The Library of Work and Play: Electricity and Its Everyday Uses

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Title: The Library of Work and Play: Electricity and Its Everyday Uses

Author: John F. Woodhull

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Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK THE LIBRARY OF WORK AND PLAY: ELECTRICITY AND ITS EVERYDAY USES ***

THE LIBRARY OF WORK AND PLAY

CARPENTRY AND WOODWORK
By Edwin W. Foster

ELECTRICITY AND ITS EVERYDAY USES
By John F. Woodhull, Ph.D.

GARDENING AND FARMING
By Ellen Eddy Shaw

HOME DECORATION
By Charles Franklin Warner, Sc.D.

HOUSEKEEPING
By Elizabeth Hale Gilman

MECHANICS, INDOORS AND OUT
By Fred T. Hodgson

NEEDLECRAFT
By Effie Archer Archer

OUTDOOR SPORTS, AND GAMES
By Claude H. Miller, Ph.B.

OUTDOOR WORK
By Mary Rogers Miller

WORKING IN METALS
By Charles Conrad Sleffel.

[Illustration: Drawing by J. Hodson Redman. Harold Sending the C. Q. D.
Message (_See page 355_).]

_The Library of Work and Play_

ELECTRICITY AND ITS
EVERYDAY USES

BY JOHN F. WOODHULL, PH.D.

[Illustration]

McGOWEN-MAIER & CO.
CHICAGO, ILL.

PREFACE

Why do we pursue one method when instructing an individual boy out of
school, and a very different method when teaching a class of boys in
school?

The school method of teaching the dynamo is to begin with the bar
magnet and, through a series of thirty or forty lessons on fundamental
principles, lead up to the dynamo, which is then presented, with
considerable attention to detail, as a composite application of
principles. This might be styled the synthetic method. He who teaches
a boy out of school is pretty likely to reverse this order and pursue
the analytic method. The class in school has very little influence in
determining the order of procedure. The lone pupil with his questions
almost wholly determines the order of procedure. Out of school no
one has the courage to deny information to a hungry boy; in school
we profess to put a ban upon information giving, and we do quite
effectually deaden his sense of hunger. The school method rarely yields
fruit which lasts beyond the examination period; on the other hand, a
considerable number of boys have become electrical experts without the
aid of a school. This book is the story of how my boy and I studied
_electricity_ together. We have had no other method than to attack our
problems directly, and _principles_ have come in only when they were
needed.

My boy had learned to read when very young by having stories read to
him while he watched the printed pages. The construction of sentences
out of words and words out of letters had come to him very incidentally
but all in due time, and when he first went to school rather late in
life for a beginner he found himself more proficient than the other
boys of his own age both in reading and in understanding the printed
pages. I could see no good reason why he should not pursue the same
method in studying electricity.

We live in a modern apartment house in a great city. My boy likes to
visit engine rooms and talk with the engineers about their machinery.
His mother and I always encourage him to talk with us about the things
in which he is most interested. If the family is alone at dinner, he is
quite likely to lead the conversation into the field of electricity.
When particularly burdened with my work I have learned to find relief
by giving an afternoon to Harold, who generally takes me to some
electrical store or power station or to ride by electric train out into
the country.

CONTENTS

CHAPTER PAGE

I. The Dynamo and The Power Station 3

II. Dynamo continued--The Magnet 11

III. The Ammeter 25

IV. The Wattmeter 35

V. The Electric Motor 43

VI. Applications of the Electro-magnet 57

VII. Electric Heating 97

VIII. Applications of Electric Heating 107

IX. Lighting a Summer Camp by Electricity 160

X. How Electricity Feels 168

XI. The Electric Sparking Equipment for a Gasolene
Engine 178

XII. Electricity From Central Stations 204

XIII. Electricity From an Old Mill 218

XIV. Doing Chores by Electricity 240

XV. Electric currents from Chemical Action and Chemical
Action from Electric Currents 248

XVI. Electrocution at Millville 271

XVII. The Telephone 274

XVIII. Electric Bell Outfit for the Cottage 296

XIX. Using Electricity to Aid the Memory 300

XX. The Electric Brick Oven 305

XXI. Electric Waves 309

XXII. Ringing Bells and Lighting Lamps by Electric
Waves 324

XXIII. Telegraphing by Electric Waves 329

XXIV. Halley's Comet and Electric Waves 333

XXV. How the Idea of a Universal Ether Developed 339

XXVI. Electric Currents Cannot Be Confined to Wires 349

XXVII. Wireless Telegraphy In Earnest 355

ILLUSTRATIONS

Harold Sending the C. Q. D. Message _Frontispiece_

FACING PAGE

Testing a Generator 8

Wiring 16

Wattmeter 40

Testing the Telegraphy Outfit 62

Electric Bell 72

Feeling Electricity 174

Operating the Switchboard 204

Induction Coil of a Wireless 330

ELECTRICITY AND ITS EVERYDAY USES

THE DYNAMO AND THE POWER STATION

One day Harold expressed a desire to see the dynamos, five miles away,
which furnish the electric light in our apartment. So I told him to
invite his best friend to accompany us and we would go.

When we were some distance from the station the boys noticed the very
tall chimneys and inquired why tall chimneys were needed for dynamos. I
explained that the dynamos were run by steam-engines, and steam-engines
required the burning of coal. "Oh!" said Ernest, Harold's friend, "I
read in the paper that electricity is the rival of steam and is going
to drive out the steam-engine." I suggested that we were about to see
some steam-engines driving electricity out of that power station. But
more seriously, I explained that steam-engines were used for many
years as locomotives to draw the trains on the elevated railroads of
New York City, and when at last they were displaced by electric trains
some people thought that it was a case of electricity driving out
steam, whereas what had really happened was that the steam power for
running those trains had been concentrated at a central station, and
its power was merely transmitted to the trains by means of electricity.
The trains were, therefore, run by steam power quite as much as ever.
In like manner, the surface cars of New York a few years ago were run
by a cable, which was merely a very long belt used to transmit to the
cars the power of steam-engines located at a central station. When they
were changed to electric cars, electricity became the successful rival
of nothing else than a twisted wire cable. The cars still run by steam
power as before, but that power is transmitted by electricity instead
of the discarded cable. Steam has driven out the horse as a power for
drawing street cars, and electricity has enabled us to gather all the
steam engines into central stations, where now they are furnishing the
power for moving surface, elevated, and subway cars for street traffic,
as also trains for suburban travel. Central station steam-engines
are producing a vast amount of power, distributed all over the city
by means of electricity, for doing a great variety of work and for
furnishing electric light and heat, all of which we shall presently
study. "Just before we go into this central station, can you tell me
how the elevator is run in our apartment house?" "It is an electric
elevator," said Harold. "And where does the electricity come from?" I
inquired. "Well, I know that it comes from the street mains, but do
they come from this power station?" "Yes," said I, "and we will now go
in and see the steam-engines which lift you up stairs many times each
day by sending electricity to run that elevator. If you choose to do
so, you may claim for purposes of discussion that your elevator is run
by steam."

As we entered the building we came first to the dynamo room and both
boys noticed that the tone which met their ears was that which I had
produced for them in the telephone the night before. "I shall try to
show you before we get through," I said, "that these dynamos are doing
something which makes iron pulsate sixty times a second and that that
is the cause of the pitch of this tone. But let us begin with the coal
which is the source of all this power.

"This particular station at the present time is burning forty tons of
coal an hour. That is as much as Mr. ---- uses to heat his twelve-room
house for a whole year. One pound of coal is capable of liberating
enough energy to supply 5-3/4 horse-power for an hour. (Written for
short 5-3/4 H.P.H.) One ton of coal is capable of furnishing (2,000 ×
5-3/4) 11,500 H.P.H. Forty tons would yield 460,000 H.P.H. But the best
furnaces, boilers, and steam-engines are terribly wasteful of energy.
About nine tenths of all this energy is wasted and only one tenth, or
about 46,000 horse-power per hour, is delivered by the steam-engines to
the dynamos.

"Coal is already scarce in the world and the supply is rapidly being
exhausted. Meanwhile we are growing more dependent upon coal. A century
ago we used scarcely any power except that of men, horses, and oxen,
and what little heat men then used came chiefly from wood. They lived
in cold houses, attended cold churches and schools, did not ride in
steam or electric cars, and did not have power plants. Our wood is
nearly all gone, our coal is going, and we are very rapidly growing
more dependent upon heat and power, our chief source of which is coal.
Wind power is too uncertain to depend upon, and we turned our backs
upon water-power when we began to crowd into cities. What little
water-power there is, however, is nearly all in use.

"There is great need both that we learn how to save the major part
of the energy of the coal which we now waste, and that we find a
substitute for the coal to use when that is gone.

"A part of the heat from the forty tons of coal which is being burned
in this particular power plant goes into the water in the boilers. It
converts this water into steam. The steam, if free to expand into the
air, would occupy about one thousand seven hundred times the volume
of the water. We compel it to expand through the cylinders of the
steam-engine, using its force of expansion to make wheels go around--to
make the dynamo revolve. These dynamos are not _devices for producing
power but merely for transmitting_ the power of these steam-engines
to far away places where it may be used, as, for instance, in our
apartment house, where we are unwilling to walk upstairs and want some
power to carry us.

"Our own apartment is fifty feet above the street. I weigh one hundred
and sixty-five pounds. If I walk up stairs from the street to our
apartment in one minute, which is the rate of a rather slow elevator,
I work at the rate of one quarter of a horse-power. One hundred and
sixty-five pounds raised two hundred feet in one minute requires one
horse-power. You boys each weigh about half as much as I do, and if one
of you walks up the same stairs in one minute you exert half the power
that I do, or if you run up the stairs in half a minute you exert the
same power, that is, one quarter of a horse-power. When we three walk
up together in one minute we exert one half horse-power. If we all
three run up the stairs in half a minute we expend one horse-power.
Now, the speed of elevators for apartment houses is about one hundred
feet a minute. We are unwilling to walk up stairs, not because we are
lazy but because we have the New York haste, and so we employ elevators
which run at the rate of about one hundred feet a minute.

[Illustration: Photograph by Helen W. Cooke. Testing a Generator]

"These dynamos enable us to employ the power of this central station
to run the elevator in our apartment house. Here is a dynamo rolling
over now in the act of sending out power, some of which goes to that
elevator; and standing beside it is another waiting to be used when
necessary. Examining these dynamos, we find that they are composed
of nothing else than iron and copper. About all that we can say of
these mysterious machines is that the moving iron generates the
electricity and the copper leads it away.

[Illustration: Fig. 1]

"Each one of these dynamos has many hundred tons of iron in it. A
huge wheel of iron, thirty-two feet in diameter, one hundred feet in
circumference, portions of which are surrounded by insulated copper
conductors, forms the centre-piece of the machine. This movable part
weighs four hundred tons. Around about this is a fixed ring of iron,
portions of which are surrounded by insulated copper conductors.
Ordinarily the ring which is stationary is called 'the field,' and the
wheel, which rotates, is called 'the armature,' although these terms
are sometimes reversed for certain reasons. The movable part in these
machines rotates about once a second, that is, its circumference moves
a little faster than a mile a minute. The iron moving at this high rate
of speed creates ether streams or electric currents, which are led off
by the copper conductors. The generation of electricity on a large
scale requires large masses of iron and high velocity."

I noticed that the boys stood before this machine in a state of
utter bewilderment, bewildered as a man who is told that what he had
considered north is really south, bewildered as a man who, having
wandered through a maze of city streets, looks up at length and
unexpectedly finds the building he has been seeking towering before
him. The questions they asked were entirely without thought. "What is
inside of it?" "Simply more iron and copper, such as you see on the
surface," I replied. "But what makes it go?" "The steam engines, of
course, four of which you see, are coupled directly to each dynamo."
"But where does it get its electricity?" "Don't forget that you are
looking at _a generator_ of electricity. Big mass of iron--rapid
motion! That is the whole truth. But it cannot satisfy you as an answer
until you have become used to it. We have seen all that we ought to
see here to-day. Let us drop the whole matter now, but return to my
laboratory to-morrow, and I will give you the next step which will help
you."

The boys did no talking upon their return journey. Whether one may say
they were thinking or not I cannot tell, but certainly their ideas were
incubating.

THE DYNAMO, CONTINUED--THE MAGNET

When we had gathered at my laboratory the next day I took down a spool
of one pound No. 24 cotton-covered copper wire (Fig. 2 _A_), which had
its centre filled with wire nails. The boys had seen it before and
remembered it. With flexible wires I connected the two ends of the wire
on this spool to a sensitive ammeter, _B_, which had its zero in the
middle of the scale, and I laid down upon the table a bar magnet, _C_.

[Illustration: Fig. 2]

"Here," I said, "is a dynamo complete." The bar magnet furnishes the
'field' and this spool of copper wire, _A_, which I will move back and
forth immediately over the magnet from end to end, is 'the armature.'
_D_ and _e_ are the line wires and the circuit is completed through
the ammeter to show whether we are generating electricity. And now as
I move this armature along the field you see the needle of the ammeter
move to the right from zero to ten. When the armature is moved in the
opposite direction along the field the needle moves in the opposite
direction past zero and on to ten at the left. The moving of the needle
in the ammeter shows that we are generating electricity. The swinging
to and fro of the needle shows that we are generating an alternating
current of electricity. It is a mere matter of detail whether we move
the armature or the field, as I will show you by letting the spool A
rest quietly upon the table and moving the magnet to and fro lengthwise
across the end of the spool. Or I may accomplish the same results by
moving them both in opposite directions. It is simply necessary that
they move _with reference to each other_. Some dynamos are made with
stationary fields and rotating armatures, some with stationary armature
and rotating fields, and some with both parts designed to rotate in
opposite directions.

"Magnetism is not confined to the magnet. It extends more or less
widely into the region about it. It is this region affected by the
magnet that we designate its magnetic field. By bringing this sensitive
compass needle into the region of this bar magnet from all directions,
I show you that it has a slight power to change the direction of the
needle when about a foot away. This power grows rapidly greater as the
distance grows less. Of course its field extends rather indefinitely,
but we may say that this particular magnet has an appreciable field
extending about one foot in all directions from it. We find upon
examination that some magnets have bigger and stronger fields than
others, that all have their strongest fields when first magnetized
and lose their strength gradually, _but never entirely_. We find that
hardened iron and steel hold magnetism longer than soft iron, _but
all iron is magnetized somewhat at all times_. Iron that is feebly
magnetized can be made into a strong magnet by bringing it into a
strong magnetic field. The earth is a feeble magnet, and that is why
it gives direction to the compass needle. That is also probably the
reason why every piece of iron upon the earth is a magnet, or, to put
the cause back another step, we may say that whatever causes the earth
to be a magnet also causes every piece of iron upon the earth to be
likewise a magnet.

