the better practice, since
it leaves the sections standard, and makes it easier to extend the
switchboard in length, as it grows, by the mere addition of new sections
without disturbing any of the old multiple.
=Influence of Traffic.= We wish again to emphasize the fact that it is
the traffic during the busiest time of day and not the number of lines
that determine the size of a multiple switchboard so far as its length
is concerned. The number of lines determines the size of the multiple in
any one section, but it is the amount of traffic, the number of calls
that are made in the busiest period, that determines the number of
operators required, and thus the number of positions. Had this now very
obvious fact been more fully realized in the past, some companies would
be operating at less expense, and some manufacturers would have sold
less expensive switchboards.
The whole question as to the number of positions boils down to how many
answering jacks and line signals may be placed at each operator's
position without overburdening the operator with incoming traffic at the
busy time of day. Obviously, some lines will call more frequently than
others, and hence the proper number of answering jacks at the different
positions will vary. Obviously, also, due to changes in the personnel of
the subscribers, the rates of calling of different groups of lines will
change from time to time, and this may necessitate a regrouping of the
line signals and answering jacks on the positions; and changes in the
personnel of the operators or in their skill also demand such
regrouping.
_Intermediate Frame._ The intermediate distributing frame is provided
for this purpose, and will be more fully discussed in subsequent
chapters. Suffice it to say here that the intermediate distributing
frame permits the answering jacks and line signals to be shifted about
among the operators' positions, so that each position will have just
enough originating traffic to keep each of the operators economically
busy during the busiest time of the day.
CHAPTER XXV
THE MAGNETO MULTIPLE SWITCHBOARD
=Field of Utility.= The principles of the multiple switchboard set forth
in the last chapter were all developed long before the common-battery
system came into existence, and consequently all of the first multiple
switchboards were of the magneto type. Although once very widely used,
the magneto multiple switchboard has almost passed out of existence,
since it has become almost universal practice to equip exchanges large
enough to employ multiple boards with common-battery systems.
Nevertheless there is a field for magneto multiple switchboards, and in
this field it has recently been coming into increasing favor. In those
towns equipped with magneto systems employing simple switchboards or
transfer switchboards, and which require new switchboards by virtue of
having outgrown or worn out their old ones, the magneto multiple
switchboard is frequently found to best fit the requirements of economy
and good practice. The reason for this is that by its use the magneto
telephones already in service may be continued, no change being required
outside of the central office. Furthermore, with the magneto multiple
switchboard no provision need be made for a power plant, which, in towns
of small size, is often an important consideration. Again, many
companies operate over a considerable area, involving a collection of
towns and hamlets. It may be that all of these towns except one are
clearly of a size to demand magneto equipment and that magneto equipment
is the standard throughout the entire territory of the company. If,
however, one of the towns, by virtue of growth, demands a multiple
switchboard, this condition affords an additional argument for the
employment of the magneto multiple switchboard, since the same standards
of equipment and construction may be maintained throughout the entire
territory of the operating company, a manifest advantage. On the other
hand, it may be said that the magneto multiple switchboard has no proper
place in modern exchanges of considerable size--say, having upward of
one thousand subscribers--at least under conditions found in the United
States.
Notwithstanding the obsolescence of the magneto multiple switchboard for
large exchanges, a brief discussion of some of the early magneto
multiple switchboards, and particularly of one of the large ones, is
worth while, in that a consideration of the defects of those early
efforts will give one a better understanding and appreciation of the
modern multiple switchboard, and particularly of the modern multiple
common-battery switchboard, the most highly organized of all the manual
switching systems. Brief reference will, therefore, be made to the
so-called series multiple switchboard, and then to the branch terminal
multiple switchboard, which latter was the highest type of switchboard
development at the time of the advent of common-battery working.
[Illustration: Fig. 337. Series Magneto Multiple Switchboard]
=Series-Multiple Board.= In Fig. 337 are shown the circuits of a series
magneto multiple switchboard as developed by the engineers of the
Western Electric Company during the eighties. As is usual, two
subscribers' lines and a single cord circuit are shown. One side of each
line passes directly from the subscriber's station to one side of the
drop, and also branches off to the sleeve contact of each of the jacks.
The other side of the line passes first to the tip spring of the first
jack, thence to the anvil of that jack and to the tip spring of the next
jack, and so on in series through all of the jacks belonging in that
line to the other terminal of the drop coil. Normally, therefore, the
drop is connected across the line ready to be responsive to the signal
sent from the subscriber's generator. The cord circuit is of the
two-conductor type, the plugs being provided with tip and sleeve
contacts, the tips being connected by one of the flexible conductors
through the proper ringing and listening key springs, and the sleeve
being likewise connected through the other flexible conductor and the
other springs of the ringing and listening keys. It is obvious that when
any plug is inserted into a jack, the circuit of the line will be
continued to the cord circuit and at the same time the line drop will be
cut out of the circuit, because of the lifting of the tip spring of the
jack from its anvil. Permanently connected between the sleeve side of
the cord circuit and ground is a retardation coil _1_ and a battery.
Another retardation coil _2_ is connected between the ground and a
point on the operator's telephone circuit between the operator's head
receiver and the secondary of her induction coil. These two retardation
coils have to do with the busy test, the action of which is as follows:
normally, or when a line is not switched at the central office, the test
thimbles will all be at substantially ground potential, _i. e._, they
are supposed to be. The point on the operator's receiver circuit which
is grounded through the retardation coil _2_ will also be of ground
potential because of that connection to ground. In order to test, the
operator always has to throw her listening key _L.K._ into the listening
position. She also has to touch the tip of the calling plug _P_c to a
sleeve or jack of the line that is being tested. If, therefore, a test
is made of an idle or non-busy line, the touching of the tip of the
calling plug with the test thimble of that line will result in no flow
of current through the operator's receiver, because there will be no
difference of potential anywhere in the test circuit, which test circuit
may be traced from the test thimble of the line under test to the tip of
the calling plug, thence through the tip strand of the cord to the
listening key, thence to the outer anvil of the listening key on that
side, through the operator's receiver to ground through the impedance
coil _2_. If, however, the line had already been switched at some other
section by the insertion of either a calling or answering plug, all of
the test thimbles of that line would have been raised to a potential
above that of the ground, by virtue of the battery connected with the
sleeve side of the cord circuit through the retardation coil _1_. If the
operator had made a test of such a line, the tip of her testing plug
would have found the thimble raised to the potential of the battery and,
therefore, a flow of current would occur which would give her the busy
click. The complete test circuit thus formed in testing a busy line
would be from the ungrounded pole of the battery through the impedance
coil _1_ associated with the cord that was already in connection with
the line, thence to the sleeve strand of that cord to the sleeve of the
jack at which the line was already switched, thence through that portion
of the line circuit to which all of the sleeve contacts were connected,
and therefore to the sleeve or test thimble of the jack at which the
test is made, thence through the tip of the calling plug employed in
making the test through the tip side of that cord circuit to the outer
listening key contact of the operator making the test, and thence to
ground through the operator's receiver and the impedance coil _2_. The
resultant click would be an indication to the operator that the line was
already in use and that, therefore, she must not make the connection.
The condenser _3_ is associated with the operator's talking set and with
the extra spring in the listening key _L.K._ in such a manner that when
the listening key is thrown, the tip strand of the cord circuit is
divided and the condenser included between them. This is for the purpose
of preventing any potentials, which might exist on the line with which
the answering plug _P_a was connected, from affecting the busy-test
conditions.
_Operation._ The operation of the system aside from the busy-test
feature is just like that described in connection with the simple
magneto switchboard. Assuming that the subscriber at Station _A_ makes
the call, he turns his hand generator, which throws the drop on his line
at the central office. The operator, seeing the signal, inserts the
answering plug of one of her idle pairs of cords into the answering jack
and throws her listening key _L.K._ This enables the operator to talk
with the calling subscriber, and having found that he desires a
connection with the line extending to Station _B_, she touches the tip
of her calling plug to the multiple jack of that line that is within her
reach, it being remembered that each one of the multiple jacks shown is
on a different section. She leaves the listening key in the listening
position when she does this. If the line is busy, the click will notify
her that she must not make the connection, in which case she informs the
calling subscriber that the line is busy and requests him to call again.
If, however, she received no click, she would insert the calling plug
into the jack, thus completing the connection between the two lines. She
would then press the ringing key associated with the calling plug and
that momentarily disconnects the calling plug from the answering plug
and at the same time establishes connection between the ringing
generator and the called line. The release of the ringing key again
connects the calling and answering plugs and, therefore, connects the
two subscribers' lines ready for conversation. All that is then
necessary is that the called subscriber shall respond and remove his
receiver from its hook, the calling subscriber already having done this.
When the conversation is finished, both of the subscribers (if they
remember it) will operate their ringing generators, which will throw the
clearing-out drop as a signal to the operator for disconnection. If it
should become necessary for the operator to ring back on the line of the
calling subscriber, she may do so by pressing the ringing key associated
with the calling plug.
Frequently this multiple switchboard arrangement was used with grounded
lines, in which case the single line wire extending from the
subscriber's station to the switchboard was connected with the tip
spring of the first jack, the circuit being continued in series through
the jack to the drop and thence to ground through a high non-inductive
resistance.
_Defects._ This series multiple magneto system was used with a great
many variations, and it had a good many defects. One of these defects
was due to the necessary extending of one limb of the line through a
large number of series contacts in the jacks. This is not to be desired
in any case, but it was particularly objectionable in the early days
before jacks had been developed to their present high state of
perfection. A particle of dust or other insulating matter, lodging
between the tip spring and its anvil in any one of the jacks, would
leave the line open, thus disabling the line to incoming signals, and
also for conversation in case the break happened to occur between the
subscriber and the jack that was used in connecting with the line.
Another defect due to the same cause was that the line through the
switchboard was always unbalanced by the insertion of a plug, one limb
of the line always extending clear through the switchboard to the drop
and the other, when the plug was inserted, extending only part way
through the switchboard and being cut off at the jack where the
connection was made. The objection will be apparent when it is
remembered that the wires in the line circuit connecting the multiple
jacks are necessarily very closely bunched together and, therefore,
there is very likely to be cross-talk between two adjacent lines unless
the two limbs of each line are exactly balanced throughout their entire
length.
Again the busy-test conditions of this circuit were not ideal. The fact
that the test rings of the line were connected permanently with the
outside line circuit subjected these test rings to whatever potentials
might exist on the outside lines, due to any causes whatever, such as a
cross with some other wire; thus the test rings of an idle line might by
some exterior cause be raised to such a potential that the line would
test busy. It may be laid down as a fundamental principle in good
multiple switchboard practice that the busy-test condition should be
made independent of any conditions on the line circuit outside of the
central office, and such is not the case in this circuit just described.
[Illustration: CABLE RUN FROM INTERMEDIATE FRAME TO MULTIPLE Cortlandt
Office, New York Telephone Co.]
=Branch-Terminal Multiple Board.= The next important step in the
development of the magneto multiple switchboard was that which produced
the so-called branch-terminal board. This came into wide use in the
various Bell operating companies before the advent of the common-battery
systems. Its circuits and the principles of operation may be understood
in connection with Fig. 338. In the branch-terminal system there are no
series contacts in the jacks and no unbalancing of the line due to a
cutting off of a portion of the line circuit when a connection was made
with it. Furthermore, the test circuits were entirely local to the
central office and were not likely to be affected by outside conditions
on the line. This switchboard also added the feature of the automatic
restoration of the drops, thus relieving the operator of the burden of
doing that manually, and also permitting the drops to be mounted on a
portion of the switchboard that was not available for the mounting of
jacks, and thus permitting a greater capacity in jack equipment.
[Illustration: Fig. 338. Branch-Terminal Magneto Multiple Switchboard]
Each jack has five contacts, and the answering and multiple jacks are
alike, both in respect to their construction and their connection with
the line. The drops are the electrically self-restoring type shown in
Fig. 263. The line circuits extended permanently from the subscriber's
station to the line winding of the drop and the two limbs of the line
branched off to the tip and sleeve contacts _1_ and _2_ respectively of
each jack. Another pair of wires extended through the multiple parallel
to the line wires and these branched off respectively to the contact
springs _3_ and _4_ of each of the jacks. This pair of wires formed
portions of the drop-restoring circuit, including the restoring coil _6_
and the battery _7_, as indicated. The test thimble _5_ of each of the
jacks is connected permanently with the spring _3_ of the corresponding
jack and, therefore, with the wire which connects through the restoring
coil _6_ of the corresponding drop to ground through the battery _7_.
The plugs were each provided with three contacts. Two of these were the
usual tip and sleeve contacts connected with the two strands of the cord
circuit. The third contact _8_ was not connected with any portion of the
cord circuit, being merely an insulated contact on the plug adapted,
when the plug was fully inserted, to connect together the springs _3_
and _4_. The cord circuit itself is readily understood from the drawing,
having two features, however, which merit attention. One is the
establishing of a grounded battery connection to the center portion of
the winding of the receiver for the purposes of the busy test, and the
other is the provision of a restoring coil and restoring circuit for the
clearing-out drop, this circuit being closed by an additional contact on
the listening key so as to restore the clearing-out drop whenever the
listening key was operated.
_Operation._ An understanding of the operation of this system is easy.
The turning of the subscriber's generator, when the line was in its
normal condition, caused the display of the line signal. The insertion
of the answering plug, in response to this call, did three things: (1)
It extended the line circuit to the tip and sleeve strand of the cord
circuit. (2) It energized the restoring coil _6_ of the drop by
establishing the circuit from the contact spring _3_ through the plug
contact _8_ to the other contact spring _4_, thus completing the circuit
between the two normally open auxiliary wires. (3) The connecting of the
springs _3_ and _4_ established a connection from ground to the test
thimbles of all the jacks on a line, the spring _4_ being always
grounded and the spring _3_ being always connected to the test thimble
_5_.
It is to be noted that on idle lines the test rings are always at the
same potential as the ungrounded pole of the battery _7_, being
connected thereto through the winding _6_ of the restoring coil. On all
busy lines, however, the test rings are dead grounded through the
contact _8_ of the plug that is connected with the line.
The tip of the testing plug at the time of making a test will also be at
the same potential as that of the ungrounded pole of the battery _7_,
since this pole of the battery _7_ is always connected to the center
portion of the operator's receiver winding, and when the listening key
is thrown the tip of the calling plug is connected therewith and is at
the same potential. When, therefore, the operator touches the tip of
the calling plug to the test thimble of an idle line, she will get no
click, since the tip of the plug and the test thimble will be at the
same potential. If, however, the line has already been switched at
another section of the board, there will be a difference of potential,
because the test thimble will be grounded, and the circuit, through
which the current which causes the click flows, may be traced from the
ungrounded pole of the battery _7_ to the center portion of the
operator's receiver, thence through one-half of the winding to the tip
of the calling plug, thence to the test thimble of the jack under test,
thence to the spring _3_ of the jack on another section at which the
connection exists, through the contact _8_ on the plug of that jack to
the spring _4_, and thence to ground and back to the other terminal of
the battery _7_.
_Magnet Windings._ Coils of the line and clearing-out drops by which
these drops are thrown, are wound to such high resistance and impedance
as to make it proper to leave them permanently bridged across the
talking circuit. The necessity for cutting them out is, therefore, done
away with, with a consequent avoidance, in the case of the line drops,
of the provision of series contacts in the jacks.
_Arrangement of Apparatus._ In boards of this type the line and
clearing-out drops were mounted in the extreme upper portion of the
switchboard face so as to be within the range of vision of the operator,
but yet out of her reach. Therefore, the whole face of the board that
was within the limit of the operator's reach was available for the
answering and multiple jacks. A front view of a little over one of the
sections of the switchboard, involving three complete operator's
positions, is shown in Fig. 339, which is a portion of the switchboard
installed by the Western Electric Company in one of the large exchanges
in Paris, France. (This has recently been replaced by a common-battery
multiple board.) In this the line drops may be seen at the extreme top
of the face of the switchboard, and immediately beneath these the
clearing-out drops. Beneath these are the multiple jacks arranged in
banks of one hundred, each hundred consisting of five strips of twenty.
At the extreme lower portion of the jack space are shown the answering
jacks and beneath these on the horizontal shelf, the plugs and keys.
These jacks were mounted on 1/2-inch centers, both vertically and
horizontally and each section had in multiple 90 banks of 100 each,
making 9,000 in all. Subsequent practice has shown that this involves
too large a reach for the operators and that, therefore, 9,000 is too
large a number of jacks to place on one section if the jacks are not
spaced closer than on 1/2-inch centers. With the jack involving as many
parts as that required by this branch terminal system, it was hardly
feasible to make them smaller than this without sacrificing their
durability, and one of the important features of the common-battery
multiple system which has supplanted this branch-terminal magneto system
is that the jacks are of such a simple nature as to lend themselves to
mounting on 3/8-inch centers, and in some cases on 3/10-inch centers.
[Illustration: Fig. 339. Face of Magneto Multiple Switchboard]
=Modern Magneto Multiple Board.= Coming now to a consideration of modern
magneto multiple switchboards, and bearing in mind that such boards are
to be found in modern practice only in comparatively small installations
and then only under rather peculiar conditions, as already set forth, we
will consider the switchboard of the Monarch Telephone Manufacturing
Company as typical of good practice in this respect.
[Illustration: Fig. 340. Monarch Magneto Multiple Switchboard Circuits]
_Line Circuit._ The line and cord circuits of the Monarch system are
shown in Fig. 340. It will be seen that each jack has in all five
contacts, numbered from _1_ to _5_ respectively, of which _1_ and _4_
are the springs which register with the tip and ring contacts of the
plug and through which the talking circuit is continued, while _2_ and
_3_ are series contacts for cutting off the line drop when a plug is
inserted, and _5_ is the test contact or thimble adapted to register
with the sleeve contact on the plug when the plug is fully inserted. The
line circuit through the drop may be traced normally from one side of
the line through the drop coil, thence through all of the pairs of
springs _2_ and _3_ in the jacks of that line, and thence to spring _1_
of the last jack, this spring always being strapped to the spring _2_ in
the last jack, and thence to the other side of the line. All the ring
springs _1_ are permanently tapped on to one side of the line, and all
of the tip springs _4_ are permanently tapped to the other side of the
line. This system may, therefore, properly be called a branch-terminal
system. It is seen that as soon as a plug is inserted into any of the
jacks, the circuit through the drop will be broken by the opening of the
springs _2_ and _3_ in that jack. The drop shown immediately above the
answering jack is so associated mechanically with that jack as to be
mechanically self-restored when the answering plug is inserted into the
answering jack in response to a call. The arrangement in this respect is
the same as that shown in Fig. 259, illustrating the Monarch combined
drop and jack.
_Cord Circuit._ The cord circuit needs little explanation. The tip and
ring strands are the ones which carry the talking current and across
these is bridged the double-wound clearing-out drop, a condenser being
included in series in the tip strand between the two drop windings in
the manner already explained in connection with Fig. 284. The third or
sleeve strand of the cord is continuous from plug to plug, and between
it and the ground there is permanently connected a retardation coil.
_Test._ The test is dependent on the presence or absence of a path to
ground from the test thimbles through some retardation coil associated
with a cord circuit. Obviously, in the case of an idle line there will
be no path to ground from the test thimbles, since normally they are
merely connected to each other and are insulated from everything else.
When, however, a plug is inserted into a multiple or answering jack, the
test thimbles of that line are connected to ground through the
retardation coil associated with the third strand of the plug used in
making the connection. When the operator applies the tip of the calling
plug to a test contact of a multiple jack there will be no path to
ground afforded if the line is idle, while if it is busy the potential
of the tip of the test plug will cause a current to flow to ground
through the impedance coil associated with the plug used in making the
connection. This will be made clearer by tracing the test circuit. With
the listening key thrown this may be traced from the live side of the
battery through the retardation coil _6_, which is common to an
operator's position, thence through the tip side of the listening key to
the tip conductor of the calling cord, and thence to the tip of the
calling plug and the thimble of the jack under test. If the line is idle
there will be no path to ground from this point and no click will
result, but if the line is busy, current will flow from the tip of the
test plug to the thimble of the jack tested, thence by the test wire in
the multiple to the thimble of the jack at which a connection already
exists, and thence to ground through the third strand of the cord used
in making that connection and the impedance coil associated therewith.