"But thanks to Oersted in Denmark in 1819 and Faraday in England in
1821 and Joseph Henry in Albany, N. Y., in 1827, we have learned to
make exceedingly powerful magnets by sending a current of electricity
in a whirl around the iron. This is the meaning of the coils of copper
wire around iron cores in the dynamo, in electric bells, in telegraph
sounders, in motors, etc., etc. To prevent the electric current from
taking the shortest route, through the iron core or through the
successive layers of copper wire, the iron core and the wire must
be covered with something like wood or paper or cotton or silk or
rubber--such things as electricity does not readily pass through--that
is, insulating material.

"Joseph Henry, while teaching in the Albany Academy, was the first to
make electro-magnets. There was no such thing as wire covered with an
insulating material then in the market, and he wound all his wire
with silk ribbon. But in the year 1834 he made magnets which lifted
thirty-five hundred pounds, to the astonishment of every one. A pair of
such electro-magnets as I have here (Fig. 3), each consisting of one
pound of No. 24 cotton covered copper wire, eight hundred feet long,
wound in one thousand turns about an iron core two inches in diameter,
will lift several hundred pounds: much more than we three can lift, as
I shall now show you."

[Illustration: Fig. 3]

The cores of the two magnets were bolted fast to an iron beam, and a
large bar of iron with a ring in it was laid across the other free ends
of the magnet cores. I made connections with the electric lighting
circuit (that in my laboratory is what is called a direct current),
and sent a current of electricity around the coils. The two boys and
I tugged at the ring in the iron bar to no avail. We were unable to
pull the iron bar away from the magnet. But when I opened the switch
and cut off the electric current, one boy with one finger in the ring
lifted the bar with perfect ease.

"Electro-magnets are now made with a magnetic intensity 90,700
times that of the earth's magnetism. Electro-magnets are used for
hoisting iron castings weighing many tons. Here is a picture of an
electro-magnet lifting a whole wagon load of kegs of nails from the
wagon to the hold of a ship.

"Electro-magnets are our only means of utilizing electricity for
power. It is the pull of electro-magnets that moves the electric car.
Electro-magnets are now used for pulling all the trains out of the
Grand Central Depot in New York City.

"Let us now compare the strength of our electro-magnet with that of the
bar magnet used in our former experiment."

I opened and closed the switch, which sent the electric current through
my magnet coils at frequent intervals, and the two boys, each with a
compass needle, searched the field for magnetic effects. They found
that the magnetic field extended six or eight feet, but this piece of
research was broken up by a new idea which appeared to strike them both
at the same instant, for they shouted both together, "Let's use this
electro-magnet in place of the bar magnet for our dynamo experiment!"

[Illustration: Photograph by Helen W. Cooke. Wiring]

"That is surely the next step in our programme," said I, "but you will
need a steam-engine to move an armature in this magnetic field, will
you not, judging from the struggle we had with that iron bar a few
minutes ago?" The boys looked quite hopeless until I said, "The best
thing about the electro-magnet remains yet to be told. You have perfect
control of its strength by changing the amount of electricity which you
send around the coil.

"By means of an instrument which works like the motorman's controller
on the electric car, I may control the amount of electricity which
flows, just as well as you may control the flow of water by a faucet or
stop-cock. By this means I will control the strength of the magnet so
that you may move the armature in your dynamo experiment.

"In 1821, Faraday, at the Royal Institution, London, learned that he
could produce magnetism by means of the electric current, and, in
1831, he learned that the reverse was also true, namely, that he could
produce electricity from magnetism. This idea coming as the result of
ten years of incessant search made him shout and dance like a child.
You are feeling a little of the pleasure of his discovery."

[Illustration: Fig. 4]

I then fastened one of the coils upon the table underneath a small
bench (Fig. 4) and sent an electric current around it. The other coil,
_B_, connected with the ammeter was pushed back and forth along the
surface of the bench over this coil. The boys found that the more
electric current I sent around the coil _A_, that is, the stronger I
made the magnetic field, the harder it was to move the coil _B_. They
found that the nearer _B_ was to _A_ the harder it was to move it. They
found that the faster they moved _B_ the more electricity was produced.
They tried laying _B_ upon its side upon the bench and thus moving
it. They tried taking _B_ off the bench and moving it on all sides of
_A_. They found it much harder to move in some ways than in others,
but in all cases they found that the harder they had to work the more
electricity was developed, as was shown by the ammeter.

"The dynamo is any machine which will convert mechanical work into
electricity. The magneto is one form of a dynamo which you have used
much at the summer cottage, but have never seen the inside of. Here are
several (see Figs. 5, 6, and 8) which I will let you examine inside and
out, and with these I must leave you to yourselves for a time."

When I returned I asked the boys why these dynamos were called
_magnetos_. "Because they have steel magnets for their fields," they
replied. "There are several magnets bent in the shape of a horseshoe."

"Yes," I said, "in this case the field is made stronger by taking
several magnets. Have you noticed any armature?" "Yes, it is made of
iron with insulated copper wire wound around it."

"Please recall that the amount of energy you expend in going upstairs
depends on two things: (1) your weight and (2) the speed with which you
move. Also recall that the amount of electricity you could generate
with a dynamo depended upon the amount of energy you expended.
Therefore, the strength of the electric current which this machine
may produce depends upon two things: (1) the strength of the magnetic
field against which you must pull and (2) the speed of the motion of
the armature. Evidently this field is made as strong as it is possible
to make it with steel magnets. Now is there any device for giving high
speed to the armature?"

"Yes, indeed," said the boys, "one has a pulley so that it may be
connected by a belt with a gas engine, and the others have each a large
cog-wheel working into a smaller one. We found in one of them that a
single revolution of the crank gave six revolutions to the armature."

I found that the boys had made large-sized drawings of the parts, and
were preparing to report on the magneto as a form of dynamo at the next
meeting of the Science Club, which we had started among the boys in
school.

[Illustration: Fig. 5]

"I will loan you some apparatus so that you may give a very interesting
demonstration on that subject," said I, "only let me show you how to
use it first. Connect the binding posts _D_ and _E_ of this magneto
(Fig. 5) with my ammeter. Turn the crank _very_ slowly and notice that
the needle of the ammeter swings to and fro with each revolution of
the armature. That shows that you have not only a _dynamo_, but an
_alternating current_ dynamo.

[Illustration: Fig. 6]

"Now connect the binding posts _d_ and _e_ of this magneto (Fig. 6)
with a short piece of copper wire. Turn the crank and you notice that
this dynamo rings two electric bells. Turn slowly and you notice
that the alternations of the current are numbered by the strokes on
the bells. The hammer swings to and fro just as the needle of the
ammeter did. Each bell therefore receives one stroke of the hammer for
each revolution of the armature. Now try to turn the crank steadily
at the rate of one revolution per second. The armature is making
six revolutions, or cycles, per second and you now have not only
an alternating current dynamo but a _six-cycle alternating current
dynamo_. The lighting circuit used in our apartment is a _sixty_-cycle
alternating current. To be sure the armature of the dynamo which
generates that current revolves only once a second, but it carries
coils enough upon its rim to make that number of alternations.

"Now connect this telephone receiver with the binding posts _D_ and
_E_ of this magneto (Fig. 7). Unscrew the cap of the receiver. Move to
one side the iron diaphragm and turn slowly the crank of the magneto.
Notice that the diaphragm vibrates in time with the alternations of
the dynamo. Replace the diaphragm, screw on the cap, hold the receiver
to your ear and turn the crank as fast as you can. You will probably
be able to make about sixteen cycles per second. The receiver in that
case is giving forth a sound of the same pitch as a sixteen-foot closed
organ-pipe.

[Illustration: Fig. 7]

"Connect the telephone receiver to the binding posts _D_ and _E_ of
this magneto (Fig. 8), and by means of a belt connect the pulley to
this series of cog-wheels. Now you may turn the crank and readily make
the armature revolve at the rate of sixty cycles per second, and you
notice that you get the same tone that we heard in the dynamo room of
the power station and the same tone the telephone receiver gave when I
connected it to a coil in our apartment. The tone which is produced by
sixty vibrations per second is very nearly that of the _C_ two octaves
below middle _C_ on the piano. Try it along with the piano and you will
find it a little flat. This string on the piano is making sixty-four
_vibrations per second_.

[Illustration: Fig. 8]

[Illustration: Fig. 9]

"Now connect this miniature telephone switchboard lamp with the magneto
(Fig. 9) and turn the crank fast. The lamp lights up to full brilliancy
and you notice that the light is steady, although it is made by an
alternating current passing through the filament in one direction,
stopping entirely, and then passing in the opposite direction. The
filament has no time to cool off, provided you turn fast enough, but
try turning a little slower and you will notice the flickering of the
lamp."

III

THE AMMETER

[Illustration: Fig. 10]

At the last meeting of the Science Club so many questions were asked,
which the demonstrators could not answer, that a programme committee,
to whom such questions might be referred thereafter, was appointed.
It was made the duty of this committee to assign to various members
the task of searching for satisfactory answers, and when the material
was ready to be reported to the club, the programme committee
determined the time and order of presentation. I found that I had been
made an honorary member of this committee and that it was expected
that I should steer the committee. I told them that I accepted this
appointment with the understanding that the fellow who steers is always
the smallest man in the crew, and if they would do all the work I would
enjoy the honorary title of cockswain. Secretly, however, I appreciated
that this was in effect adding several courses to my already rather
heavy programme. I must, under the régime, direct a large number of
inexperienced students in library research, in laboratory research, and
in the art of giving demonstrations with apparatus and experiments to
audiences.

The most urgent questions, as also those which were next in the natural
order, concerned the _ammeter_. I told the committee to make that the
subject of the next meeting and to send to my laboratory on a certain
day the person or persons whom they might appoint to report upon it.

[Illustration: Fig. 11]

I find that the boys never come singly, but generally in pairs. When
the boys came they found lying upon the table an ammeter (Fig. 11).

I told one of them to take out the three screws in the front and remove
the face of the instrument. I had told the boys that the instrument
cost sixty dollars and that letting them open it was like letting
them open my watch. As soon as the face came off one of the boys
exclaimed that from my reference to the watch he had expected to see
very complicated machinery with many wheels, but from the exceeding
simplicity of the mechanism he could not see why it should cost sixty
dollars. I told him that although it was a fine piece of workmanship
it was fortunately very easy to understand, and I asked them if it
reminded them of anything else that they had ever seen. After a few
moments of reflection they agreed that it was very much like one of the
magnetos. "Well," said I, "where is the field?"

[Illustration: Fig. 12]

"Is this horseshoe arrangement a magnet?" they inquired.

"There is a compass needle right at your hand waiting to answer that
question," I replied. They immediately found that it was a magnet.
"Well," I said, "to be really sure that it is a magnet you must find
a portion of it that will _repel_ a portion of your compass needle as
well as other portions in both horseshoe and needles which attract each
other." Whereupon, they found that the portion marked _N_ (Fig. 13)
repelled the blue end of the compass needle and attracted strongly the
bright end of the needle, while the portion marked _S_ did the reverse.
"We will call _N_ and _S_ the poles of the magnet. This is simply a
steel bar magnet bent into the shape of a horseshoe."

[Illustration: Fig. 13]

"You told us," remarked one of the boys, "that steel magnets gradually
lose their strength. How then can this be correct as a measuring
instrument?"

"It is the purpose of the iron case to enable this magnet to retain its
magnetism, and if you will examine its field, as we did that of another
magnet upon a former occasion, you will find that although this is a
strong steel magnet its field does not extend outside of the iron case.
It is as though we could box up magnetism and keep it from escaping.

"Now if this is like the magneto, where is the armature? The spool-like
thing between the poles of the magnet looks just like the armature in
one of the magnetos.

"Yes, it has an iron core with a coil of insulated wire around it, and
you remember that when an electric current is sent around a piece of
iron, that iron is made into a magnet, and if it is a magnet it must
have poles. It is very delicately poised upon a pivot and will act
exactly like your compass needle, which is also a little magnet with
poles. I will send an electric current through the wire which surrounds
this armature, and you notice that the needle which it carries moves
to the right. Notice that the lower end of this armature acts like the
blue end of your compass needle in that it is repelled from the pole
_N_ of the field and is attracted toward _S_ of the field. In like
manner, the upper end or pole of the armature is repelled from _S_
and attracted to _N_ of the field. The blue end of the compass needle
is called its north pole because it points north under the magnetic
influence of the earth, and so we may call the lower end of the
armature its north pole.

"The electric current which I am sending through the armature comes
first through one ordinary 16-candle-power electric lamp which you
see lighted on this 'resistance board,' as it is called, and you
notice that the needle points to .5. This means that half an ampere of
electricity is passing through this lamp. I will now send the current
through a 32-candle-power lamp, and you notice that the needle points
to one, indicating that one ampere is required to light that lamp. But
what prevents the needle from going farther, and what brings it back to
zero each time?" The boys discovered a very small spring, like the hair
spring of a watch, coiled around the pivot of the armature. "So, then,
one ampere of electricity gives magnetism to this armature so that it
may pull against its coiled spring hard enough to carry the needle to
the point one. Twice as much electricity will give it magnetism enough
to carry it to two, and so on across the scale.

"The full name of this instrument is Ampere meter, which by usage
has been shortened to ammeter. It was named in honour of André Marie
Ampère, who was born at Lyons, in France, in 1775, the year our
Revolutionary War broke out. He died in 1836. When Oersted made his
famous discovery of the action of an electric current upon a magnetic
needle, in 1819, Ampère was in middle life (forty-four), and took up
the same line of research with great vigour. The next year, 1820, he
discovered what you will doubtless enjoy rediscovering now.