The current which flows in this test circuit changes momentarily the
potential of the tip side of the operator's telephone circuit, thus
unbalancing her talking circuit and causing a click.
[Illustration: Fig. 341. Magneto Multiple Switchboard]
If this test system were used in a very large board where the multiple
would extend through a great many sections, there would be some
liability of a false test due to the static capacity of the test
contacts and the test wire running through the multiple. For small
boards, however, where the multiple is short, this system has proven
reliable. A multiple magneto switchboard employing the form of circuits
just described is shown in Fig. 341. This switchboard consists of three
sections of two positions each. The combined answering jacks and drops
may be seen at the lower part of the face of the switchboard and
occupying somewhat over one-half of the jack and drop space. The
multiple jacks are above the answering jacks and drops and it may be
noted that the same arrangement and number of these jacks is repeated in
each section. This switchboard may be extended by adding more sections
and increasing the multiple in those already installed to serve 1,600
lines.
_Assembly._ In connection with the assembly of these magneto multiple
switchboards, as installed by the Monarch Company, Fig. 342 shows the
details of the cord rack at the back of the board. It shows how the ends
of the switchboard cords opposite to the ends that are fastened to the
plugs are connected permanently to terminals on the cord rack, at which
point the flexible conductors are brought out to terminal clips or
binding posts, to which the wires leading from the other portions of the
cord circuit are led. In order to relieve the conductors in the cords
from strain, the outer braiding of the cord at the rack end is usually
extended to form what is called a _strain cord_, and this attached to an
eyelet under the cord rack, so that the weight of the cord and the cord
weights will be borne by the braiding rather than by the conductors.
This leaves the insulated conductors extending from the ends of the
cords free to hang loose without strain and be connected to the
terminals as shown. This method of connecting cords, with variations in
form and detail, is practically universal in all types of switchboards.
[Illustration: Fig. 342. Cord-Rack Connectors]
A detail of the assembly of the drops and jacks in such a switchboard
is shown in Fig. 343. The single pair of clearing-out drops is mounted
in the lower part of the vertical face of the switchboard just above the
space occupied by the plug shelf. Vertical stile strips extend above the
clearing-out drop space for supporting the drops and jacks. A single row
of 10 answering jacks and the corresponding line drops are shown in
place. Above these there would be placed, in the completely assembled
board, the other answering jacks and line signals that were to occupy
this panel, and above these the strips of multiple jacks. The rearwardly
projecting pins from the stile strips are for the support of the
multiple jack strips, these pins supporting the strips horizontally by
suitable multiple clips at the ends of the jack strips; the jack strips
being fastened from the rear by means of nuts engaging the screw
threads on these pins. This method of supporting drops and jacks is one
that is equally adaptable for use in other forms of boards, such as the
simple magneto switchboard.
[Illustration: Fig. 343. Drop and Jack Mounting]
[Illustration: Fig. 344. Keyboard Wiring]
In Fig. 344 is shown a detail photograph of the key shelf wiring in one
of these Monarch magneto switchboards. In this the under side of the
keys is shown, the key shelf being raised on its hinge for that purpose.
The cable, containing all of the insulated wires leading to these keys,
enters the space under the key shelf at the extreme left and from the
rear. It then passes to the right of this space where a "knee" is
formed, after which the cable is securely strapped to the under side of
the key shelf. By this construction sufficient flexibility is provided
for in the cable to permit the raising and lowering of the key shelf,
the long reach of the cable between the "knee" and the point of entry at
the left serving as a torsion member, so that the raising of the shelf
will give the cable a slight twist rather than bend it at a sharp angle.
CHAPTER XXVI
THE COMMON-BATTERY MULTIPLE SWITCHBOARD
=Western Electric No. 1 Relay Board.= The common-battery multiple
switchboard differs from the simple or non-multiple common-battery
switchboard mainly in the provision of multiple jacks and in the added
features which are involved in the provision for a busy test. The
principles of signaling and of supplying current to the subscribers for
talking are the same as in the non-multiple common-battery board. For
purposes of illustrating the practical workings of the common-battery
multiple switchboard, we will take the standard form of the Western
Electric Company, choosing this only because it is the standard with
nearly all the Bell operating companies throughout the United States.
[Illustration: Fig. 345. Line Circuit Western Electric No. 1. Board]
_Line Circuit._ We will first consider the line circuit in simplified
form, as shown in Fig. 345. At the left in this figure the
common-battery circuit is shown at the subscriber's station, and at the
right the central-office apparatus is indicated so far as equipment of a
single line is concerned. In this simplified diagram no attempt has been
made to show the relative positions of the various parts, these having
been grouped in this figure in such a way as to give as clear and simple
an idea as possible of the circuit arrangements. It is seen at a glance
that this is a branch terminal board, the three contacts of each jack
being connected by separate taps or legs to three wires running
throughout the length of the board, these three wires being individual
to the jacks of one line. On this account this line circuit is commonly
referred to as a three-wire circuit. By the same considerations it will
be seen that the switchboard line circuit of the branch-terminal
multiple magneto system, shown in Fig. 338, would be called a four-wire
circuit. It will be shown later that other multiple switchboards in wide
use have a still further reduction in the number of wires running
through the jacks, or through the multiple as it is called, such being
referred to as two-wire switchboards.
The two limbs of the line which extend from the subscriber's circuit,
beside being connected by taps to the tip and sleeve contacts of the
jack respectively, connect with the two back contacts of a cut-off
relay, and when this relay is in its normal or unenergized condition,
these two limbs of the line are continued through the windings of the
line relay and thence one to the ungrounded or negative side of the
common-battery and the other to the grounded side. The subscriber's
station circuit being normally open, no current flows through the line,
but when the subscriber removes his receiver for the purpose of making a
call the line circuit is completed and current flows through the coil of
the line relay, thus energizing that relay and causing it to complete
the circuit of the line lamp. The cut-off relay plays no part in the
operation of the subscriber's calling, but merely leaves the circuit of
the line connected through to the calling relay and battery. The coil of
the cut-off relay is connected to ground on one side and on the other
side to the third wire running through the switchboard multiple and
which is tapped off to each of the test rings on the jacks. As will be
shown later, when the operator plugs into the jack of a line, such a
connection is established that the test ring of that jack will be
connected to the live or negative pole of the common battery, which will
cause current to flow through the coil of the cut-off relay, which will
then operate to _cut off_ both of the limbs of the line from their
normal connection with ground and the battery and the line relay. Hence
the name _cut-off relay_.
The use of the cut-off relay to sever the calling apparatus from the
line at all times when the line is switched serves to make possible a
very much simpler jack than would otherwise be required, as will be
obvious to anyone who tries to design a common-battery multiple system
without a cut-off relay. The additional complication introduced by the
cut-off relay is more than offset by the saving in complexity of the
jacks. It is desirable, on account of the great number of jacks
necessarily employed in a multiple switchboard, that the jacks be of the
simplest possible construction, thus reducing to a minimum their first
cost and making them much less likely to get out of order.
_Cord Circuit._ The cord circuit of the Western Electric standard
multiple common-battery switchboard is shown in Fig. 346. This cord
circuit involves the use of three strands in the flexible cords of both
the calling and the answering plugs. Two of these are the ordinary tip
and ring conductors over which speech is transmitted to the connected
subscriber's wire. The third, the sleeve strand, carries the supervisory
lamps and has associated with it other apparatus for the control of
these lamps and of the test circuit.
[Illustration: Fig. 346. Cord Circuit Western Electric No. 1 Board]
The system of battery feed is the well-known split repeating-coil
arrangement already discussed. The tip strand runs straight through to
the repeating coil, while the ring strand contains, in each case, the
winding of the supervisory relay corresponding to either the calling or
the answering plug. In order that the presence in the talking circuit of
a magnet winding possessing considerable impedance may not interfere
with the talking efficiency, each of these supervisory relay windings is
shunted by a non-inductive resistance. In practice the supervisory relay
windings have each a resistance of about 20 ohms and the shunt around
them each a resistance of about 31 ohms. In the third strand of each
cord is placed a 12-volt supervisory lamp, and in series with it a
resistance of about 80 ohms. Each supervisory relay is adapted, when
energized, to close a 40-ohm shunt about its supervisory lamp. The
arrangement and proportion of these resistances is such that when a plug
is inserted into the jack of a line the lamp will receive current from a
circuit traced from the negative pole of the battery in the center of
the cord circuit through the lamp and the 80-ohm series resistance,
through the third strand of the cord to the test thimble of the jack,
and thence to the positive or grounded pole of the battery through the
third conductor in the multiple and the winding of the cut-off relay.
This current always flows as long as the plug is inserted, and it is
just sufficient to illuminate the lamp when the supervisory relay
armature is not attracted. When, however, the supervisory relay armature
is attracted, the shunting of the lamp by the 40-ohm resistance cuts
down the current to such a degree as to prevent the illumination of the
lamp, although some current still flows through it.
The usual ringing and listening key is associated with the calling plug,
and in some cases a ring-back key is associated with the answering plug,
but this is not standard practice.
_Operation._ The operation of this cord circuit in conjunction with the
line circuit of Fig. 345 may best be understood by reference to Fig.
347. This figure employs a little different arrangement of the line
circuit in order more clearly to indicate how the two lines may be
connected by a cord; a study of the two line circuits, however, will
show that they are identical in actual connections. It is to be
remembered that all of the battery symbols shown in this figure
represent in reality the same battery, separate symbols being shown for
greater simplicity in circuit connections.