"You will notice that the binding posts on the bottom of this ammeter
are marked, one positive, +, and the other, negative -. The electric
current now enters the instrument by the post marked + and after
passing around the armature leaves by the post marked -. I will reverse
the connections and thus send the current around the armature in the
other direction, and you notice that its poles are now reversed. The
lower end which was formerly the north pole of the armature has now
become the south pole, as proven by the fact that it is repelled
from the south pole of the field and attracted to its north pole.
This carried the needle to the left, and inasmuch as the zero is in
the middle of the scale we may with this instrument both measure the
amount of current and tell its direction. You will recall that when
we connected the magneto with this instrument, it indicated that
the magneto sent the current first in one direction and then in the
other, which we call an 'alternating current.' But you notice that
the current which I am using in this laboratory flows continuously
in one direction. This is called the 'direct current.' We shall find
out how a dynamo may produce a direct current at another time. Let
us not forget, however, that we have repeated Ampère's discovery, and
found out that the direction in which we send the current around an
electro-magnet determines which end shall be its north and which its
south pole. If you will note carefully which way the wire is wound
around the armature you will see that when I send the current in at
the positive post it is passing around the north pole of the armature
opposite to the direction in which the hands of a clock move. If I
reverse the current it passes around the lower end of the armature
_in the same direction as the hands of a clock move_ and then this
end becomes a south pole. This is 'Ampère's rule,' and it is what
candidates for admission to college are very careful to learn.

"Before we replace the face of this ammeter I must call your attention
to a wire running by a short cut from one binding post to the other,
_s_ (Fig. 14). Suppose _a_ represents the wire around the armature.
Electricity, like water, goes more readily through a big conductor than
a small one and more readily through a short than a long conductor. If
_s_ and _a_ were water pipes, each having a stop-cock, we might easily
adjust the cocks so that one tenth of the water would go through _a_
and nine tenths through _s_. Or, indeed, without stop-cocks, the size
and length of _s_ and _a_ might be so apportioned that one tenth of
the water would flow through _a_ and nine tenths through _s_. This is
precisely the adjustment which has been made with reference to the flow
of electricity through this instrument. _s_ is called a 'shunt.' When
the shunt is out all the current goes through _a_ and when the shunt
is in only one tenth of the current goes through _a_. I have two other
shunts, each of which may be put in the place of _s_. With the second
only one hundredth of the current goes through _a_ and with the third
only one thousandth of the current goes through _a_. Thus I have an
instrument which will measure anything from one thousandth of an ampere
up to ten amperes.

[Illustration: Fig. 14]

"In this laboratory we pay about one cent for an ampere of electricity
for one hour. Twice as much coal must be consumed to furnish two
amperes as one, and twice as much coal must be consumed to furnish
an ampere for two hours as for one hour. Hence we need an instrument
which will keep account of time as well as amount of current. Such an
instrument we must look into next.

"Just before we pass to that, however, let me ask if you have ever
heard of a 'shunt-wound' dynamo. Can you guess from the way we have
just used the word 'shunt' what the expression could mean with
reference to a dynamo?" Without hesitation the boys told me that it
meant that the field and armature were wound parallel to one another,
as shown by diagram in Fig. 15. In which case the electric current
which the machine generates divides, part of it going around the
field and part around the armature. Another type, called series-wound
dynamos, is indicated by diagram in Fig. 16, in which case the electric
current goes through field and armature in succession. Under either of
these circumstances, how can the armature move with reference to the
field? The answer will appear in the next chapter.

[Illustration: Fig. 15]

[Illustration: Fig. 16]

THE WATTMETER

We were able to maintain connections between the binding posts of the
ammeter and the movable armature of flexible wires because the armature
never moves more than one third of a revolution, but we now wish to
examine an instrument in which the armature must not only make a
complete revolution but must continue to revolve in the same direction
indefinitely. How are connections made so that an electric current may
pass from the fixed binding posts to the wire of the moving coil? I
will lift the cover off this instrument, which is called a wattmeter,
and let you find the answer to that question.

I sent through the instrument the current from a 32-candle-power lamp.
According to the ammeter, which was also in circuit, the amount was one
ampere.

The armature of the wattmeter revolved slowly and it was not long
before the boys reported that connections for the current were made
by strips of metal sliding on metal plates. The ends of the armature
wire were fastened one to one plate and the other to the other plate,
and the metal strips brush along over the surfaces of the plates. (That
is why they are called "brushes," I said.) And the brushes slide from
one plate to the other each time the armature makes half a revolution.
(That is, the brushes change the connection and thus change the poles
of the armature at the proper instant so that they are always attracted
to the poles of the field toward which they are moving.) This is called
a commutator.

Notice that while the ammeter was like the magneto in having a steel
magnet for its field, the wattmeter is like the dynamo in having
electro-magnets for both armature and field. Notice in the second place
that this instrument is an _electric motor_ since it is made to revolve
by an electric current. If it were made to revolve by some other power
it would generate electricity and would then be called a dynamo.
Indeed, let me tell you something which must at present be nothing
more than a puzzle to you. _Every machine, while it is being driven
by an electric current as an electric motor, is, at the same time,
acting as a dynamo to generate a current in the opposite direction._
Notice in the third place that this is a shunt-wound instrument. The
current which is sent into the instrument divides, and part of it goes
through the field, while part goes through the armature. Motors, as
well as dynamos, are either shunt-wound or series-wound. But notice
finally that the axle on which the armature is carried has a cyclometer
arrangement which keeps account of the number of revolutions. The
armature is going slowly enough for us to count the revolutions. With
watch in hand we found that it made one hundred and twenty revolutions
per minute. I next brought the current to the wattmeter through a
16-candle-power lamp and the ammeter, connected in series, showed that
half an ampere was passing. We counted the revolutions of the wattmeter
and found them to be sixty per minute.

Here, then, is a simple electric motor which will register the amount
of electricity we use. It will register the same amount whether we use
one ampere for one hour or half an ampere for two hours or two amperes
for half an hour. In any case this product is called _one ampere hour_.
But the words printed upon the dials of this instrument are not _ampere
hours_, but _watt hours_ and the name of the instrument is _wattmeter_.
This next requires explanation. Follow me in a little roundabout
journey and the matter will be readily understood when viewed from
another approach.

[Illustration: Fig. 17]

When we were estimating the energy required to climb the stairs of an
apartment house, we needed to take into account two factors, (1) our
weight and (2) the time which we took in climbing them. The amount of
coal burned, steam generated, electricity produced, to run our elevator
depends upon two factors, (1) its weight and (2) its speed. That idea
is fundamental. Let us get at it in still another way. Suppose we have
a mill pond, (Fig. 17, _A_). We construct a penstock _p_ and install
a water-wheel, _S_, to operate a mill. Our business increases and we
install more machinery in our mill and must have more power to run
it. We have two ways of getting it, (1) we may lengthen our wheel and
enlarge our penstock so that a greater weight of water will fall upon
the wheel, or (2) we may lengthen our penstock and move the wheel
farther down so that the water will fall upon the wheel with greater
velocity. It is just so with the electric current. Like water it is
driven on in its course by pressure. The unit for electric pressure is
called a volt. If we wish to drive the wattmeter or any other electric
motor twice as fast as now, we may choose whether we shall do so by
doubling the volts of pressure or by doubling the amperes of quantity.

The electric pressure on our mains is about one-hundred and ten volts.
We three together weigh 330 pounds. Our elevator brought us up stairs
at the speed of 100 feet per minute. It requires one horse-power to
raise 330 pounds 100 feet in a minute. The ammeter in the engine room
showed that 7 amperes of electricity were sent through the motor of the
elevator to bring us up. That is, seven amperes at 110-volt pressure
give one horse-power. In the office building across the street where
they use a 220-volt current 3-1/2 amperes are required to take us up
stairs at the same speed. It is necessary that the same amount of coal
be consumed to furnish the horse-power of energy whether we supply it
by means of seven amperes at 110 volts or 3-1/2 amperes at 220 volts.
You notice that the product is 770 in each case. The name given to
this product is _watts_. More accurately 746 watts of electrical power
are equivalent to one horse-power. The name of this unit commemorates
the famous inventor of the steam engine, James Watt (1736-1819). His
monument now overlooks the Clyde at his native town, Greenock, Scotland.

To light a certain lamp, to heat a certain laundry iron, to furnish a
certain amount of power for an electric motor, we must have a definite
number of watts. We may choose whether we will have it at high or low
voltage with correspondingly low or high number of amperes.

[Illustration: Fig. 18]

We will now connect with our laboratory current a 32-candle-power
lamp, an ammeter, and a wattmeter, all in series, Fig. 18, and in
parallel with these a volt meter. This last instrument indicates
the electric pressure. Its mechanism will be examined later. The
volt meter indicates 110 volts and the ammeter shows that one ampere
is passing. The filament in the lamp resists the passage of the
current. It gets quite hot and gives forth as much light as thirty-two
candles. Its resistance is just such that 110 volts of pressure send
one ampere through it. We will now take the reading of the wattmeter,
note the time and read it again later. One hour later its index showed
that 110 watt hours of electrical energy had been converted into light
and heat. This at the usual rate, costs 1.1 cents, one cent per hundred
watt hours or ten cents per thousand watt hours, called a kilowatt
hour. The more common 16-candle-power lamp costs about half a cent an
hour to operate. It requires one horse-power to keep fourteen of them
burning.

[Illustration: Photograph by Helen W. Cooke. Wattmeter]

I will now take you to see the wattmeter which measures all the
electric energy used in this building. You note down its reading and
the date and the next time you come we will read it again and thus
find out how much electricity has been used for electric lights, for
electric ventilating fans, for electric elevators, for electric ovens,
and electric irons in the school of household arts, for electric motors
to run lathes and other machines in the school of technical arts, for
electric experiments in my laboratories and lecture room, for electric
vacuum cleaners and, lastly, for pumping the pipe organ in chapel.

I saw by the boys' faces as they departed what would be the next
question that they would bring to me. Knowing, however, that the hour
was up, they were too polite to press it then.

THE ELECTRIC MOTOR

In a few days I received a telephone message, asking if I could appoint
an hour to meet the programme committee in my laboratory. I must
confess that my pleasure in these meetings had increased so much that I
was quite ready to slight other duties, if need be, to engage in them.
Moreover, since my business was education it was not difficult for me
to regard these meetings in the light of a duty quite as important as
my regular class instruction--perhaps more effective. At any rate the
boys and I managed to get together. May God forgive the man who essays
to teach boys, but does not love to be with them.

Of course at the last meeting of the Science Club every one wanted
to know how we ran a pipe organ by electricity. Moreover the
Electrical Show was coming on in the city, and cows were to be
milked by electricity, dishes were to be washed by electricity, rugs
and furniture were to be cleaned by electricity, and innumerable
distracting and distressing things were to take place. I told the boys
that really only two kinds of things were to be done by electricity at
the show, and if they would give me two one-hour appointments I would
furnish them with the key to the whole show. We might as well begin
to-day with the pipe organ question.

A pipe organ is operated by air. It has bellows which are simply one
form of an air pump. A boy is often employed to turn a crank which
works the bellows. Down in the basement underneath our pipe organ I
will show you how a half-horse-power electric motor takes the place of
a boy. We found a dark and dirty corner where a boy used to stand and
turn a crank every time æsthetically inclined people enjoyed an organ
recital in the room above. Science, which has not been given credit for
being _humanitarian_, put an electric motor into that dark corner and
sent the boy up stairs to hear the music. The motor _grumbled_ at the
dirt in the corner and compelled the janitor to keep it clean.

The electric motor, better than any device I know, enforces justice,
but never requires mercy, or at least rarely receives it. It comes
nearer than any other machine to paying back all that you put into it.
It is most economical when working up to its full capacity. I recommend
that you look it over carefully and after a few minutes tell me what
you have seen in it.

[Illustration: Fig. 19]

The boys said that it looked just like a dynamo. We must not forget
that it is a dynamo, but is here used as a motor by sending an electric
current through it. This fact, that a dynamo might be driven by an
electric current and serve as a mover of other machinery, was first
publicly exhibited in 1873 at the Vienna Exhibition, and by many
believed to have been discovered by accident at that exhibit. But why
does it look like a dynamo? It has a field whose magnetism is produced
by an electric current sent through coils of wire, and it has an
armature whose magnetism is likewise produced by the electric current.
If it were used as a dynamo, where would it get the electric current to
magnetize its field? From its own moving armature. Is it adapted for
direct current? Yes. It has a commutator and brushes. Is it shunt- or
series-wound? Shunt-wound, as shown by diagram in Fig. 20.

[Illustration: Fig. 20]

Suppose we treat the machine as a dynamo. Bring the ends of the line
wire together, thus, as we say, closing the circuit. By some external
force let us cause the armature to rotate and under the influence
of the magnetic field it will generate an electric current, part of
which will pass through the field and part through the line circuit.
We may adjust the relative amount of wire in field and line so that
any portion of the current we choose will pass through the field. The
amount of current it will generate depends, (1) upon the strength of
the field and (2) upon the speed of the armature. Its field, although
never entirely without magnetism, is very feeble at first, and hence
in the first instance a very small current will be generated in the
moving armature. This, however, will strengthen the field slightly, and
as the field is strengthened the armature will generate more current,
and thus by a mutual reaction the machine gradually "builds up" to full
strength.

[Illustration: Fig. 21]

When now we use the machine as a motor, an electric current must be
sent along the line wires in the opposite direction (Fig. 21) from
which it would come out of the machine when acting as a dynamo. It will
then be noticed that, although the direction of the current through
the field is the same, whether the machine is used as a dynamo or a
motor, the direction through the armature, when used as a motor, is the
reverse of that when used as a dynamo.

You may perhaps be able to notice that the amount of wire on the field
is considerably more than that on the armature. Now if you will trace
the wires carefully you will find that there is provided a way of
supplementing the wire of the armature with some more wire in what
is called the rheostat, Fig. 22. This wire, or portions of it, is
introduced into the armature circuit when the machine first starts.
When, however, the machine has started and the armature is moving
within the influence of a magnetic field, it plays the part of a dynamo
at the same time that it is acting as a motor. Two conflicting and
opposite electro-motive forces therefore exist in the armature at the
same time. In Fig. 22 the arrow _a_ represents the direction of the
electro-motive force which is impressed upon the armature, and the
arrow _b_ represents the counter-electro-motive force which the moving
armature develops.

[Illustration: Fig. 22]

This counter-electro-motive force, which develops while the machine is
in motion, makes it unnecessary to hold back the current longer by the
extra resistance of the rheostat and hence that is usually cut out.
Being used only for starting purposes and looking like a box, it is
generally called the "starting box." If now it was intended that this
motor should run at a constant speed, as is often the case, no other
governor would be needed than this counter-electro-motive force, for
whenever the machine begins to go faster, on account of reduced load,
its counter-electro-motive force increases as the speed and holds in
check the impressed electro-motive force. This acts very perfectly as
a governor, and motors operate with notoriously constant speed under
variable loads. But, of course, in this present instance the motor is
required to work at a variable speed. It must pump air slowly for the
soft passages of music, and it must work the pump to its utmost for the
very strong passages.

[Illustration: Fig. 23]

To understand how an electric motor may pump an organ and have its
speed automatically controlled, let us examine the diagram in Fig.
23. The motor _m_ causes the shaft _S_ to revolve, carrying the crank
_C_ around with it. The rod _r_ causes _a b_, the lower side of the
bellows, to rise and fall, this side being hinged at _b_. The side _b
c_, is fixed. When the side _a b_ is pushed upward by the crank rod the
valve _f_ closes and the air in the compartment _h_ pushes open the
valve _g_ and enters the compartment _j_. The upper side _d e_, of this
compartment rises as it is filled with air. Weights _K_, _K_, _K_,
rest on the top of this and air ducts lead from this compartment to
the pipes of the organ. The keys of the organ operate air cocks which
open and close the air ducts connected with the organ-pipes. A chain
connected with _e_ passes around the axle of the wheel _l_ and has a
weight _W_ upon its lower end. The wheel _l_ carries a strip of brass
_n_, which slides over metal points _p_, _p_, _p_, etc. The successive
points are connected by coils of wire to furnish resistance. This
series of coils is called a rheostat. The wires _t_ and _u_ form a loop
from the armature of the motor and connect this rheostat in series with
the armature. _u_ is connected with the brass strip _n_. Notice that
when the compartment _j_ is full of air and the side _d e_, is lifted
to its greatest height the strip _n_ is moved to the lowest point _p_,
and the electric current must pass from _u_ through all the resistance
of the rheostat in order to get back to the armature by the wire _t_.
This makes the motor go very slowly. When _d e_ sinks down, the strip
_n_ moves to the upper points _p_, and the resistance is reduced step
by step, enabling the motor to quicken its speed and pump faster as
more air is required.

Small motors in order to be effective must travel at high speed. This
motor when moving at its highest speed makes 1,800 revolutions per
minute. The bellows on the other hand needs to be large and move slowly
in order to be efficient. Hence the motor is not in reality connected
directly to the shaft _S_, but causes the shaft to revolve by means of
a series of pulleys and belts. The pulley on the motor is three inches
in diameter. It is connected by a flat leather belt with a wheel thirty
inches in diameter. When the motor therefore, makes 1,800 revolutions
per minute this wheel makes 180 revolutions per minute. The axle of
this wheel carries a small cog-wheel three inches in diameter and it
is connected by a chain belt with a cog wheel on the shaft _S_ (Fig.
23). Thus this shaft revolves thirty times per minute, that is, the rod
_r_ rises and falls each second. A pull of one pound on the rim of the
motor pulley will cause a pull of sixty pounds on the cogs of the wheel
upon the shaft _S_. If the second belt were leather, a sixty-pound pull
would cause it to slip on the smaller pulley. Hence the second belt is
a steel chain and the wheels have cogs, or sprockets, like a bicycle.

[Illustration: Fig. 24]

The organist before beginning to play closes a double-pole,
single-throw switch (Fig. 24), which sends the electric current to the
motor.

The motor pumps air until the bellows is full, and if the organist
delays playing, the strip of brass _n_ (Fig. 23) is carried below the
lowest point _p_, thus cutting off the current and stopping the motor.
As soon as he uses some of the air in the bellows, however, _n_ rises
and makes contact with the points _p_ and the motor starts.

This suggests that a somewhat similar thing is accomplished under
electric cars which have air brakes. An electric motor pumps the air
and compresses it in a tank. When the pressure reaches a certain
point, say sixty pounds per square inch, it automatically shuts off
the electric current from the motor which works the pump. But when the
motorman uses some of the air to apply the brakes to the wheels, and
the pressure in the tank falls below sixty pounds, the electric current
is again automatically turned on to the motor.

Of course if an electric motor can operate a pump to compress air it
may also work a pump to exhaust air. This is what is done in a vacuum
cleaner. The electric pump as it is called (which means a pump worked
by an electric motor), exhausts some of the air from a compartment in
the machine, and the atmosphere pressing in through nozzle and hose
carries dust from rugs and furniture with it into the compartment. The
best vacuum cleaners will produce a pressure of seven or eight pounds
per square inch, about half an atmosphere. This will remove dust from
the warp and woof of a rug better than our greatest hurricanes can when
the rugs are hung upon a line. There are three kinds of air pumps in
use with vacuum cleaners: (1) bellows, (2) rotating disk or fan, (3)
piston.

To milk cows by electricity is simply to apply the vacuum-cleaner idea
to the process, and, in general, doing things by electricity usually
means doing them by some machine that is made to go by an electric
motor. This then is the first key to the Electrical Show, and if you
will remember to look first for the motor it may remove much of the
mystery from some of the exhibits. In many cases it is not necessary
to have a complete electric motor, but simply an electro-magnet to do
the work. In booth No. 56 you will find a piano played by electricity.
Its keys are moving, but no hands strike them. There is no ghost at
work here. A little strip of iron has been placed upon the under side
of each key and a small electro-magnet is placed under that. It is
only necessary that wires should run from these electro-magnets to
two dry-battery cells and to push buttons, and a person far away may
play the piano. In reality, however, it is not a person but a roll of
punctured paper that opens and closes the electric circuits to these
various magnets underneath the keys.

It often happens that you see a person playing a pipe organ with his
keyboard far removed from the organ itself. In this case the keys
simply act as push buttons to close the electric circuit through
electro-magnets placed in the organ itself. These electro-magnets
operate the air valves of the various pipes.

[Illustration: Fig. 25]

You call at some apartment house where there is no hall boy, but a
row of push buttons labelled with the names of the tenants. You push
a button and the door which was locked opens apparently of its own
accord. To say that the door opens by electricity is only to add
mystery. What does happen is that an electric bell up in the apartment
rings in response to your push of the button, and in reply the tenant
pushes a button and the door is unlatched by an electro-magnet
concealed in the door casing (Fig. 25).

So I would say that the first key to the Electric Show or to the
multitude of electrical appliances which you meet in life is the
electro-magnet. Consider the motor as one illustration of its use.

If you are really to understand the Electric Show you should go
twice. I advise going with this key alone first and note down all the
applications of electro-magnets which you can find there. When you have
done so I shall be glad to have your report.

APPLICATIONS OF THE ELECTRO-MAGNET

It became quite the rage now among the boys to find as many uses of
electro-magnets as possible. These were reported and explained to the
club and a list kept. This list included:

1. Dynamo.
2. Magneto.
3. Ammeter.
4. Wattmeter.
5. Motor.
6. Electric piano and organ players.
7. Electric door openers.

Already noticed in the preceding pages, and the following:

8. _The Electric Spinner_ (Fig. 26).--A toy full of instruction. The
standard is a steel magnet which produces a magnetic field. Inside of
this is an electro-magnet which serves as an armature. Plainly visible
on its shaft is a commutator to which the electric current from a dry
cell is sent. This causes the armature to revolve and carry with it a
series of colour disks which may be adjusted so as to show what tint or
shade results from mixing colours in various proportions.

[Illustration: Fig. 26]

[Illustration: Fig. 27]

[Illustration: Fig. 28]

9. _The Electric Engine_ (Fig. 27).--This toy, with one dry battery
cell, develops power enough to run several other toy machines. The
diagram in Fig. 28 will make its plan of operation plain. _B_ is the
battery cell, _c_ the electro-magnets, _a_ an armature of iron. By a
rod this armature is connected with a crank on the axle which carries
the fly wheel _f_. Another crank, _d_, upon the same axle serves like
a push button to close the electric circuit at the right instant. The
wire _g_ from the battery cell encircles the electro-magnet _c_ and
then is connected to the iron base of the toy. When the crank _d_
touches the conductor _e_, which is a spring, the electric current
passes around the magnet, the magnet pulls the iron armature _a_, and
this gives an impulse to the wheel _f_ whose momentum carries it around
during that portion of the revolution when _d_ is separated from _e_
and _a_ is receding from the magnet.

It is customary to say that the circuit is closed through the base of
the machine, but this language requires interpretation. It means that
a way is provided for the electric current to pass through the base. A
person who is expert in language but not in electricity might expect us
to say "the circuit is open through the base."

[Illustration: Fig. 29]

10. _The Telegraph Sounder_ (Fig. 29).--This was a toy half a century
ago, but since the days of Samuel Finley Breese Morse it has become of
vast commercial importance. The Western Union Telegraph Company in 1909
had 211,513 miles of poles and cables, 1,382,500 miles of wire, 24,321
offices, sent 68,053,439 messages, received $30,541,072.55, expended
$23,193,965.66, and had $7,347,106.89 in profits. In the United States
more than 93,000,000 and in the world at large more than 600,000,000
messages are sent annually, and there are men still living who scoffed
at Morse's ideas as _impracticable_.

It is interesting to contemplate what would happen to the Stock
Exchange, to the newspapers, to the railroads, to the congressman
addressing his constituents from the floor of a legislative chamber, to
business in general, if the world were deprived of the telegraph.

A few years ago a telegraph despatch was sent from New York to San
Francisco, Tokio, London, and back to New York, 42,872 miles, in three
minutes less than an hour. Electricity can travel around the world in a
fraction of a second, the time was consumed in repeating the message.
I once sent a message from New York to New Haven to announce that I
was coming, and afterward took my train and reached New Haven in time
to receive my own message and pay the messenger boy. But I have never
lost faith in the beneficent results of Morse's labours.

Morse (1791-1872) was an artist and the first President of the
National Academy of Design. He was likewise a professor in New York
University and constructed his first experimental telegraph line upon
the University campus in 1835. His first public line was built from
Washington to Baltimore in 1844. The Western Union Telegraph Company
was incorporated in 1856. Of course the work of Morse rested upon that
of Oersted, in Copenhagen, who, in 1819, discovered electro-magnetism,
and upon that of Joseph Henry of Albany, who in 1827 first insulated
the wires.

[Illustration: Fig. 30]

The application of the electro-magnet to producing telegraphic signals
will be understood by referring to Fig. 30. _B_ is the generator of
an electric current--sometimes a battery and sometimes a dynamo. One
wire from this goes to the earth, _E_. The other wire goes through a
key, which, like a push button or a switch, serves to open or close
the circuit. This is normally closed when not in use. Through this
the current passes around the electro-magnet _S_, which attracts the
armature _a_, causing it to click against a metal stop, hence it is
called the sounder. From this the current passes along the line wire to
a distant station and there through the sounder and closed key to the
earth. There is likely to be a generator at each station. The current
must run continually through the system. If a battery is employed, the
copper sulphate, or gravity cell, to be described later, is chosen,
because it will endure continued usage better than any other.

The operator, in sending signals, opens the circuit, the magnets cease
to hold down the armatures, and they are raised by springs and strike
against metallic stops above. It is customary to say that the circuit
is completed through the earth. This statement misleads some persons
into imagining an electric current capable of corroding water pipes
and decomposing chemical compounds, passing through the earth between
stations.

[Illustration: Photograph by Helen W. Cooke. Testing the Telegraphy
Outfit]

Perhaps it will help to a better understanding of the truth if we
think of a city pumping water out of the ocean, say to fight fire,
and disposing of it again into the ocean. The ocean currents thus
produced are not likely to be destructive. Indeed, just as we measure
height from the ocean level as zero, so we measure electric pressures
as from the zero level of the earth's electrical state.

[Illustration: Fig. 31]

The key used by telegraphers is represented in Fig. 31. It has
connected with it a switch to keep the circuit closed when the key
is not in operation. The Morse code of signals consists of dots and
dashes, when printed, as follows:

a . -
b - . . .
c . . .
etc.

Operators learn to read the message by the intervals between sounds. A
dot consists of two taps of the sounder with a short interval between,
and a dash consists of two taps with a longer interval between. One tap
of the sounder is caused by its descending upon the metal stop below
and another by its rising against the upper stop.

Telegraph sounders are operated on about a quarter of an ampere of
current if from a battery circuit, or on about one tenth of an ampere
from a dynamo circuit. The dynamo circuit is supplied with more volts
of electric pressure, and hence its power is ample to cause the
armature to strike the metal stops hard enough to be heard by the
operator.

For example a battery circuit may supply to the sounder a current with
these characteristics:

2 volts × .25 amperes = .5 watts,

while a dynamo circuit may give:

6 volts × .1 ampere = .6 watts.

Telegraph line wires are usually bare, the insulation being merely the
glass knobs at the poles. Clean water is a very good insulator but
dirty water is a fairly good conductor. A wet telegraph pole may bring
so much current to earth as to prevent all sounders on the line from
operating. Hence the line is separated from the poles by glass. The
poles are about one hundred and thirty-two feet apart, making forty to
the mile. The wires are usually galvanized iron one sixth of an inch
in diameter. Copper conducts six times as well as iron, and is now
replacing iron in the lines.

Morse laid a submarine telegraph line in New York Harbour and suggested
a cable across the ocean. But that gigantic undertaking had to await
the masterful intelligence of Lord Kelvin and the indomitable will of
Cyrus W. Field. A submarine cable was laid across the Strait of Dover
in 1850. It was cut by the anchor of a fisherman a few hours after
it was laid. The first attempt to lay a submarine cable across the
Atlantic Ocean was made in 1857. Two ships of war, the _Agamemnon_ of
Great Britain and the _Niagara_ of the United States, engaged in this
undertaking. Three hundred miles had been laid when the cable parted
where the ocean was more than two miles deep. William Thomson was on
board the _Agamemnon_ as electrical expert. He went home to study and
improve the methods. The next year, 1858, the _Agamemnon_ and the
_Niagara_ met in midocean each with a portion of the cable on board.
The splice was made, and the _Agamemnon_ started toward Ireland and the
_Niagara_ toward Newfoundland. When six miles apart the cable broke.
The ships met again, made a new splice and again started in opposite
directions. They laid eighty miles and the cable parted a second time.
They met again, spliced and laid two hundred miles when it parted for
the third time. They met a fourth time, made the splice and succeeded
in laying the first cable from Ireland to Newfoundland on August 5,
1858.