We will assume the subscriber at Station _A_ calls for the subscriber at
Station _B_. The operation of the line relay and the consequent lighting
of the line lamp, and also the operation of the pilot relay will be
obvious from what has been stated. The response of the operator by
inserting the answering plug into the answering jack, and the throwing
of her listening key so as to bridge her talking circuit across the cord
in order to place herself in communication with the subscriber, is also
obvious. The insertion of the answering plug into the answering jack
completed the circuit through the third strand of the cord and the
winding of the cut-off relay of the calling line, and this accomplishes
three desirable results. The circuit so completed may be traced from the
negative or ungrounded side of the battery to the center portion of the
cord circuit, thence through the supervisory lamp _1_, resistance _2_,
to the third conductor on the plug, test thimble on the jack, thence
through the winding of the cut-off relay to ground, which forms the
other terminal of the battery. The results accomplished by the closing
of this circuit are: first, the energizing of the cut-off relay to cut
off the signaling portion of the line; second, the flowing of current
through the lamp that is almost sufficient to illuminate it, but not
quite so because of the closure of the shunt about it, for the reason
that will be described; third, the raising of the potential of all the
contact thimbles on the jacks from zero to a potential different from
that of the ground and equal in amount to the fall of potential through
the winding of the cut-off relay. A condition is thus established at the
test rings such that some other operator at some other section in
testing the line will find it busy and will not connect with it.
[Illustration: Fig. 347. Western Electric No. 1 Board]
The reason why the lamp _1_, connected with the answering plug, was not
lighted was that the supervisory relay _3_, associated with the
answering plug, became energized when the operator plugged in, due to
the flow of current from the battery through the calling subscriber's
talking apparatus, this flow of current being permitted by the removal
of the calling subscriber's receiver from its hook. The energizing of
this relay magnet by causing the attraction of its armature, closed the
shunt about the lamp _1_, which shunt contains the 40-ohm resistance
_4_, and thus prevents the lamp from receiving enough current to
illuminate it. Obviously, as soon as the calling subscriber replaces his
receiver on its hook, the supervisory relay _3_ will be de-energized,
the shunt around the lamp will be broken, and the lamp will be
illuminated to indicate to the operator the fact that the subscriber
with whose line her calling plug is connected has replaced his receiver
on its hook.
_Testing--Called Line Idle._ Having now shown how the operator connects
with the calling subscriber's line and how that line automatically
becomes guarded as soon as it is connected with, so that no other
operator will connect with it, we will discuss how the operator tests
the called line and subsequently connects with that line, if it is found
proper to do so. If, on making the test with one of the multiple jacks
of the line leading to Station _B_, that line is idle and free to be
connected with, its test rings will all be at zero potential because of
the fact that they are connected with ground through the cut-off relay
winding with no source of current connected with them. The tip of the
calling plug will also be at zero potential in making this test, because
it is connected to ground through the tip side of the calling-plug
circuit and one winding of the cord-circuit repeating coil. As a result
no flow of current will occur, the operator will receive no click, and
she will know that she is free to connect with the line. As soon as she
does so, by inserting the plug, the third strand of the cord will be
connected with the test thimble of the calling line and the resulting
flow of current will bring about three results, two of which are the
same, and one of which is slightly different from those described as
resulting from the insertion of the answering plug into the jack of the
calling line. First, the cut-off relay will be operated and cut off the
line signaling apparatus from the called line; second, a flow of current
will result through the calling supervisory lamp _5_, which in this case
will be sufficient to illuminate that lamp for the reason that the
called subscriber has not yet responded, the calling supervisory relay
_6_ has, therefore, not yet been energized, and the lamp has not,
therefore, been shunted by its associated resistance _7_; third, the
test thimbles of the called line will be raised to a potential above
that of the earth, and thus the line will be guarded against connection
at another section of the switchboard. As soon as the called subscriber
responds to the ringing current sent out by the operator, current will
flow over the cord circuit and over his line through his transmitter.
This will cause the calling supervisory relay to be energized and the
calling lamp to be extinguished. Both lamps _1_ and _5_ remain
extinguished as long as the connected subscribers are in conversation,
but as soon as either one of them hangs up his receiver the
corresponding lamp will be lighted, due to the de-energization of the
supervisory relay and the breaking of the shunt around the lamp. The
lighting of both lamps associated with a cord circuit is a signal to the
operator for disconnection.
[Illustration: TERMINAL ROOM IN MEDIUM-SIZED MANUAL OFFICE Relay Rack at
Right. This Employs the Kellogg Parallel Arrangement of Frames.]
_Testing--Called Line Busy._ If we now assume that the called line was
already busy, by virtue of being connected with at another section, the
test rings of that line would accordingly all be raised to a potential
above that of the earth. As a result, when the operator applied the tip
of her calling plug to a test thimble on that line, current would flow
from this test thimble through the tip of the calling plug and tip
strand of the cord and through one winding of the cord-circuit repeating
coil to ground. This would cause a slight raising of potential of the
entire tip side of the cord circuit and a consequent momentary flow of
current through the secondary of the operator's circuit bridged across
the cord circuit at that time.
_Operator's Circuit Details._ The details of the operator's talking
circuit shown in Fig. 347 deserve some attention. The battery supply to
the operator's transmitter is through an impedance coil _9_. The
condenser _12_ is bridged around the transmitter and the two primary
windings _10_ and _11_, which windings are in parallel so as to afford a
local circuit for the passage of fluctuating currents set up by the
transmitter. The two primary windings _10_ and _11_ are on separate
induction coils, the secondary windings _13_ and _14_ being, therefore,
on separate cores. The winding _15_, in circuit with the secondary
winding _14_ and the receiver, is a non-inductive winding and is
supposed to have a resistance about equal to the effective resistance to
fluctuating currents of a subscriber's line of average length. Owing to
the respective directions of the primary and secondary windings _10_ and
_11_, _13_ and _14_, the result is that the outgoing currents set up by
the operator's transmitter are largely neutralized in the operator's
receiver. Incoming currents from either of the connected subscribers,
however, pass, in the main, through the secondary coil _13_ and the
operator's receiver, rather than through the shunt path formed by the
secondary _14_, and the non-inductive resistance _15_. This is known as
an "anti-side tone" arrangement, and its object is to prevent the
operator from receiving her own voice transmission so loudly as to make
her ear insensitive to the feebler voice currents coming in from the
subscribers.
_Order-Wire Circuits._ The two keys _16_ and _17_, shown in connection
with the operator's talking circuit in Fig. 347, play no part in the
regular operation of connecting two local lines, as described above.
They are order-wire keys, and the circuits with which they connect lead
to the telephone sets of other operators at distant central offices, and
by pressing either one of these keys the operator is enabled to place
herself in communication over these so-called order-wire circuits with
such other operators. The function and mode of operation of these
order-wire circuits will be described in the next chapter, wherein
inter-office connections will be discussed.
_Wiring of Line Circuit._ The line circuits shown in Figs. 345 and 347
are, as stated, simplified to facilitate understanding, although the
connections shown are those which actually exist. The more complete
wiring of a single line circuit is shown in Fig. 348. The line wires are
shown entering at the left. They pass immediately, upon entering the
central office, through the main distributing frame, the functions and
construction of which will be considered in detail in a subsequent
chapter. The dotted portions of the circuit shown in connection with
this main distributing frame indicate the path from the terminals on one
side of the frame to those on the other through so-called jumper wires.
The two limbs of the line then pass to terminals _1_ and _2_ on one side
of the so-called intermediate distributing frame. Here the circuit of
each limb of the line divides, passing, on the one hand, to the tip and
sleeve springs of all the multiple jacks belonging to that line; and, on
the other hand, through the jumper wires indicated by dotted lines on
the intermediate distributing frame, and thence to the tip and ring
contacts of the answering jack. A consideration of this connection will
show that the actual electrical connections so far as already described
are exactly those of Figs. 345 and 347, although those figures omitted
the main and intermediate distributing frames. Only two limbs of the
line are involved in the main frame. In the intermediate frame the test
wire running through the multiple is also involved. This test wire, it
will be seen, leads from the test thimbles of all the multiple jacks to
the terminal _3_ on the intermediate frame, thence through the jumper
wire to the terminal _6_ of this frame, and to the test thimble of the
answering jack. Here again the electrical connections are exactly those
represented in Figs. 345 and 347, although those figures do not show the
intermediate frame.
The two terminals _4_ and _5_ of the intermediate frame, besides being
connected to the tip and sleeve springs of the answering jack, are
connected to the contacts of the cut-off relay, and thence through the
coils of the line relay to ground on one side and to battery on the
other. Thus the line relay and battery are normally included in the
circuit of the line. The contact _6_ on the intermediate distributing
frame, besides being connected to the test thimble of all the jacks, is
connected through the coil of the cut-off relay to ground, thus
establishing a path by which current is supplied to the cut-off relay
when connection is made to the line at any jack. There is another
contact _7_ on the intermediate distributing frame which merely forms a
terminal for joining one side of the line lamp to the back contact of
the line relay.
_Functions of Distributing Frames._ Since the line circuit thus far
described in connection with Fig. 348 is exactly the same as that of
Fig. 345 in its electrical connections, it becomes obvious that the main
and intermediate distributing frames play no part in the operation of
the circuit any more than a binding post of a telephone plays a part in
its operation. These frames carry terminals for facilitating the
connection of the various wires in the line circuit and, as will be
shown later, for facilitating certain changes in the line connection.
[Illustration: Fig. 348. Line Circuit No. 1 Board]
Remembering that the dotted lines in Fig. 348 indicate jumper wires of
the main and intermediate distributing frames, and that these are in the
nature of temporary or readily changeable connections, and that the full
lines, whether heavy or light, are permanent connections not readily
changeable, it will be seen that the wires leading through the multiple
jacks of a certain line are permanently associated with each other, and
with certain terminals on the main distributing frame and certain other
terminals on the intermediate distributing frame. It will also be seen
that the line lamp and the answering jack, together with the cut-off
relay and line relay, are permanently associated with each other and
with another group of terminals _4_, _5_, _6_, and _7_ on the
intermediate distributing frame. It will also be apparent that by
changing the jumper wires on the main frame, any outside line may be
connected with any different set of line switchboard equipment, and also
that by making changes in the jumper wires on the intermediate frame,
any given answering jack and line lamp with its associated line cut-off
relay may be associated with any set of multiple jacks.
_Pilot Signals._ In a portion of the circuit leading from the battery
that is common to a group of line lamps is the winding of the pilot
relay, which is common to this group of line lamps. This controls, as
already described, the circuit of the pilot lamp common to the same
group of line lamps. In addition, a night-bell circuit is sometimes
provided, this usually being in the form of an ordinary polarized
ringer, the circuit of which is controlled by a night-bell relay common
to the entire office. Normally, this relay is shunted out of the circuit
of the common portion of the lead to the pilot relay contacts by the key
_8_, but when the key _8_ is opened all current that is fed to the pilot
lamps passes through the night-bell relay, and thus, whenever any pilot
lamp is lighted, the night-bell relay will attract its armature and thus
close the circuit of the calling generator through the night bell.
A study of this figure will make clear to the student how the portions
of the circuit that are individual to the line are associated with such
things as the battery, that are common to the entire office, and such as
the pilot relay and lamp, that are common to a group of lines
terminating in one position.
_Modified Relay Windings._ In some cases, the line relay instead of
being double wound, as shown, is made with a single winding, this
winding being normally included between the ring side of the cut-off
relay and the battery, the tip side of the cut-off relay being run
direct to ground. The present practice of the Western Electric Company
is towards the double-wound relay, however, and that is considered
standard in all of their large No. 1 multiple boards, except where the
customer, owing to special reasons, demands a single wound relay on the
ring side of the line. The prime reason for the two-winding line relay
is the lessened click in the calling-subscriber's receiver which occurs
when the operator answers. All line relays prior to 1902 were
single-wound, but after that they were made double and used some turns
of resistance wire to limit the normal calling current.
_Relay Mounting._ In the standard No. 1 relay board of the Western
Electric Company and, in fact, in nearly all common-battery multiple
boards that are manufactured by other companies, the line and cut-off
relays are mounted on separate racks outside the switchboard room and
adjacent to the main and intermediate distributing frames, the wiring
being extended from the relays to the jacks and lamps on the switchboard
proper by means of suitable cables. The Western Electric Company has
recently instituted a departure from this practice in the case of some
of their smaller No. 1 switchboard installations. Where it is thought
that the ultimate capacity required by the board will not be above 3,000
lines, the relay rack is dispensed with and all of the line and cut-off
relays, as well as the supervisory relays, are mounted in the rear of
the switchboard frame. For this purpose the line and cut-off relays are
specially made with the view to securing the utmost compactness. In
still other cases, in switchboards of relatively small ultimate
capacity, they use this small line and cut-off relay mounted on a
separate relay rack, in which case the board is the standard No. 1 board
except for the type of relays. In all of these modifications of the No.
1 board adapted for the use of the smaller and cheaper relays, the line
relay has but a single winding, the small size of the relay winding not
lending itself readily to double winding with the added necessary coil
terminals.
_Capacity Range._ The No. 1 Western Electric board is made in standard
sizes up to an ultimate capacity of 9,600 lines. For all capacities
above 4,900 lines, a 3/8-inch jack, vertical and horizontal face
dimensions, is employed. For this capacity the smaller types of cut-off
and line relays are not employed. Up to ultimate capacities of 4,900
lines, 1/2-inch jacks are employed, and either the small or the large
relays mounted on a separate rack are available. Up to 3,000 lines
ultimate capacity, the 1/2-inch jack is employed, and either the small
or the large cut-off and line relays are available, but in case the
small type is used the purchaser has the option of mounting them on a
separate relay rack, as in ordinary practice, or mounting them in the
switchboard cabinet and dispensing with the relay rack.
=Western Electric No. 10 Board.= The No. 1 common-battery multiple
switchboard, regardless of its size and type of arrangement of line and
cut-off relays, involves two relays for each line, the line relay
energized by the taking of the receiver off its hook, and the cut-off
relay energized by the act of the operator on plugging in and serving to
remove the line relay from the circuit whenever and as long as a plug is
inserted into any jack of the line. This seems to involve a considerable
expense in relays, but this, as has been stated, is warranted by the
greater simplicity in jacks which the use of the cut-off relay makes
possible. In addition to this expense of investment in the line and
cut-off relays, the amount of current required to hold up the cut-off
relays during conversations foots up to a considerable item of expense,
particularly as the system of supervisory signals is one in which the
supervisory lamp takes current not only while burning, but its circuit
takes even more current when the lamp is extinguished during the time of
a connection. For all of these reasons, and some other minor ones, it
was deemed expedient by the engineers of the Western Electric Company to
design a common-battery multiple switchboard for small and medium-sized
exchanges in which certain sacrifices might be made to the end of
accomplishing certain savings. The result has been a type of
switchboard, designated the No. 10, which may be found in a number of
Bell exchanges, it being considered particularly adaptable to
installations of from 500 to 3,000 lines. Although this board has been
subject to a good deal of adverse criticism, and although it seems
probable that even for the cheaper boards the No. 1 type with some of
the modifications just described will eventually supersede this No. 10
board, yet the present extent of use of the No. 10 board and the
instructive features which its type displays warrant its discussion
here.
_Circuits._ The circuits of this switchboard are shown in Fig. 349, this
indicating two-line circuits and a connecting cord circuit, together
with the auxiliary apparatus employed in connection with the operator's
telephone circuit, the pilot and night alarm circuits. The most
noticeable feature is that cut-off jacks are employed, the circuit of
the line normally extending through the sets of jack springs in the
multiple, and answering jacks to the line relay and battery on one side
of the line, and to ground on the other side. Obviously, the additional
complexity of the jack saves the use of a cut-off relay and the relay
equipment of each line consists, therefore, of but a single line relay,
which controls the lamp in an obvious manner.
[Illustration: Fig. 349. Western Electric No. 10 Board]
The cord circuit is of the three-conductor type, the two talking strands
extending to the usual split repeating-coil arrangement, and battery
current for talking purposes being fed through these windings as in the
standard No. 1 board. The supervisory relay is included in the ring
strand of the cord circuit and is shunted by a non-inductive resistance,
so that its impedance will not interfere with the talking currents. The
armature of the supervisory relay closes the lamp contact on its back
stroke, so that the lamp is always held extinguished when the relay is
energized. The supervisory lamp is included in a connection between the
back contact of the supervisory relay and ground, this connection
including the central-office battery. As a result, the illumination of
the supervisory lamp is impossible until a plug has been inserted into a
jack, in which case, assuming the supervisory relay to be de-energized,
the lamp circuit is completed through the wire connecting all of the
test thimbles and the resistance permanently bridged to ground from that
wire.
_Test._ For purposes of the test it is evident that the test rings of an
idle line are always at ground potential, due to their connection to
ground through the resistance coil. It is also evident that the tip of
an unused calling plug will always be at ground potential and,
therefore, that the testing of an idle line will result in no click in
the operator's receiver. When a line is switched, however, the potential
of all the test rings will be raised due to their being connected with
the live pole of the battery through the third strand of the cord. When
the operator in testing touches the test contact of the jack of a busy
line, a current will, therefore, flow from this test contact to the tip
strand of the cord and thence to ground through one of the repeating
coil windings. The potential of the tip side of the cord will,
therefore, be momentarily altered, and this will result in a click in
the operator's receiver bridged across the cord circuit at the time. The
details of the operator's cord circuit and of the pilot lamp and night
alarm circuits will be clear from the diagram.
_Operation._ A brief summary of the operation of this system is as
follows:
The subscriber removes his receiver from its hook, thus drawing up the
armature of the line relay and lighting his line lamp. The operator
answers. The line lamp is extinguished by the falling back of the
line-relay armature, due to the breaking of the relay circuit at the
jack contacts. The subscriber then receives current for his transmitter
through the cord-circuit battery connections. The supervisory relay
connected with the answering cord is not lighted, because, although the
lamp-circuit connection is completed at the jack, the supervisory relay
is operated to hold the lamp circuit open. Conversation ensues between
the operator and the subscriber, after which the operator tests the line
called for with the tip of the calling plug of the pair used in
answering. If the called line is not busy, no click will ensue, because
both the tested ring and the calling plug are at the same potential.
Finding no click, the operator will insert the plug and ring by means of
the ringing key. When the operator plugs in, the supervisory lamp,
associated with the calling plug, becomes lighted because the circuit is
completed at the jack and the supervisory relay remains de-energized,
since the line circuit is open at the subscriber's station. When the
called subscriber responds, the calling supervisory lamp goes out
because of the energization of the supervisory relay. Both lamps remain
out during the conversation, but when either subscriber hangs up, the
corresponding supervisory lamp will be lighted because of the falling
back of the supervisory relay armature.
If the called line is busy, a click will be heard, for the reason
described, and the operator will so inform the calling subscriber. It
goes without saying, that in any multiple-switchboard system a plug may
be found in the actual multiple jack that is reached for, in which case,
although no test will be made, the busy condition will be reported back
to the calling subscriber.
_Economy._ It has been the belief of the Western Electric engineers that
a real economy is accomplished in this type of board by the saving in
relay equipment. It is, of course, apparent at a glance that with a
switchboard long enough and of sections enough, the cost of extra jack
springs and their platinum contacts must become great enough to offset
the saving accomplished by omitting the cut-off relay. This makes it
apparent that if there is any economy in this type of multiple
switchboard, it must be found in the very small boards where there are
but few jacks per line and where the extra cost of the cut-off jack is
not enough to offset the extra cost of an added relay. It is the growing
belief, however, among engineers, that the multiple switchboard must be
very small indeed in order that the added complexity of the cut-off
jacks and wiring may be able to save anything over the two-relay type of
line; and it is believed that where economy is necessary in small
boards, it may be best effected by employing cheaper and more compact
forms of relays and mounting them, if necessary, directly in the
switchboard cabinet.
NOTE. These two standard types of common-battery multiple
switchboards of the Western Electric Company represent the
development through long years of careful work on the part of
the Western Electric and Bell engineers, credit being
particularly due to Scribner, McBerty, and McQuarrie of the
Western Electric Company, and Hayes of the American Telephone
and Telegraph Company.