In a few weeks the insulation failed and no more messages could be
sent. Seven years were spent in studying the problem, and again in
1865 the _Great Eastern_, a mammoth ship, started to lay the cable.
William Thomson was again on board as the expert. When twelve hundred
miles had been laid the cable parted in deep water. Three times the
cable was grappled and brought part way to the surface and lost again.
The _Great Eastern_ returned to land. The next year, 1866, the _Great
Eastern_, having on board William Thomson (Lord Kelvin), Mr. Canning,
the engineer of the expedition, and Captain Anderson, in command,
laid the cable which has worked successfully ever since. Thomson,
Canning, and Anderson were knighted as a result of their labours. Sir
William Thomson (1824-1907), afterward Lord Kelvin, is credited with
having solved the difficult electrical problems connected with this
enterprise. Cyrus W. Field (1819-1892), born in Stockbridge, Mass.,
helped to secure the many millions of dollars necessary to carry the
work to completion.

There are now seventy-three cables connecting Europe and America, and
two across the Pacific Ocean. Cable rates are: New York to England,
France, Germany, or Holland twenty-five cents a word, to Switzerland
thirty cents a word, and to Japan one dollar and thirty-three cents a
word.

[Illustration: Fig. 32]

The boys were kept very busy now looking up historical and biographical
sketches, as well as working up the many applications of the
electro-magnet. The next to be reported was:

11. _The Relay_ (Fig. 32).--Telegraphing from 3,000 to 10,000 miles
under the ocean is full of difficulties not now to be explained.

Of course when we attempt to telegraph many miles upon land we
find that the resistance of the wire cuts down the strength of the
current so that it will not move the sounder. This, however, is
readily obviated by the relay devised by Morse. It simply serves as
an automatic key to close a circuit. A diagram will make this clear
(Fig. 33). Suppose the line wire to be very long and on account of its
resistance the current is too feeble to operate a sounder. It is likely
to be about .025 ampere where the local sounder may require .25 ampere
or ten times as much. It is easily possible to wind a magnet (Fig. 33),
_R_, such that .025 ampere will close the armature _a_, so that it
may complete a local circuit when it would not make noise enough for
a sounder. _B_ may represent a local battery of any desired strength
which may operate the sounder _S_ of that station as loudly as may be
desired.

[Illustration: Fig. 33]

[Illustration: Fig. 34]

12. _Annunciator_ (Fig. 34).--We live in a fifth-floor apartment.
When we push the button to call the elevator a No. 5 appears in the
annunciator in the elevator car. This tells the elevator boy where
the call comes from. Take out two or three screws and the annunciator
opens, revealing a series of electro-magnets like the one shown in Fig.
35. When an electric current passes around the coil it pulls back an
iron catch and allows a number to drop so as to show through a small
window. The elevator boy, having noted that the call is from the fifth
floor, pushes up the number and the iron catch holds it until the coil
is magnetized again by an electric current.

[Illustration: Fig. 35]

[Illustration: Fig. 36]

[Illustration: Fig. 37]

The annunciator has a bell to call attention. A cable of six wires
enters this annunciator (Fig. 36). One wire goes direct to the bell
and the other five reach the bell through the separate coils of the
electro-magnets which control the drops. But how are electrical
connections made between a moving elevator car and the push buttons on
various floors? The diagram in Fig. 37 shows this in elevation. _B_
represents a battery of several dry cells located in the basement. One
wire from it runs direct to the push buttons 1, 2, 3, 4, 5, located
upon the five floors of the house. The other wire from the battery,
together with wires from each of the five push buttons, all run to a
point, _A_, half-way up the elevator shaft. Here the six wires are
gathered into a cable long enough to reach either to the top or the
bottom of the elevator shaft. The other end of this cable enters the
elevator car and runs to the annunciator. The wire from the battery
goes direct to the bell. The wires from the various push buttons go
through correspondingly numbered electro-magnets to the bell. When,
therefore, we pushed the button on the fifth floor, we closed the gap
in the electric circuit at that point. The current came up from the
battery, passed through the button, went down the cable to the car,
went through electro-magnet No. 5, went through the bell, and returned
direct to the battery, thus completing the circuit. Annunciators are
used about buildings to call other attendants, besides the elevator
boy. They are likewise used in burglar alarms to inform the householder
which door or window is being forced. They are used in the fire
department to tell what part of the city the call came from.

[Illustration: Fig. 38]

13. _The Electric Bell and Buzzer_ (Fig. 38).--So common a thing as an
electric bell really belongs to the present generation. Bells were
either novelties or toys when I was your age. They cost then many times
what they do now and then were poorly made. Nobody dared to trust them
for front-door bells. It was necessary to have a card permanently
posted over the push button saying, "If the bell does not ring, knock."
In those days batteries were troublesome to care for, houses were not
wired when built, and no one had learned the art of concealing the
wires neatly.

The buzzer is simply a bell minus gong and hammer. Those shown in Fig.
38 ring well on a single dry cell. A cell costing twelve cents operated
one for two years while it was used as a call bell from dining room to
kitchen, the current required being .15 ampere.

[Illustration: Fig. 39]

[Illustration: Electric Bell]

The connections are shown in the diagram (Fig. 39). Suppose the current
to enter at the binding post _a_, pass around the magnets _b_ and
then to the post _c_. The armature _d_ normally rests against the
post _c_ and the current finds its way along this to the post _e_ and
thence back to the battery. But as soon as the current passes, _b_
becomes a magnet and pulls the armature _d_ away from the post
_c_, thus breaking the circuit, when _b_ ceases to be a magnet and a
spring pushes the armature _d_ back against the post _c_ to repeat the
operation. The armature _d_ carries a hammer which strikes the gong
_f_. If the wire, which is usually connected with the binding post _e_,
is connected with the post _c_, the "clatter" bell is changed to a
"single-stroke" bell, and if the gong and hammer are removed the "bell"
is changed to a "buzzer."

[Illustration: Fig. 40]

In the case of the buzzer, by changing the length of the armature or
by weighting it, we may change the time of its vibrations and its
tone. The connections between battery push button and bell form a
complete circuit. In Fig. 40 _B_ represents a battery, usually of dry
cells, _B'_ represents the bell, and _P_ represents the push button.
The electric circuit is "open," (that is, there is a break in the
conductor) at _P_ until some one "pushes the button," that is, simply
pushes against a spring so as to cause a piece of metal to bridge the
gap in the conductor. Then we say the circuit is "closed."

[Illustration: Fig. 41]

[Illustration: Fig. 42]

Push button devices and switches are innumerable. In every case they
are simply devices for pushing one piece of metal against another
and completing the circuit for an electric current. Every one should
unscrew and examine a few of them, both for the pleasure of seeing how
they work and to learn how to make them work when they sometimes fail.
Not only in bells but in all other instruments where electro-magnets
are used, the magnets are placed in pairs, fastened together upon an
iron base. They are wound so that the free ends are made opposite poles
by the electric current. Like a horseshoe magnet, they form one magnet.
The two poles thus placed are mutually helpful and each is stronger
than it would be if separated from the other.

[Illustration: Fig. 43]

14. _Electric Clocks, Self-winding Clocks, Programme Clocks._--A
pretentious-looking thing which appeared like a dish pan with a glass
bottom was opened by the boys and found to be the simplest of all
clocks. It had an electro-magnet like that in Fig. 44. A strip of iron
acting as an armature across the free ends of this magnet, pushed like
a finger against the cogs of a wheel. This wheel was on the axle of
the minute hand and it had sixty cogs. The electric circuit was closed
through the magnet for an instant each minute and the armature pushed
the wheel ahead one cog. Thus it made one complete revolution in an
hour. A train of four other cog-wheels caused the hour hand to trail
after at one twelfth the speed of the minute hand. This machinery made
simply a small handful in an eighteen-inch stamped-metal "dish-pan"
costing fifteen dollars.

[Illustration: Fig. 44]

A self-winding clock was opened and found to contain two dry battery
cells, an electro-magnet which operated very much like that of a
"clatter" bell, the hammer like a finger poking against the cogs of
a wheel. Once an hour the long hand closed the circuit through the
battery and the magnet and its armature swung back and forth long
enough to give the cog wheel one complete revolution and wind a
spring, which it carried upon its axle. This spring kept the clock
running one hour, until the next winding.

[Illustration: Fig. 45]

The programme clocks which were examined were self-winding clocks, but
were connected by wires to the master clock which corrected them each
hour. Each time the long hand of the master clock came to twelve it
closed an electric circuit through all the clocks in the system. In
each clock the current passed around an electro-magnet and caused it to
pull an armature against a metal stop and set each long hand exactly at
twelve. This master clock is sometimes situated many miles away and may
correct the time for a whole city. Thus a master clock at Washington,
D. C., furnishes standard time to all parts of the United States. The
master clock which we examined also closed the circuit at proper
intervals through a series of programme bells placed in the various
class rooms, and these called and dismissed classes automatically.

15. _Watchman's Time Detector_ (Fig. 45).--This is a device to compel
a watchman to make his appointed trips. Push buttons or switches are
distributed about the building at various points, and it is made his
duty to close the circuits at these points at stated times. When he
does so, the fact is recorded by electro-magnets puncturing, or, in
some way, marking a revolving time card in the clock.

[Illustration: Fig. 46]

16. _Circuit Breakers_ (Fig. 46).--Electro-magnets are used to open
switches and thus protect dynamos and other machines against a larger
electric current than they are able to carry. The switch is held
closed by a spring which, by an adjusting device, may be tightened or
loosened. A dynamo which we examined had its circuit breaker adjusted
so that it would remain closed if any current under 1500 amperes
passed, but if a greater current than that passed it would strengthen
the magnet sufficiently to open the switch and thus break the circuit.

17. _Separating Iron from Ore._--In 1897 Edison first proposed to
use an electro-magnet to separate iron from crushed earth. Fig. 47
represents the process. _E_ is an electro-magnet. _S_ is the stream of
crushed ore containing iron. Gravity would cause all the material to
fall into bin _A_, but the electro-magnet _E_ pulls that portion of the
material which is magnetic to one side so that it falls into the bin
_B_.

[Illustration: Fig. 47]

[Illustration: Fig. 48]

18. _Lifting Magnets._--Electro-magnets are made for use with hoisting
apparatus to save the trouble of manipulating grappling hooks, etc.
They may lift barrels and boxes of iron, the wood of the barrel or
box being transparent, we say, to the magnetic influence. That is, the
magnet will attract iron through the wood just as light will shine
through glass. Such magnets are used to pick up from the bottom of
the sea cases of hardware from wrecked ships. (See the accompanying
illustration, Fig. 48.) In such cases the electric conductors which
lead to and encircle the magnets must be well insulated from the water
of the sea, otherwise the electric current would take the shorter
path from one line wire through the sea water, which is a fairly good
conductor, and back by the other line wire, rather than go the path of
greater resistance around the magnet. Electro-magnets are coming into
use in foundries, etc., for lifting heavy iron castings.

[Illustration: Fig. 49]

19. _Electro-Magnet on Starting Box._--As was explained under _electric
motors_, a starting box is simply a series of resistance coils _r_,
_r_, _r_, _r_, _r_, in Fig. 49. When the motor is not in use the switch
_l_ rests upon the point 1 and no electric current passes. When the
switch is moved to point 2, the current entering at _a_ passes to the
pivot of the switch and up the metal strip _l_ to the point 2, then
around the series of coils, _r_, _r_, _r_, _r_, _r_, to the post _b_
and thence back to the generator. As the switch is moved to the right,
the current passes through less and less of this resistance until, when
it reaches point 7, all the coils of resistance are "cut out," that is,
they are not in the path of the current. Now the motor has reached its
full speed and is developing enough counter-electro-motive force to
protect itself against too much current. Through a shunt, however, a
portion of the current passes from _a_ to _b_ around the electro-magnet
_e_, the two poles of which are presented to the metal strip _l_, which
must be of iron. This magnet holds the switch over so long as the
current is on, but when the current is cut off, by opening a switch in
the line wire, _e_ ceases to be a magnet and _l_ is carried back to
point 1 by a spring. Thus an extra resistance must always be in circuit
when the motor is first started. Those who start motors are expected
to move the lever _l_ of the starting box slowly from point to point,
pausing a second or two on each to give the motor time to acquire
proper speed for its protection. How too great a current would "burn
out" a motor will be explained later.

The motor man handles a lever for starting his car, which works like
that of the "starting box." His "starting box," however, is called a
"controller." Although it accomplishes the same result as the starting
box it has a wholly different and vastly more complex mechanism than
that already described.

The elevator boy, who runs our electric elevator, handles a lever which
also does the same thing through far different mechanism. Indeed, in
his case electro-magnets are used to prevent him from cutting out
resistance too fast if he should move his lever too quickly.

20. _Starting Switches for Electric Elevators._--The motor man has
to be instructed particularly how he should handle the lever of his
controller, and he is trusted to follow his directions to some extent,
however lacking in intelligence and integrity he may be. But the
elevator boy receives scarcely any instructions about his machine, and,
indeed, his machine has been constructed pretty nearly "foolproof." It
will automatically correct his errors of management. If he throws the
handle from one extreme to the other, all resistance cannot be thrown
out instantly, but this is accomplished by a series of electro-magnets
closing one switch after another and thus cutting out resistance
gradually.

21. _Arc Lamp Feed._--As will be explained later, an arc lamp must
have its carbons touching one another when the current is first thrown
on, and then the carbons must be drawn apart from a quarter to half an
inch. The upper carbon is lifted away from the lower one by a portion
of the current passing by means of a shunt around an electro-magnet.