=Kellogg Two-Wire Multiple Board.= The simplicity in the jacks permitted
by the use of the cut-off relay in the Western Electric common-battery
multiple switchboard for larger exchanges was carried a step further by
Dunbar and Miller in the development of the so-called two-wire
common-battery multiple switchboard, which for many years has been the
standard of the Kellogg Switchboard and Supply Company. The particular
condition which led to the development of the two-wire system was the
demand at that time on the Kellogg Company for certain very large
multiple switchboards, involving as many as 18,000 lines in the
multiple. Obviously, this necessitated a small jack, and obviously a
jack having only two contacts, a tip spring and a sleeve, could be made
more easily and with greater durability of this very small size than a
jack requiring three or more contacts. Other reasons that were
considered were, of course, cheapness in cost of construction and
extreme simplicity, which, other things being equal, lends itself to low
cost of maintenance.
_Line Circuit._ Like the standard Western Electric board for large
offices, the Kellogg two-wire board employs two relays for each line,
the line relay under the control of the subscriber and in turn
controlling the lamp, and a cut-off relay under the control of the
operator and in turn controlling the connection of the line relay with
the line. The line circuit as originally developed and as widely used by
the Kellogg Company is shown in Fig. 350. The extreme simplicity of the
jacks is apparent, as is also the fact that but two wires lead through
the multiple. Another distinguishing feature is, that all of the
multiple and answering jacks are normally cut off from the line at the
cut-off relay, but when the cut-off relay operates it serves, in
addition to cutting off the line relay, to attach the two limbs of the
line to the two wires leading through the multiple and answering jacks.
The control of the line relay by the subscriber's switch hook is clear
from the figure. The control of the cut-off relay is secured by
attaching one terminal of the cut-off relay winding permanently to that
wire leading through the multiple which connects with the sleeve
contacts of the jack, the other terminal of the cut-off relay being
grounded. The way in which this relay is operated will be understood
when it is stated that the sleeve contacts of both the answering and
calling plugs always carry full battery potential and, therefore,
whenever any plug is inserted into any jack, current flows from the
sleeve of the jack through the sleeve contact of the jack to ground,
through the winding of the cut-off relay, which relay becomes energized
and performs the functions just stated. It is seen that the wire
running through the multiple to which the sleeve jack contacts are
attached, is thus made to serve the double purpose of answering as one
side of the talking circuit, and also of performing the functions
carried out by the separate or third wire in the three-wire system. It
will be shown also that, in addition, this wire is made to lend itself
to the purposes of the busy test without any of these functions
interfering with each other in any way.
[Illustration: Fig. 350. Two-Wire Line Circuit]
_Cord Circuit._ The cord circuit in somewhat simplified form is shown in
Fig. 351. Here again there are but two conductors to the plugs and two
strands to the cords. This greater simplicity is in some measure offset
by the fact that four relays are required, two for each plug. This
so-called four-relay cord circuit may be most readily understood by
considering half of it at a time, since the two relays associated with
the answering plug act in exactly the same way as those connected with
the calling plug.
[Illustration: Fig. 351. Two-Wire Cord Circuit]
Associated with each plug of a pair are two relays _1_ and _2_, in the
case of the answering cord, and _3_ and _4_ in the case of the calling
cord. The coils of the relays _1_ and _2_ are connected in series and
bridged across the answering cord, a battery being included between the
coils in this circuit. The coils of the relays _3_ and _4_ are similarly
connected across the calling cord. A peculiar feature of the Kellogg
system is that two batteries are used in connection with the cord
circuit, one of them being common to all answering cords and the other
to all calling cords. The operation of the system would, however, be
exactly the same if a single battery were substituted for the two.
_Supervisory Signals._ Considering the relays associated with the
answering cord, it is obvious that these two relays _1_ and _2_ together
control the circuit of the supervisory lamp _5_, the circuit of this
lamp being closed only when the relay _1_ is de-energized and the relay
_2_ is energized. We will find in discussing the operation of these that
the relay _2_ is wholly under the control of the operator, and that the
relay _1_, after its plug has been connected with a line, is wholly
under the control of the subscriber on that line. It is through the
windings of these two relays that current is fed to the line of the
subscriber connected with the corresponding cord.
When a plug--the answering plug, for instance--is inserted into a jack,
current at once flows from the positive pole of the left-hand battery
through the winding of the relay _2_ to the sleeve of the plug, thence
to the sleeve of the jack and through the cut-off relay to ground. This
at once energizes the supervisory relay _2_ and the cut-off relay
associated with the line. The cut-off relay acts, as stated, to continue
the tip and sleeve wires associated with the jacks to the line leading
to the subscriber, and also to cut off the line relay. The supervisory
relay _2_ acts at the same time to attract its armature and thus
complete its part in closing the circuit of the supervisory lamp.
Whether or not the lamp will be lighted at this time depends on whether
the relay _1_ is energized or not, and this, it will be seen, depends on
whether the subscriber's receiver is off or on its hook. If off its
hook, current will flow through the metallic circuit of the line for
energizing the subscriber's transmitter, and as whatever current goes to
the subscriber's line must flow through the relay _1_, that relay will
be energized and prevent the lighting of the supervisory lamp _5_. If,
on the other hand, the subscriber's receiver is on its hook, no current
will flow through the line, the supervisory relay will not be energized,
and the lamp _5_ will be lighted.
In a nutshell, the sleeve supervisory relay normally prevents the
lighting of the corresponding supervisory lamp, but as soon as the
operator inserts a plug into the jack of the line, the relay _2_
establishes such a condition as to make possible the lighting of the
supervisory lamp, and the lighting of this lamp is then controlled
entirely by the relay _1_, which is, in turn, controlled by the position
of the subscriber's switch hook.
_Battery Feed._ A 2-microfarad condenser is included in each strand of
the cord, and battery is fed through the relay windings to the calling
and called subscribers on opposite sides of these condensers, in
accordance with the combined impedance coil and condenser method
described in Chapter XIII. Here the relay windings do double duty,
serving as magnets for operating the relays and as retardation coils in
the system of battery supply.
_Complete Cord and Line Circuits._ The complete cord and line circuits
of the Kellogg two-wire system are shown in Fig. 352. In the more recent
installations of the Kellogg Company the cord and line circuits have
been slightly changed from those shown in Figs. 350 and 351, and these
changes have been incorporated in Fig. 352. The principles of operation
described in connection with the simplified figures remain, however,
exactly the same. One of the changes is, that the tip side of the lines
is permanently connected to the tips of the jacks instead of being
normally cut off by the cut-off relay, as was done in the system as
originally developed. Another change is, that the line relay is
associated with the tip side of the line, rather than with the sleeve
side, as was formerly done. The cord circuit shown in Fig. 352 shows
exactly the same arrangement of supervisory relays and exactly the same
method of battery feed as in the simplified cord circuit of Fig. 351,
but in addition to this the detailed connections of the operator's
talking set and of her order-wire keys are indicated, and also the
ringing equipment is indicated as being adapted for four-party harmonic
work.
[Illustration: Fig. 352. Kellogg Two-Wire Board]
In connection with this ringing key it may be stated that the springs
_7_, _8_, _9_, and _10_ are individually operated by the pressure of one
of the ringing key buttons, while the spring _17_, connected with the
sleeve side of the calling plug, is always operated simultaneously with
the operation of any one of the other springs. As a result the proper
ringing circuit is established, it being understood that the upper
contacts of the springs _7_, _8_, _9_, and _10_ lead to the terminals of
their respective ringing generators, the other terminals of which are
grounded. The circuit is, therefore, from the generator, through the
ringing key, out through the tip side of the line, back over the sleeve
side of the line, and to ground through the spring _17_, resistance
_11_, and the battery, which is one of the cord-circuit batteries. The
object of this coil _11_ and the battery connection through it to the
ringing-key spring is to prevent the falling back of the cut-off relay
when the ringing key is operated. This will be clear when it is
remembered that the cut-off relay is energized by battery current fed
over the sleeve strand of the cord, and obviously, since it is necessary
when the ringing key is operated to cut off the supply wire back of the
key, this would de-energize the cut-off relay when the ringing key was
depressed, and the falling back of the cut-off relay contacts would make
it impossible to ring because the sleeve side of the line would be cut
off. The battery supply through the resistance _11_ is, therefore,
substituted on the sleeve strand of the cord for the battery supply
through the normal connection.
_Busy Test._ The busy test depends on all of the test rings being at
zero potential on an idle line and at a higher potential on a busy line.
Obviously, when the line is not switched, the test rings are at zero
potential on account of a ground through the cut-off relay. When,
however, a plug is inserted in either the answering or multiple jacks,
the test rings will all be raised in potential due to being connected
with the live side of the battery through the sleeve strand of the cord.
Conditions on the line external to the central office cannot make an
idle line test busy because, owing to the presence of the cut-off relay,
the sleeve contacts of all the jacks are disconnected from the line when
it is idle. The test circuit from the tip of the calling plug to ground
at the operator's set passes through the tip strand of the cord, thence
through a pair of normally closed extra contacts on the supervisory
relay _4_, thence in series through all the ringing key springs _10_,
_9_, _8_, and _7_, thence through an extra pair of springs _12_ and _13_
on the listening key--closed only when the listening key is
operated--and thence to ground through a retardation coil _14_. No
battery or other source of potential exists in this circuit between
ground and the tip of the calling plug and, therefore, the tip is
normally at ground potential. The sleeve ring of the jack being at
ground potential if the line is idle, no current will flow and no click
will be produced in testing such a line. If, however, the line is busy,
the test ring will be at a higher potential and, therefore, current will
flow from the tip of the calling plug to ground over the path just
traced, and this will cause a rise in potential at the terminal of the
condenser _15_ and a momentary flow of current through the tertiary
winding _16_ of the operator's induction coil; hence the click.
[Illustration: SWITCH ROOM OF CITIZENS' TELEPHONE COMPANY, GRAND RAPIDS,
MICH. One of the Earliest Large Automatic Offices.]
Obviously the testing circuit from the tip of the calling plug to ground
at the operator's set is only useful during the time when the calling
plug is not in a jack, and as the tip strand of the calling plug has to
do double duty in testing and in serving as a part of the talking
circuit, the arrangement is made that the testing circuit will be
automatically broken and the talking circuit through the tip strand
automatically completed when the plug is inserted into a jack in
establishing a connection. This is accomplished by means of the extra
contact on the relay _4_, which relay, it will be remembered, is held
energized when its corresponding plug is inserted in a jack. During the
time when the plug is not inserted, this relay is not energized and the
test circuit is completed through the back contact of its right-hand
armature. When connection is made at the jack, this relay becomes
energized and the tip strand of the cord circuit is made complete by the
right-hand lever being pulled against the front contact of this relay.
The keys shown to the right of the operator's set are order-wire keys.
_Summary of Operation._ We may give a brief summary of the operation of
this system as shown in Fig. 352. The left-hand station calls and the
line relay pulls up, lighting the lamp. The operator inserts an
answering plug in the answering jack, thus energizing the cut-off relay
which operates to cut off the line relay and to complete the connection
between the jacks and the external line. The act of plugging in by the
operator also raises the potential of all the test rings so as to guard
the line against intrusion by other callers. The supervisory lamp _5_
remains unlighted because, although the relay _2_ is operated, the relay
_1_ is also operated, due to the calling subscriber's receiver being off
its hook. The operator throws her listening key, communicates with the
subscriber, and, learning that the right-hand station is wanted,
proceeds to test that line. If the line is idle, she will get no click,
because the tip of her calling plug and the tested ring will be at the
same ground potential. She then plugs in and presses the proper
ringing-key button to send out the proper frequency to ring the
particular subscriber on the line--if there be more than one--the
current from the battery through the coil _11_ and spring _17_ serving
during this operation to hold up the cut-off relay.
As soon as the operator plugs in with the calling plug, the supervisory
lamp _6_ lights, assuming that the called subscriber had not already
removed his receiver from its hook, due to the fact that the relay _4_
is energized and the relay _3_ is not. As soon as the called subscriber
responds, the relay _3_ becomes energized and the supervisory lamp goes
out. If the line called for had been busy by virtue of being plugged at
another section, the tip of the operator's plug in testing would have
found the test ring raised to a potential above the ground, and, as a
consequence, current would have flowed from the tip of this plug through
the back contact of the right-hand lever of relay _4_, thence through
the ringing key springs and the auxiliary listening-key springs to
ground through the retardation coil _14_. This would have produced a
click by causing a momentary flow of current through the tertiary
winding _16_ of the operator's set.
_Wiring of Line Circuit._ The more complete wiring diagram of a single
subscriber's line, Fig. 353, shows the placing in the circuits of the
terminals and jumper wires of the main distributing frame and of the
intermediate distributing frame, and also shows how the pilot lamps and
night-alarm circuits are associated with a group of lines. The main
distributing frame occupies the same relative position in this line
circuit as in the Western Electric, being located in the main line
circuit outside of all the switchboard apparatus. The intermediate
distributing frame occupies a different relative position from that in
the Western Electric line. It will be recalled by reference to Fig. 348
that the line lamp and the answering jack were permanently associated
with the line and cut-off relays, such mutations of arrangement as were
possible at the intermediate distributing frame serving only to vary the
connection between the multiple of a line and one of the various groups
of apparatus consisting of an answering jack and line lamp and
associated relays. In the Kellogg arrangement, Fig. 353, the line and
cut-off relays, instead of being permanently associated with the
answering jack and line lamp, are permanently associated with the
multiple jacks, no changes, of which the intermediate or main frames are
capable, being able to alter the relation between a group of multiple
jacks and its associated line and cut-off relays. In this Kellogg
arrangement the intermediate distributing frame may only alter the
connection of an answering jack and line lamp with the multiple and its
permanently associated relays. The pilot and night alarm arrangements of
Fig. 353 should be obvious from the description already given of other
similar systems.
[Illustration: Fig. 353. Kellogg Two-Wire Line Circuit]
=Dean Multiple Board.= In Fig. 354 are shown the circuits of the
multiple switchboard of the Dean Electric Company. The subscriber's
station equipment shown at Station _A_ and Station _B_ will be
recognized as the Wheatstone-bridge circuit of the Dean Company.
_Line Circuit._ The line circuit is easily understood in view of what
has been said concerning the Western Electric line circuit, the line
relay _1_ being single wound and between the live side of the battery
and the ring side of the line. The cut-off relay _2_ is operated
whenever a plug is inserted in a jack and serves to sever the connection
of the line with the normal line signaling apparatus.
_Cord Circuit._ The cord circuit is of the four-relay type, but employs
three conductors instead of two, as in the two-wire system. The relay
_3_, being in series between the battery and the sleeve contact on the
plug, is energized whenever a plug is inserted in the jack, its winding
being placed in series with the cut-off relay of the line with which the
plug is connected. This completes the circuit through the associated
supervisory lamp unless the relay _4_ is energized, the local lamp
circuit being controlled by the back contact of relay _4_ and the front
contact of relay _3_. It is through the two windings of the relay _4_
that current is fed to the subscriber's station, and, therefore, the
armature of this relay is responsive to the movements of the
subscriber's hook. As the relay _3_ holds the supervisory lamp circuit
closed as long as a plug is inserted in a jack of the line, it follows
that during a connection the relay _4_ will have entire control of the
supervisory lamp.
_Listening Key._ The listening key, as usual, serves to connect the
operator's set across the talking strands of the cord circuit, and the
action of this in connection with the operator's set needs no further
explanation.
_Ringing Keys._ The ringing-key arrangement illustrated is adapted for
use with harmonic ringing, the single springs _5_, _6_, _7_, and _8_
each being controlled by a separate button and serving to select the
particular frequency that is to be sent to line. The two springs _9_ and
_10_ always act to open the cord circuit back of the ringing keys,
whenever any one of the selective buttons is depressed, in order to
prevent interference by ringing current with the other operations of the
circuit.
Two views of these ringing keys are shown in Figs. 355 and 356. Fig. 356
is an end view of the entire set. In Fig. 355 the listening key is shown
at the extreme right and the four selective buttons at the left. When a
button is released it rises far enough to cause the disengagement of the
contacts, but remains partially depressed to serve as an indication that
it was last used. The group of springs at the extreme left of Fig. 355
are the ones represented at _9_ and _10_ in Fig. 354 and by the anvils
with which those springs co-operate.
[Illustration: Fig. 354. Dean Multiple Board Circuits]
_Test._ The test in this Dean system is simple, and, like the Western
Electric and Kellogg systems, it depends on the raising of the
potential of the test thimbles of all the line jacks of a line when a
connection is made with that line by a plug at any position. When an
operator makes a test by applying the tip of the calling plug to the
test thimble of a busy line, current passes from the test thimble
through the tip strand of the cord to ground through the left-hand
winding of the calling supervisory relay _4_. The drop of potential
through this winding causes the tip strand of the cord to be raised to a
higher potential than it was before, and as a result the upper plate of
the condenser _11_ is thus altered in potential and this change in
potential across the condenser results in a click in the operator's ear.
[Illustration: Fig. 355. Dean Party Line Ringing Key]
[Illustration: Fig. 356. Dean Party Line Ringing Key]
=Stromberg-Carlson Multiple Board.= _Line Circuit._ In Fig. 357 is shown
the multiple common-battery switchboard circuits employed by the
Stromberg-Carlson Telephone Manufacturing Company. The subscriber's line
circuits shown in this drawing are of the three-wire type and, with the
exception of the subscriber's station, are the same as already described
for the Western Electric Company's system.
_Cord Circuit._ The cord circuit employed is of the two-conductor type,
the plugs being so constructed as to connect the ring and thimble
contacts of the jack when inserted. This cord circuit is somewhat
similar to that employed by the Kellogg Switchboard and Supply Company,
shown in Fig. 352, except that only one battery is employed, and that
certain functions of this circuit are performed mechanically by the
inter-action of the armatures of the relays.
_Supervisory Signals._ When the answering plug is inserted in a jack, in
response to a call, the current passing to the subscriber's station and
also through the cut-off relay must flow through the relay _1_, thus
energizing it. As the calling subscriber's receiver is at this time
removed from the hook switch, the path for current will be completed
through the tip of the jack, thence through the tip of the plug, through
relay _2_ to ground, causing relay _2_ to be operated and to break the
circuit of the answering supervisory lamp. The two relays _1_ and _2_
are so associated mechanically that the armature of _1_ controls the
armature of _2_ in such a manner as to normally hold the circuit of the
answering supervisory lamp open. But, however, when the plug is inserted
in a jack, relay _1_ is operated and allows the operation of relay _2_
to be controlled by the hook switch at the subscriber's station. The
supervisory relay _3_ associated with the calling cord is operated when
the calling plug is placed in a jack, and this relay normally holds the
armature of relay _4_ in an operated position in a similar manner as the
armature of relay _1_ controlled that of relay _2_. Supervisory relay
_4_ is under the control of the hook switch at the called subscriber's
station.
_Test._ In this circuit, as in several previously described, when a plug
is inserted in a jack of a line, the thimble contacts of the jacks
associated with that line are raised to a higher potential than that
which they normally have. The operator in testing a busy line, of course
having previously moved the listening key to the listening position,
closes a path from the test thimble of the jack, through the tip of the
calling plug, through the contacts of the relay _4_, the inside springs
of the listening key, thence through a winding of the induction coil
associated with her set to ground. The circuit thus established allows
current to flow from the test thimble of the jack through the winding of
her induction coil to ground, causing a click in her telephone receiver.
The arrangement of the ringing circuit does not differ materially from
that already described for other systems and, therefore, needs no
further explanation.
[Illustration: Fig. 357. Stromberg-Carlson Multiple Board Circuits]
=Multiple Switchboard Apparatus.= Coming now to a discussion of the
details of apparatus employed in multiple switchboards, it may be
stated that much of the apparatus used in the simpler types is capable
of doing duty in multiple switchboards, although, of course,
modification in detail is often necessary to make the apparatus fit the
particular demands of the system in which it is to be used.