[Illustration: Fig. 50]

[Illustration: Fig. 51]

[Illustration: Fig. 52]

22. _Volt meter._--The volt meter measures the pressure of an electric
current. The volt meter which we examined looked outside like our
ammeter, and when we removed the face it appeared inside like an
ammeter. There was the steel magnet of horseshoe shape to furnish a
field (Fig. 51), and there was an electro-magnet poised between its
poles for an armature. The armature in the volt meter, however, had
wound upon it finer wire and more of it than was the case in the
ammeter. There was no shunt wire in the volt meter as there was in
the ammeter. We connected in series a fluid cell (to be described
later), the ammeter, and the volt meter (Fig. 52). The ammeter shunt
was removed so that all the current went through its armature. The
volt meter needle went to one which was two thirds of the scale (Fig.
53), and the ammeter needle indicated .016. That is, this particular
cell can push sixteen thousandths of an ampere through the resistance
of this volt meter, and .016 ampere passing through the armature of
this volt meter will magnetize it sufficiently to move it against its
spring, say sixty degrees.

[Illustration: Fig. 53]

[Illustration: Fig. 54]

[Illustration: Fig. 55]

We put into the circuit a lot more fine wire for resistance, _R_ (Fig.
54), so that the volt meter needle went only half as far as before,
that is to .5. The ammeter indicated only half as much as before,
that is .008 ampere. We put in resistance enough to bring the volt meter
needle down to .25 and the ammeter indicated one quarter of
the original current. We put in less resistance, bringing the volt meter
needle to .75, and the ammeter indicated three fourths of the
original current. Evidently the volt meter is merely an ammeter with
a different scale marked upon its card. With a pen we marked upon the
card of the volt meter a true ammeter scale (Fig. 55).

[Illustration: Fig. 56]

In order to understand the volt meter, let us turn our attention for a
moment to Fig. 56. I have arranged the water tank _T_ at such a height
above the faucet _F_ that when the faucet is opened one quart of water
will flow in a minute. If I partially close the faucet, making the
opening one half as large (that is, offering twice the resistance to
the flow), half a quart will flow in a minute. If I make the resistance
four times as great only one quarter of a quart will flow in a minute.
It is evident that I could arrange a scale underneath the handle of the
faucet to indicate the quantity of water flowing, just as the ammeter
and volt meter indicate the quantity of electricity which flows. If now
that much is understood, it will be easy to learn how the water faucet
may be used to measure water pressure and the volt meter in like manner
used to measure electric pressure.

Having set the faucet so that a quart will flow per minute, let us put
on a longer tube _p_, and move the tank up to another shelf so that the
distance from the water level in the tank to the faucet is twice as
great as before. Under the increased pressure water runs through the
faucet twice as fast and we now get two quarts per minute.

I purposely placed the tank out of sight behind a partition so that you
might practise judging the water pressure by the flow at the faucet.
We cannot very well talk about pressure in quarts. We might talk about
it in pounds, but if we used this apparatus much we should probably
get into the habit of talking about the pressure from one shelf, two
shelves, three shelves, etc.

In order that the pressure might remain nearly constant during the
experiment we would probably introduce resistance (that is, partially
close the faucet) so that the water level should not fall much. We
might, for example, set the faucet so that half a pint would flow in
a minute when the tank was on the first shelf. Then a pint per minute
would flow when the tank was on the second shelf and one and a half
pints per minute when the tank was on the third shelf, etc. Thus we
should infer the pressure by measuring the quantity.

One more illustration and the case will be clear. To save the trouble
of measuring the quantity of water which flows through the faucet,
suppose I introduce the device represented in Fig. 57. _W_ is a small
water wheel comparable to the armature of the volt meter. It carries a
pointer which moves over a scale just as in the case of the volt meter.

[Illustration: Fig. 57]

It has a spring coiled around its axle which tends to keep the pointer
at _0_, as in the case of the volt meter. The tank is placed upon the
first shelf, the faucet is fixed so that a small amount of water flows
and the needle moves to a certain figure upon the scale. We will mark
this point one and call it "first-shelf pressure." The tank is lifted
to the second shelf and the index moves to another point, which we will
mark two and call it "second-shelf pressure." The tank is lifted to the
third shelf and the index moves to a third point, which we will mark
three and call it "third-shelf pressure," etc.

Ordinarily we measure water pressure with an instrument which allows
no water to run to waste, but in measuring electric pressure by the
volt meter some current must pass through the instrument, just as in
the case of our water-wheel illustration in Fig. 57. We put in large
resistance so as to make this current as small as possible, while we
let enough pass to move the armature.

[Illustration: Fig. 58]

Now let us return to the volt meter itself. By referring to Fig. 55,
we see that it requires .024 ampere to move the needle of the volt meter
clear across the scale, and we have found that one fluid cell was
able to send enough current through the resistance of the armature to
move the needle two thirds of the way across the scale. At this point
we find Fig. 1, which might be read "one-cell pressure." We prefer to
commemorate the name of one of the workers in the field of electricity
and call this pressure a "volt" after Alessandro Volta (1745-1827),
born at Como, Italy. It is the electric pressure which is produced by
one fluid cell of a certain kind. We say, then, that one volt pushes
through the resistance of this armature .016 ampere. Half a volt would
push through the resistance of the armature half as much current or
.008 ampere. At this point we put .5. Thus each of the figures in the
lower row (Fig. 55) shows what part of a volt is required to send
enough current through this particular armature to move the needle to
that point.

[Illustration: Fig. 59]

We found out how much wire was wound upon the armature and put exactly
the same amount in the outside resistance, _R_ (Fig. 59). The needle
now showed that one volt is able to push through twice the resistance
of the armature only half as much current, and the needle stopped
at .008 ampere. If this were to be the resistance in the volt meter
circuit one volt should stand under .008 ampere and two under .016 and
three under .024. It is evident then, that, if we know the internal
resistance of a volt meter, we may make it capable of measuring greater
electrical pressures by adding the proper amount of resistance. By
putting at _R_, (Fig. 59) nine times the internal resistance of the
instrument, thus multiplying the total resistance tenfold, the figures
upon the scale of volts may be read as whole numbers from one to
fifteen. In this case it will require fifteen cells to push the needle
clear across the scale and ten cells to push it two thirds of the
way across. If now we add enough external resistance to multiply the
resistance of the armature a hundred fold it will require 150 volts to
push .024 of an ampere through the armature and pull its needle clear
across the scale. In this case the figures upon the scale of volts are
multiplied by one hundred and read from ten to one hundred and fifty.
Such a scale would adapt this volt meter for use with our 110-volt
lighting circuit. Volt meters are made with a series of such external
resistances, called "multipliers," attached so that they may be easily
thrown into the circuit.

It is evident that we need some term so that we may speak of quantities
of resistance. This need has given rise to a unit of resistance called
an ohm, after George Simon Ohm (1789-1854) born at Erlanger in Bavaria.
Two inches of No. 36 German silver wire, such as is wound upon the
armature of this volt meter, gives one ohm of resistance. There are 125
inches of this wire upon the armature. Its resistance is, therefore,
62.5 ohms, and we may, therefore, say that one volt of electric
pressure can push through 62.5 ohms of resistance .016 of an ampere of
current. Ohm discovered this relationship in 1827, and formulated it
as follows:

volts/ohms = amperes (not, however, using these words).

(1 volt)/(62.5 ohms) = .016 ampere.

62.5) 1.0000 (.016
625
----
3750
3750
----

This is called Ohm's law, as every candidate for college admission will
hear and hear again.

[Illustration: Fig. 60]

Volt meters and armatures for the alternating current have
electro-magnets for their fields as well as for their armatures. Such
instruments are equally well adapted for either direct or alternating
currents. For when the current reverses its direction it reverses in
field and armature alike, and thus a repulsion between like poles is
maintained. Such an instrument, however, cannot respond to as slight
a current as those previously described, since they must consume some
energy in both field and armature.

23. _Telephone Receiver_ (Fig. 61).--It requires a stretch neither
of the imagination nor of the truth to call a telephone receiver an
electro-magnet, although perhaps it has never been called that before.
We took it apart and found that it consisted of a steel-bar magnet _m_
(Fig. 62), with a small spool of wire _w_ around one end of it. The
ends of the wire on the spool run along inside the hard rubber shell
to the two binding posts _a_ and _b_ at the other end. A disk of sheet
iron _S_ is held in the large end of the case very near to, but not
quite touching, the end of the magnet. When an alternating current is
sent through the wire upon the spool it causes rapid changes in the
strength of the magnetic field, if not reversals of the poles of the
field, and the iron disk is made to vibrate, keeping time with the
alternations of the current.

[Illustration: Fig. 61]

[Illustration: Fig. 62]

In this laboratory we have seen that our current has sixty alternations
per second. When it is connected with the receiver the disk, therefore,
makes sixty vibrations per second, and produces a tone which has very
nearly the pitch of C two octaves below the middle C upon the piano.

24. _Spark Coil_ (Fig. 63).--The automobile spark coil which we have
already used is an electro-magnet. The battery sends a current through
wire coiled around an iron core. At one end of this iron core is an
iron armature which is made to vibrate in precisely the same manner as
the armature of an electric bell. This makes and breaks the current and
causes rapid changes in the strength of the field. A rapidly changing
magnetic field may be used to develop electricity in a conductor, as we
have already seen in the case of the dynamo.

[Illustration: Fig. 63]

How it is used in the automobile spark coil will be shown later. It is
sufficient now to mention it as a case of a magnetic field produced by
an electric current passing through a wire coiled around an iron core,
or, in short, an electro-magnet.

Induction coils, Ruhmkorff coils, and transformers, to be described
later, are closely related to this. They all create magnetic fields in
the same way and are all electro-magnets.

[Illustration: Fig. 64. Transformers]

VII

ELECTRIC HEATING

It was Washington's birthday. The schools were to have a holiday and
the Science Club was to hold a special, open meeting at which I had
been asked to present the subject of electricity in the household. I
replied to the programme committee that that was too large a subject,
but that I would talk upon electric heating. I warned them, however,
that it would be a dry study, and not an entertainment. They replied
that the father of his country had been born at a time of the year
when the weather was unfavourable to outdoor sports, and that February
usually found them acclimated to vigorous study. Neither they nor their
friends objected to study if it seemed to have a motive.

I found an audience composed of old and young, men and women, girls and
boys. Most of them had left school--many of them because their teachers
thought they were incompetent to continue.

[Illustration: Fig. 65]

Not far from here is "a wheel in the middle of a wheel ... as for their
rings they are so high that they are dreadful ... and the spirit of the
living creature is in the wheels." Those wheels are now sending the
electric current to this room for our experiments. I propose to show
that we convert electricity into heat by offering resistance to its
flow. Experience teaches us that resistance to motion always produces
heat. At Niagara Falls thousands of tons of water descend at the rate
of one hundred and sixty feet in three seconds. When the water reaches
the bottom of the falls, it is moving a little faster than a mile a
minute. The resistance which this mass meets after its fall retards its
motion and generates heat.

Hundreds of meteors fall into our atmosphere daily, travelling a
thousand times as fast as the waters of Niagara Falls. The resistance
to their motion, which our atmosphere offers, heats them white hot,
melts them, vaporizes them, burns them up, so that very few of them
reach the solid earth in a solid condition.

An iron spile driver, measuring two cubic feet, weighs about half a
ton. When it falls sixteen feet upon the end of a spile it is moving
at the rate of twenty miles an hour. The energy of this moving mass
depends upon both its weight and its velocity, and when its motion is
arrested by the spile that energy of motion is largely converted into
heat energy, from which both the spile and the spile driver get hot.

A piece of iron may be made red hot by pounding it with a trip hammer.

Count Rumford found, in 1798, while boring cannon in the arsenal at
Munich, that the resistance which the iron offered to the motion of the
boring tool furnished heat enough to boil water.

Seven hundred and seventy-eight foot pounds of mechanical energy when
converted into heat would raise one pound of water (one pint) one
degree. This is called the British thermal unit. The spile driver,
weighing 1000 pounds, falling 16 feet upon a spile, produces heat
enough to raise 1 pint of water 20 degrees.

[Illustration: Fig. 66]

Here are two binding posts, _a_ and _b_, 8 feet apart (Fig. 66),
connected by copper wires with the dynamo circuit. The volt meter
indicates 112 volts of pressure. I will close the circuit by stretching
between _a_ and _b_ 8 feet of No. 24 iron wire. (This wire is about the
thickness of a common pin.) The iron wire offers resistance to the flow
of the electric current, thereby producing heat--heat enough as you see
to make the wire white hot, indeed heat enough to raise it to something
over two thousand degrees Fahr., for now you see it has melted.

We will put in a fresh piece of wire and connect also the ammeter
in the circuit (Fig. 67). As I close the circuit the needle of the
ammeter at first indicates 20 or 30 amperes, but in a second drops to
8 amperes, and remains there a second until the wire melts and falls
apart. One hundred and twelve volts of electric pressure are able to
push 8 amperes of electricity through this wire when hot.

[Illustration: Fig. 67]

(112 volts)/(14 ohms) = 8 amperes

112 volts × 8 amperes = 896 watts

746 watts = one horse-power

Hence it required about one and one fifth horse-power to melt the wire
in a second, and the heat produced was a little less than one British
thermal unit, a unit much used by engineers.

1 pound raised 1 foot = 1 foot pound

550 foot pounds per second = 1 horse-power

778 foot pounds (1.4 H.-P.) = 1 B. T. U. (British thermal unit) =
heat required to raise 1 pound of water 1° Fahrenheit

1 volt × 1 ampere = 1 watt

746 watts = 1 horse-power

In order to hold back 112 volts of electric pressure so that not more
than eight amperes of electricity should pass, the iron wire must have
offered about 14 ohms of resistance.

The behaviour of the ammeter needle showed that the wire offered very
much less resistance when cold than when hot. Indeed eight feet of No.
24 iron wire offers about one and one third ohms resistance when cold,
hence heat had increased its resistance to the passage of the electric
current tenfold.

This piece of iron wire offered resistance to the flow of the electric
current. It offered resistance to the motion of the dynamo. This
offered resistance to the steam-engine which drives the dynamo. This
caused the governor of the engine to open and pass more steam from
the boiler. This reduced the pressure at the steam gauge. This caused
the fireman to shovel more coal into the furnace. The heat of the
burning coal melts the wire, but it does it only after several changes.
First, it is converted into mechanical energy in the steam-engine with
great loss--about nine tenths being lost. Second, it is converted
into electrical energy by the dynamo, with some loss, and, third, it
is conducted to the iron wire and converted back to heat with still
further loss. It is evident that the most economical way to heat the
wire would be to take it to the furnace. Yet all electric cooking is
done by sending electric current through wires embedded in the walls of
the cooking utensils, and it is the most wasteful method of using the
energy stored in coal that has yet been devised.

[Illustration: Fig. 68]

That merely connecting the binding posts _a_ and _b_ (Fig. 67) by a
small piece of wire should throw a load upon the dynamo miles away;
should offer resistance to its motion, and make it require 1.18
horse-power more of energy to keep up its speed of revolution, is,
indeed, uncanny. I will attempt to make it seem more real. At one end
of the lecture table I have a rotary pump _P_ (Fig. 68). The end of
the rubber tube _a_, which leads to the pump is lying upon the table
outside of the tank of water, _T_. While things are in this condition
I move the crank which operates the pump with perfect ease. Now while
still turning the crank I pick up the tube _a_ and drop its free end
into the water tank. I cannot now conceal the fact, even if I were
disposed to do so, that I must work hard to keep the pump going. The
pump itself tells you by its laboured sound that it is working hard,
and the stream of water which issues from the pipe _b_ tells how much
work I am performing. The pump is discharging five and a half pints of
water per second, that is 5.5 pounds, and it raises this water 10 feet.
Hence I am doing 55 foot pounds of work per second, which requires
one tenth of a horse-power. Here is a lad who consents to try the
experiment for us. He turns the crank easily while I am holding the
tube _a_ out of the water, but when I lower it into the water he finds
the resistance so great that, tug however much he may, he is unable to
keep the pump going.

At the other end of the table I have a small hand dynamo, _D_ (Fig.
68), _M_ is an ammeter, _V_ is a volt meter, _S_ is a switch. All the
wires are good-sized copper, and offer little resistance, except that
stretched between the binding posts _a_ and _b_. This is a piece of
fine German silver wire. While the switch is open I turn the crank
of the dynamo with perfect ease. A small amount of current is going
through the volt meter, but this is too slight to offer any perceptible
resistance to the motion of the machine.

Notice that the volt meter needle moves according to the speed of
revolution. If I turn the crank once a second the needle stands at
25 volts. The electric pressure increases or decreases according to
whether I rotate the armature faster or slower. Now I will attempt
to keep the machine revolving at a constant rate while I close the
switch _S_, and surely you must see that I have hard work to do so. The
wire _a b_ has now become red hot. The volt meter shows 25 volts of
pressure, and the ammeter shows 3 amperes of current.

Twenty-five volts × 3 amperes = 75 watts, which require one tenth of
a horse-power (746 watts = 1 horse-power). The lad now takes my place
at turning the machine and finds it easy when the switch is open, but
I actually overload him by merely closing the switch. Heating the wire
red hot requires more energy than he is able to put forth.

I proposed to the president that my lecture close at this point, and
that each one in the room have a chance to _feel_ the load which was
thrown upon the dynamo each time it was required to heat the wire.
I suggested that each person should get a realizing sense of this
fact, first by doing the work himself, and second by going home and
reflecting upon this hint. When the switch is closed three amperes of
electricity pass around the circuit. This increases the magnetism in
both the field and the armature of the dynamo, and it requires one
tenth of a horse-power more to keep the armature moving within the
field against this magnetic pull.

I further desired to announce that during this hour I had delivered to
them the second key to the Electrical Show which I had promised a few
days ago. The second key is:

Heat (and light) is produced by offering resistance to the flow of the
electric current. The first key is the electro-magnet. These two unlock
all the mysteries of the show.

The president closed the formal exercises with the facetious remark
that I had warned them before the lecture that they must work, so now
each would be expected to take a turn at the cranks of the pump and
dynamo.

VIII

APPLICATIONS OF ELECTRIC HEATING

The programme committee decided that each member of the Science Club
should busy himself looking for _applications of electric heating_ and
should consult me freely about the matter. My telephone was kept busy,
my laboratory was in great demand, and we were all getting a good deal
more education than the school was giving us credit for.

The boys generally came to me in pairs, and each pair having worked up
some illustration of heat produced by electricity reported it to the
club. These were spread by the secretary in due form upon the minutes
of the club and constituted "The Proceedings of the Science Club."

[Illustration: Fig. 69]

1. _The Electric Sad Iron_ (Fig. 69).--Removing three screws the iron
comes apart, revealing a lot of No. 24 German silver wire wound upon a
sheet of mica. This is put between other sheets of mica (Fig. 70) and
tucked away within the body of the iron. German silver offers about
twice the resistance of iron when it is cold, but, at the temperature
of the sad iron when in use, there is not much difference between the
resistance of the two metals. German silver wire, however, does not
rust as iron wire would, and hence it is chosen. German silver is an
alloy of copper, zinc, and nickel.

[Illustration: Fig. 70]

We put the 112-volt current upon this wire of the iron, and according
to the ammeter it passed 4 amperes. Its resistance must therefore have
been 28 ohms.

(112 volts)/(28 ohms) = 4 amperes

Electricity costs us about 10 cents per kilowatt hour. That is 10
cents for 1000 watts for an hour, or 1 cent for a hundred watts for an
hour, or, on a 100-volt current, 1 cent for an ampere for an hour. It,
therefore, costs about 4 cents or, more accurately, 4-1/2 cents an hour
to heat this iron.

Persons sometimes carry electric irons with them, when they travel, to
iron pocket handkerchiefs and other small articles while stopping at
a hotel. Before connecting an iron in a chandelier one must know the
voltage used in the building. If the voltage in use in the building is
not the same as that stamped upon the iron, it is not safe to connect
it. Not knowing this, many persons have had the embarrassment of
"blowing a fuse" and extinguishing their own lights, and perhaps those
of others in the same building, and very likely also ruining the iron.

Suppose we take for example this iron stamped _110 V; 400 Watts_. (A
slight variation of 5 or 10 volts will not injure an iron.) The wire in
this iron we found to offer about 28 ohms resistance when hot, and it
lets pass 4 amperes. This is about all the current which it is able to
carry without melting. Now suppose a 220-volt current is used in the
building where it is proposed to connect the iron. This would force
through the wire enough current to melt it. The wire was seen to be
at a very dull-red heat when examined in a dark room. Its temperature
was about nine hundred degrees. At this temperature its resistance is
about three times what it is when cold. We estimated by measurements
that the iron contained about twenty-five feet of the wire. The boys
then took twenty-five feet of No. 24 German silver wire and stretched
it between two nails driven up in the laboratory (Fig. 71, _a b_). The
dynamo current was then sent through this. The end, _c_, of the wire
from the dynamo was provided with a metal clip which could be slid
along on the German silver wire. Sliding this to the left, and thus
shortening the distance on the German silver wire through which the
current must pass, increased the amount of current and heated the wire
hotter. The resistance decreases as the wire is shortened.

[Illustration: Fig. 71]

The boys wound this wire upon a piece of asbestos board (Fig. 72),
about nine inches square and one eighth of an inch thick, taking
care to keep the successive turns half an inch apart. Asbestos paper
was wrapped around this. The two ends of the wire were left free for
connections. This they called a "hot plate."

[Illustration: Fig. 72]

2. _Electric Hot Plate_ (Fig. 73).--This when opened was found to have
wire coiled up inside in the same manner as the sad iron. Indeed the
sad iron supported bottom side up makes a perfectly good hot plate.
The particular hot plate which we examined had a three-point switch
which gave three different heats for the plate. (See Fig. 74.) When the
switch _S_ is upon the first point the current goes through 112 ohms of
resistance and 1 ampere passes:

(112 volts)/(112 ohms) = 1 ampere

[Illustration: Fig. 73]

[Illustration: Fig. 74]

This warms the plate slightly--enough to keep food warm which has been
already cooked. This costs about one cent an hour.

When the switch is placed upon the second point the current goes
through 56 ohms of resistance and 2 amperes pass.

(112 volts)/(56 ohms) = 2 amperes.

This makes the plate warmer and is adapted to certain cooking
processes. It costs about two cents an hour.

When the switch is placed upon the third point the current goes through
28 ohms of resistance and 4 amperes pass.

(112 volts)/(28 ohms) = 4 amperes.

[Illustration: Fig. 75]

We placed upon this hot plate a basin containing 1 pint of water
(equals 1 pound) and heated it from the temperature of the room (68
degrees) to boiling (212 degrees) in 7 minutes and then put an egg in
and boiled it 3 minutes. Using 4 amperes for 10 minutes cost two thirds
of a cent. If it takes 7 minutes to boil a pint of water it would
require 1 hour to boil a gallon upon this hot plate using 4 amperes, or
448 watts. That is, it costs us about 4.5 cents a gallon to boil water
by electricity. The cost is usually put at three and a half cents per
gallon, but much depends upon conditions.

3. _Traveller's Cooker_ (Fig. 75).--This consists of a hot plate with a
covered basin permanently attached to it.

4. _Electric Coffee Percolator_ (Fig. 76) consists of a hot plate
with a coffee percolator to sit upon it. The coffee percolator might
sit upon any other hot plate or this hot plate might serve any other
purpose, but people do not seem to think of that.

[Illustration: Fig. 76]

5. _Electric Chafing-Dish_ (Fig. 77) consists merely of an electric hot
plate with a chafing-dish attached. The electric coffee percolators and
chafing dishes require from 300 to 600 watts according to size. If used
on the 110-volt current they take about 3 to 6 amperes, and if adapted
to the 220-volt current they take from 1-1/2 to 3 amperes, but cost the
same to operate in either case. They have connected with them flexible
cords and plugs to screw into the lamp sockets.

[Illustration: Fig. 77]

6. _Electric Broilers_ are merely hot plates, generally corrugated to
conduct off the melted fat. One that we examined had a switch for three
heats: low, requiring 360 watts--costs 3.6 cents per hour; medium,
requiring 600 watts--cost 6 cents per hour; high, requiring 1280
watts--cost 12.8 cents per hour.

7. _Electric Oven._--This one has double walls to retain the heat and
has two large hot plates, one on the bottom and one on the top. It is
large enough to hold four loaves of bread. It required 1520 watts for
40 minutes to heat it to the baking temperature and one hour to bake
the bread. Hence the cost of the electricity is about 25 cents, about
what the bread would cost in the market.

8. _Electric Incubator._--This is simply a well-ventilated oven warmed
by an electric hot plate and automatically controlled so that it keeps
a constant temperature of 103 degrees. Under these conditions chickens
hatch from hens' eggs in three weeks. An incubator for 5 dozen eggs was
found to take 25 cents' worth of electricity for the whole process of
incubation.

9. _Electric Toaster._--The wire coiled up in sad irons and hot plates
becomes hot enough to scorch cloth and paper, and even set fire to them
if they come in direct contact. We proved this by opening the iron and
touching paper to the wire while it was carrying the current. We also
lighted a cigar by touching it to the wire. Electric toasters have the
hot German silver wire simply covered by a screen.

10. _Electric Cigar Lighters_ (Fig. 78).--The one we examined hung by
a flexible cord from the chandelier. It had a small disk on the side
which contained a lot of fine wire covered by perforated mica. The wire
became red hot when the push button in the handle was pressed. It took
half an ampere of 110-volt current, and operated only while the button
was pushed. As near as we could calculate it cost .0003 of a cent to
light a cigar.

[Illustration: Fig. 78]

[Illustration: Fig. 79]

11. _Electric Curling Iron_ (Fig. 79).--One who has flat hair needs no
curling iron, but those who have round hair may curl it temporarily, if
they will unscrew an electric light bulb and screw into its socket the
plug of an electric curling iron. The flexible cord contains two wires
insulated from each other. One of these wires is attached to the outer
shell of the plug, the other wire is attached to the central button of
the plug. These make connections with the two separate dynamo wires in
the socket. The current comes down one of the wires in the flexible
cord, passes through a coil of fine German silver wire inside of the
curling iron, and returns by the other wire in the flexible cord. The
small wire in the curling iron offers 220 ohms of resistance when hot
and passes half an ampere of the 110-volt current.

(110 volts)/(220 ohms) = .5 ampere.

12. _Electric Soldering Irons_ (Fig. 80).--Or coppers, as they should
be called, are ideal implements for soldering. They remain continually
at the proper temperature and are free from corrosion. They require
from 55 to 220 watts. On the 110-volt current they take from one half
to two amperes.

[Illustration: Fig. 80]

13. _Electric Heating Pad_ (Fig. 81).--This consists of resistance
wire inside of a pad of soft material. It maintains a temperature of
180 degrees, and is an excellent substitute for a hot water bag. It
contains about two hundred and twenty ohms of resistance and requires
the same current as a 16-candle-power lamp.

[Illustration: Fig. 81]

14. _Electric Fuses_ (Fig. 82).--Fuses are made of short pieces of wire
or thin sheet metal. The metal is an alloy of lead and tin which melts
at a low temperature. They derive their name from the fact that they
readily fuse or melt. A building is wired in various separate circuits.
The size of the copper wires used in each circuit is determined by the
amount of current which the circuit is expected to carry. Each circuit
is protected by one or more fuses. These melt and cut off the current
whenever too much passes for the copper conductor to carry without
getting hot. The fuse wire melts at about six hundred degrees, while
the copper will not melt until it reaches nearly two thousand degrees.
This temperature is sufficient to set fire to wood, paper, and cloth.
When any fuse melts, the current is cut off from all chandeliers,
etc., in the particular circuit controlled by the fuse. This produces
consternation among people who do not understand the function of a
fuse. They become panic-stricken and begin to trample their neighbours
to death in the theatre or on the electric train when they hear that a
fuse is "blown" (which is the electrician's way of saying that it has
melted). Everyone should know that a fuse is a safety device. It is
always enclosed in a box lined with sheet iron or asbestos, so that it
is impossible for the flash, which occurs when the circuit is broken,
to set fire to anything.