_Jacks._ Probably the most important piece of apparatus in the multiple
switchboard is the jack, its importance being increased by the fact that
such very large numbers of them are sometimes necessary. Switchboards
having hundreds of thousands of jacks are not uncommon. The multiple
jacks are nearly always mounted in strips of twenty and the answering
jacks usually in strips of ten, the length of the jack strip being the
same in each case in the same board and, therefore, giving twice as wide
a spacing in the answering as in the multiple jacks. The distance
between centers in the multiple jacks varies from a quarter of an
inch--which is perhaps the extreme minimum--to half an inch, beyond
which larger limit there seems to be no need of going in any case. It is
customary that the jack strip shall be made of the same total thickness
as the distance between the centers of two of its jacks, and from this
it follows that the strips when piled one upon the other give the same
vertical distance between jack centers as the horizontal distance.
In Fig. 358 is shown a strip of multiple and a strip of answering jacks
of Western Electric make, this being the type employed in the No. 1
standard switchboards for large exchanges. In Fig. 359 are shown the
multiple and answering jacks employed in the No. 10 Western Electric
switchboard. The multiple jacks in the No. 1 switchboard are mounted on
3/8-inch centers, the jacks having three branch terminal contacts. The
multiple jacks of the No. 10 switchboard indicated in Fig. 359 are
mounted on 1/2-inch centers, each jack having five contacts as indicated
by the requirement of the circuits in Fig. 349.
In Fig. 360 are shown the answering and multiple jacks of the Kellogg
Switchboard and Supply Company's two-wire system. The extreme simplicity
of these is particularly well shown in the cut of the answering jack,
and these figures also show clearly the customary method of numbering
jacks. In very large multiple boards it has been the practice of the
Kellogg Company to space the multiple jacks on 3/10-inch centers, and in
their smaller multiple work, they employ the 1/2-inch spacing. With the
3/10-inch spacing that company has been able to build boards having a
capacity of 18,000 lines, that many jacks being placed within the reach
of each operator.
In all modern multiple switchboards the test thimble or sleeve contacts
are drawn up from sheet brass or German silver into tubular form and
inserted in properly spaced borings in strips of hard rubber forming the
faces of the jacks. These strips sometimes are reinforced by brass
strips on their under sides. The springs forming the other terminals of
the jack are mounted in milled slots in another strip of hard rubber
mounted in the rear of and parallel to the front strip and rigidly
attached thereto by a suitable metal framework. In this way desired
rigidity and high insulation between the various parts is secured.
[Illustration: Fig. 358. Answering and Multiple Jacks for No. 1 Board]
_Lamp Jacks._ The lamp jacks employed in multiple work need no further
description in view of what has been said in connection with lamp jacks
for simple common-battery boards. The lamp jack spacing is always the
same as the answering jack spacing, so that the lamps will come in the
same vertical alignment as their corresponding answering jacks when the
lamp strips and answering jack strips are mounted in alternate layers.
[Illustration: Fig. 359. Answering and Multiple Jacks for No. 10 Board]
[Illustration: Fig. 360. Answering and Multiple Jacks for Kellogg
Two-Wire Board]
_Relays._ Next in order of importance in the matter of individual parts
for multiple switchboards is the relay. The necessity for reliability of
action in these is apparent, and this means that they must not only be
well constructed, but that they must be protected from dust and moisture
and must have contact points of such a nature as not to corrode even in
the presence of considerable sparking and of the most adverse
atmospheric conditions. Economy of space is also a factor and has led to
the almost universal adoption of the single-magnet type of relay for
line and cut-off as well as supervisory purposes.
[Illustration: Fig. 361. Type of Line Relay]
[Illustration: Fig. 362. Type of Cut-Off Relay]
The Western Electric Company employs different types of relays for line,
cut-off, and supervisory purposes. This is contrary to the practice of
most of the other companies who make the same general type of relay
serve for all of these purposes. A good idea of the type of Western
Electric line relay, as employed in its No. 1 board, may be had from
Fig. 361. As is seen this is of the tilting armature type, the armature
rocking back and forth on a knife-edge contact at its base, the part on
which it rests being of iron and of such form as to practically
complete, with the armature and core, the magnetic circuit. The cut-off
relay, Fig. 362, is of an entirely different type. The armature in this
is loosely suspended by means of a flexible spring underneath two
L-shaped polar extensions, one extending up from the rear end of the
core and the other from the front end. When energized this armature is
pulled away from the core by these L-shaped pieces and imparts its
motion through a hard-rubber pin to the upper pair of springs so as to
effect the necessary changes in the circuit.
[Illustration: Fig. 363. Western Electric Combined Line and Cut-off
Relay]
[Illustration: Fig. 364. Western Electric Supervisory Relay]
[Illustration: Fig. 365. Line Relay No. 10 Board]
Much economy in space and in wiring is secured in the type of
switchboards employing cut-off as well as line relays by mounting the
two relays together and in making of them, in fact, a unitary piece of
apparatus. Since the line relay is always associated with the cut-off
relay of the same line and with no other, it is obvious that this
unitary arrangement effects a great saving in wiring and also secures a
great advantage in the matter of convenience of inspection. Such a
combined cut-off and line relay, employed in the Western Electric No. 1
relay board, is shown in Fig. 363. These are mounted in banks of ten
pairs, a common dust cap of sheet iron covering the entire group.
The Western Electric supervisory relay, Fig. 364, is of the tilting
armature type and is copper clad. The dust cap in this case fits on with
a bayonet joint as clearly indicated. In Fig. 365 is shown the line
relay employed in the Western Electric No. 10 board.
[Illustration: Fig. 366. Kellogg Line and Cut-off Relays]
[Illustration: Fig. 367. Strip of Kellogg Line and Cut-Off Relays]
The Kellogg Company employs the type of relay of which the magnetic
circuit was illustrated in Fig. 95. In its multiple boards it commonly
mounts the line and cut-off relays together, as shown in Fig. 366. A
single, soft iron shell is used to cover both of these, thus serving as
a dust shield and also as a magnetic shield to prevent cross-talk
between adjacent relays--an important feature, since it will be
remembered the cut-off relays are left permanently connected with the
talking circuit. Fig. 367, which shows a strip of twenty such pairs of
relays, from five of which the covers have been removed, is an excellent
detail view of the general practice in this respect; obviously, a very
large number of such relays may be mounted in a comparatively small
space. The mounting strip shown in this cut is of heavy rolled iron and
is provided with openings through which the connection terminals--shown
more clearly in Fig. 366--project. On the back of this mounting strip
all the wiring is done and much of this wiring--that connecting adjacent
terminals on the back of the relay strip--is made by means of thin
copper wires without insulation, the wires being so short as to support
themselves without danger of crossing with other wires. When these wires
are adjacent to ground or battery wires they may be protected by
sleeving, so as to prevent crosses.
[Illustration: Fig. 368. Monarch Relay]
An interesting feature in relay construction is found in the relay of
the Monarch Telephone Manufacturing Company shown in Figs. 368 and 369.
The assembled relay and its mounting strip and cap are shown in Fig.
368. This relay is so constructed that by the lifting of a single latch
not only the armature but the coil may be bodily removed, as shown in
Fig. 369, in which the latch is shown in its raised position. As seen,
the armature has an L-shaped projection which serves to operate the
contact springs lying on the iron plate above the coil. The simplicity
of this device is attractive, and it is of convenience not only from the
standpoint of easy repairs but also from the standpoint of factory
assembly, since by manufacturing standard coils with different
characters of windings and standard groups of springs, it is possible to
produce without special manufacture almost any combination of relay.
[Illustration: Fig. 369. Monarch Relay]
=Assembly.= The arrangement of the key and jack equipment in complete
multiple switchboard sections is clearly shown in Fig. 370, which shows
a single three-position section of one of the small multiple
switchboards of the Kellogg Switchboard and Supply Company. The
arrangement of keys and plugs on the key shelf is substantially the same
as in simple common-battery boards. As in the simple switchboards the
supervisory lamps are usually mounted on the hinged key shelf
immediately in the rear of the listening and ringing keys and with such
spacing as to lie immediately in front of the plugs to which they
correspond. The reason for mounting the supervisory lamps on the key
shelf is to make them easy of access in case of the necessity of lamp
renewals or repairs on the wiring. The space at the bottom of the
vertical panels, containing the jacks, is left blank, as this space is
obstructed by the standing plugs in front of it. Above the plugs,
however, are seen the alternate strips of line lamps and answering
jacks, the lamps in each case being directly below the corresponding
answering jacks. Above the line lamps and answering jacks in the two
positions at the right there are blank strips into which additional line
lamps and jacks may be placed in case the future needs of the system
demand it. The space above these is the multiple jack space, and it is
evident from the small number of multiple jacks in this little
switchboard that the present equipment of the board is small. It is also
evident from the amount of blank space left for future installations of
multiple jacks that a considerable growth is expected. Thus, while there
are but four banks of 100 multiple jacks, or 400 in all, there is room
in the multiple for 300 banks of 100 multiple jacks, or 3,000 in all.
The method of grouping the jacks in banks of 100 and of providing for
their future growth is clearly indicated in this figure. The next
section at the right of the one shown would contain a duplicate set of
multiple jacks and also an additional equipment of answering jacks and
lamps.
[Illustration: A MULTIPLE MANUAL SWITCHING BOARD FOR TOLL CONNECTIONS IN
AN AUTOMATIC SYSTEM Multiple Jacks are Provided for Each Line through
Which Toll Connections are Handled Directly.]
[Illustration: Fig. 370. Small Multiple Board Section]
For ordinary local service no operator would sit at the left-hand
position of the section shown, that being the end position, since the
operator there would not be able easily to reach the extreme right-hand
portion of the third position and would have nothing to reach at her
left. This end position in this particular board illustrated is provided
with toll-line equipment, a practice not uncommon in small multiple
boards. To prevent confusion let us assume that the multiple jack space
contains its full equipment of 3,
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