[Illustration: Fig. 82]

[Illustration: Fig. 83]

15. _Electric Gas Lighter_ (Fig. 83).--These usually have two or three
small, dry battery cells in the handle. By pushing a button in the
handle connection is made between this battery and a short piece of
resistance wire in the tip. This wire gets red hot and lights the gas.
It is a surprise to many that we can light illuminating gas without
bringing a flame to it, and it is equally surprising that some flames,
or at least sparks, may not be able to light the gas. The fact is that
it is wholly a matter of _temperature and kind of gas_. Iron heated to
dull red will not light the illuminating gas now being furnished in
New York City, while iron at a bright red heat will do so. Iron may be
hot enough to light illuminating gas but too cool to light gasolene
vapour, which requires a dazzling white heat. Iron which is just under
the temperature at which it gives any light may set fire to wood and
paper. After it has cooled a good deal below that, it will set fire to
sulphur, and when it has cooled so that one may hold it in the hand,
it is still hot enough to set fire to phosphorus. The glowing end of
a lighted cigar, the spark made by striking flint, or the spark from
a spark coil with a feeble battery, all fail to set fire to gasolene
vapour, simply because they are not hot enough.

Fresh battery cells must occasionally be put in the handle of the
electric gas lighter.

Four facts regarding the resistance of wires it is well to remember:

1. The longer the wire the more resistance it offers to the electric
current.

2. The smaller the diameter of the wire the more resistance it offers.

3. Some materials offer more resistance than others, for example, iron
about six times as much as copper and German silver about twelve times
as much as copper.

4. The common metals offer more resistance when hot than when cold,
about double the resistance when heated to five hundred degrees. It is
the reverse with carbon, which offers more resistance when cold than
when hot. The carbon filament lamp offers about double the resistance
when cold as when lighted to full brilliancy.

16. _Electric Flasher_ (Fig. 84).--For automatically flashing electric
lights. The one which we examined was constructed according to the
plan shown in Fig. 85. The lighting circuit is brought to the binding
posts _b_ and _c_. A small insulated wire of high resistance connects
_b_ and _c_, being wound around the metal bar _a b_. The resistance of
this wire, when added to that of lamps, permits not more than one fifth
of an ampere to pass, and this warms the wire slightly. The bar _a b_
is composed of two strips of metal, brass above and iron below. Heat
expands brass more than iron. The result is that when the current is
turned on, the bar begins to curve downward until presently it touches
the metal base of _c_. Then the full current required to light the
lamps which are in circuit passes. While the circuit is closed through
the large metal strips not enough passes through the fine wire to warm
it. On cooling, _a b_ curves upward and breaks the connection with _c_,
and now the current begins again to warm up the small wire.

[Illustration: Fig. 84]

[Illustration: Fig. 85]

The flasher that we examined was adapted to operate: one
32-candle-power lamp; or two 16-candle-power lamps; or four
8-candle-power lamps, on a one ampere circuit of 110-volt pressure.

Let us see what would happen if it were connected either with a
current of higher voltage or a circuit of more lamps. Suppose we have
a 32-candle-power carbon filament lamp in circuit. This requires one
ampere to light it. Its resistance when hot is 110 ohms.

(110 volts)/(110 ohms) = 1 ampere.

When cold its resistance is about double or 220 ohms. The German
silver wire of the electric flasher offers 330 ohms of resistance, and
together they make 550 ohms. Thus the current is cut down to .2 ampere.

(110 volts)/(330 + 220 ohms) = .2 ampere

Suppose now we should undertake to use the same flasher and the same
lamp on a 220-volt current. This might push more current through than
the small wire could carry. It might melt, or its insulation might burn
off before _a_ made contact with _b_; if not the lamp would certainly
burn out after the contact. If we undertook to operate with this
flasher several 32-candle-power lamps instead of one upon the 110-volt
circuit, the result would be the same, for in that case the resistance
would be reduced and, therefore, a greater current would pass than the
wire could carry without undue heating.

[Illustration: Fig. 86]

The boys were at first troubled to see how increasing the number of
lamps in a circuit would decrease the resistance in that circuit. Fig.
86 was drawn to explain the matter. The lamps _l_, _l_, _l_, etc., are
connected _in parallel_. Each lamp makes an independent connection from
one feed wire to the other. The flasher _a_ acts as a switch to close
the circuit for the whole.

Now if we think of these wires as pipes to conduct water we would say
that water flows from _D_ to _E_ through ten pipes more readily than
through one. It would meet with only one tenth as much resistance. The
result would be the same, if we should substitute for the ten pipes one
pipe ten times as large in cross section. So it is with wires which are
conducting electricity. Introduce two in parallel, and you allow twice
as much current to pass by reducing the resistance to one half. Ten
parallel conductors reduce the resistance to one tenth and allow ten
times as much current to pass.

[Illustration: Fig. 87]

It is to be noticed that this flasher is an automatic switch which is
opened or closed according to temperature. Remove the fine wire from
_a_ and we have precisely the device which regulated the temperature in
our electric incubator. Suppose the "thermostat" (as it is called in
that case) is placed within the egg chamber which is to be kept at 103
degrees. A screw in the metal strip _c_ underneath the end of _a_ may
be set so that it will normally touch _a_. Suppose now the brass strip
is underneath the strip of iron in _a_. As the hot plate warms up the
egg chamber, the brass will expand more than the iron, and the bar will
curve upward and break the connection with _c_. As soon as the current
stops the temperature of the chamber begins to fall, and the bar curves
downward again until connection is made. This device is capable of
adjustment so as to keep the temperature constantly at 103 degrees or
any other desired degree. The device is in use for scores of different
purposes, including the regulation of temperature in school rooms.

17. _Electric Car Heaters._--Ten or fifteen years ago there were
no heated street cars in New York City. Now they are all heated by
electricity and their maximum and minimum temperatures are regulated by
law. The resistance wire may be seen in coils underneath the car seats.
Electric street cars usually operate on a 500 or 600-volt current. The
amount of current used for heating varies from 2 to 12 amperes. Perhaps
3 amperes may be taken as an average.

500 V × 3 _a_ = 1500 _w_ = 1-1/2 kilowatts.

It costs the large electric railway companies about 1.5 cents per
kilowatt hour to generate their supply of current. Eighteen hours is
considered a car day.

1-1/2 kilowatts × 18 hours = 27 kilowatt hours.

27 kilowatt hours at 1.5 cents = 40 cents per car day.

18. _Heating Apartments by Electricity._--For heating apartments
by electricity the same sort of apparatus is used as that already
described for heating cars. A family of four adults, living in an
eight-room apartment with at least 120 cubic feet of fresh air
admitted per minute, will use on an average ten amperes of the 110-volt
current. The cost will be about two dollars and fifty cents per day
or seventy-five dollars per month. Although this is as much as the
entire rental of a perfectly comfortable apartment, the novelty and the
convenience attract tenants and the extra cost of rent does not deter
them.

[Illustration: Fig. 88]

19. _Electric Bedroom Heater._--One of the boys constructed a heater
for his own room as follows: He procured a box eight inches deep by
eighteen inches square on the bottom. This he lined with asbestos
paper. He then stood it upon its side and arranged four incandescent
light sockets as shown in Fig. 88. These were connected by a flexible
cord to a plug which he could insert in place of a lamp in the
chandelier. He placed this heater on the floor underneath the window
and usually had 16-candle-power lamps in the sockets. He claimed that
it was a jolly foot warmer and kept the room comfortable without other
heat. He turned on from one to four lamps according to his need and
replaced the 16-candle-power lamps by 32-candle-power lamps when
the weather was extremely cold. I remarked that he must have light
along with heat by this arrangement, and I should think that might be
objectionable when he desired to sleep at night. He said that he always
turned it off, and opened the window at night, always preferring a cold
room to sleep in.

[Illustration: Fig. 89]

20. _Cooking with Incandescent Lamps._--This piece of apparatus was
devised by the boys and used in my laboratory. A sheet iron basin _a_,
was inverted over four 16-candle-power incandescent lamps, shown in
elevation by Fig. 89, and shown in plan by Fig. 90. The sides of the
basin were cut so as to admit the glass globes of the lamps, but the
sockets and keys were outside, so that it was convenient to turn on
and off the lamps separately, thus using one half to two amperes of
current, as desired. This rested upon another basin, _b_. Basin _b_
was covered with asbestos for the lamps to lie on and the whole was
attached to a board base, _c_. A flexible cord and plug allowed us to
attach this to the chandelier. A pint of water was boiled upon this
stove in fifteen minutes, and refreshments have been served hot from it
repeatedly.

[Illustration: Fig. 90]

21. _Electric Fireless Cooker._--There are five indictments against
ordinary cooking processes.

1. They heat the house in summer.

2. They convert what would be pleasant flavours in the food into
noxious odours about the house.

3. They cannot be controlled with regard to time and temperature as
scientific experiments should be.

4. They confine the cook too closely and are not sufficiently automatic.

5. They are wasteful of fuel.

It would seem that electricity might enable us to cure most of these
evils. To be sure the production of heat by electricity is wasteful
of fuel, and it seems doubtful how the account will balance regarding
the fifth item. But the remaining four items furnish a very hopeful
field for research. I use the last word advisedly, and think it is just
as applicable to high school boys as to university students. After
experimenting awhile the boys and I concluded to give a dinner party
in the laboratory and invite a few friends to test the results of our
cooking.

We procured a cylinder of magnesia such as is used for covering large
steam-pipes. This was inverted over our electric stove which was
illustrated in Fig. 89. The magnesia was cut at the bottom, so as to
give access to the key sockets of the lamps, (Fig. 91). First upon the
electric stove was placed a covered dish containing a roast of lamb.
Above this was another dish containing a vegetable, and upon the top of
that was a pudding. A flat piece of magnesia was used as a cover to the
whole. Through a hole in this was suspended a thermometer.

[Illustration: Fig. 91]

This "fireless cooker" was sitting in the centre of the dinner
table when the guests gathered around it. We had these problems for
investigation:

1. Will this cooker heat the house in summer?

All testified that they did not know that there was any heat about it
until they laid their hands upon it, and then they found it only very
slightly warm.

2. Is there any smell of cooking here? The process has been carried on
from start to finish right on this table.

All agreed that no smell could be detected.

I then turned off the electric current which had been running until now
and served the meat and vegetable, leaving the pudding inside to be
kept warm by the hot walls of the cooker.

3. Regarding the control of the process: we were using 32-candle-power
lamps, which gave us a variable current, from 0 to 4 amperes, and a
watch and a thermometer. We had control, but as yet lacked knowledge of
how it should be used. In the present case we had arbitrarily decided
to begin with temperature of 400 degrees, continue it for 20 minutes,
then turn off all the electric current, and let the temperature fall
gradually. This had been done at our convenience in the morning before
school. At a quarter before twelve we had found the temperature at
200 degrees, and turned on all the current, and now, at five minutes
past twelve o'clock, all testified that the lamb was particularly
good--neither too well done nor undercooked, and that its flavour was
better than usual.

As for economy of fuel, we find at least that we get better results
from incandescent lamps than from hot plates used in the same
apparatus, and the electric equipment enables us to put the heat
exactly where it is needed and nowhere else.

22. _Incandescent Lamp._--We feel quite justified in putting the
incandescent lamp under the heading, _Applications of Electric
Heating_, since the electric lamps in general use convert 96 per cent.
of the electric energy into heat and only 4 per cent. into light.

They were originally made by introducing a short piece of fine wire
into the circuit, choosing the kind of wire, its diameter, and its
length so as to make the proper relation between resistance and
voltage, in order that enough current might pass to make it white hot,
but not quite melt it. Platinum wire was first chosen because it would
stand the highest heat without melting and without rusting.

We will pass our 112-volt current through 9 feet of the No. 24 iron
wire. The wire is heated to bright red, but does not melt as it did
when we used 8 feet in a former experiment. The increased length has
added resistance, and, as you see by the ammeter, cut the current down
from 8 to 7.5 amperes. I will now darken the room and you find that
it is giving light enough to read by. But you notice that the light
is growing dimmer, its colour is growing redder, and the ammeter
indicates that less current is passing. I will cut off the current
and let you examine the wire and you notice that a crust has formed
upon it. This is due to the oxygen of the air which unites with the
iron, forming iron rust. Iron rust does not conduct electricity. We
have converted No. 24 iron wire into a wire of smaller diameter with a
sheath of iron rust around it. We might prevent the rusting by putting
the wire in a glass globe and exhausting the air from it.

I have here a piece of No. 24 platinum wire which has about the same
resistance as iron wire when cold, but you notice that I may use a
very much shorter length than I did of the iron wire because it will
endure a very much higher heat without melting. Reducing the length
would reduce the resistance, but reducing the resistance would allow
more current to pass. If more current should pass it would make the
wire hotter, and raising the temperature would increase the resistance,
which would cut down the current, etc. By sliding the clip _c_ (Fig.
92), along, I finally reach a point where conditions balance so that I
get a very brilliant light, dangerously near the fusing point of the
platinum which is three thousand degrees above the boiling point of
water.

In 1879 Mr. Thomas A. Edison literally searched the whole world for
something better than platinum for the filament of an incandescent
lamp. He finally decided upon charred threads of a bamboo which he
found in Japan. No research was ever more timely than this. Whereas
there was practically no electric lighting before 1880, soon after
that there began a phenomenal demand for carbon filament lamps. In
1890, 800,000 of these lamps were manufactured in the United States. In
1900 the number had risen to 25,000,000. In 1909 central stations were
supplying electric current to 41,807,944 incandescent electric lights.
By far the greatest number are still made with carbon filaments.

[Illustration: Fig. 92]

[Illustration: Fig. 93]

We examined an ordinary 110-volt 16-candle-power carbon filament lamp,
(Fig. 93). As near as we could estimate, its filament measured about
eight inches in length. We broke open the bulb of this lamp by laying
it upon the table and tapping it with a board. The bulb broke with
rather a loud noise and the brittle carbon filament broke into many
pieces. We found one of these pieces and measured its diameter with a
wire gauge, (Fig. 94). It was the same size as No. 33 wire, which we
also found by the wire gauge was the size of No. 90 sewing cotton. The
diameter of No. 33 wire was given upon the wire gauge as .007 inch.
When lighted, the filament of this lamp had looked to be about the size
of No. 18 wire, which has a diameter of .04. That is, the filament
when lighted looked six times as thick as it really was. Those who
use sewing cotton learn quickly to know the size of the thread by its
number. So those who have much to do with wire easily learn the system
of designating sizes by numbers. Here are some selected figures easy
to remember. A trolley wire is about one third of an inch in diameter.
It is designated as No. 0. Notice in the following table that as the
numbers rise by six the diameters are divided by two. Notice also that
as the diameters diminish by two the resistance increases by four.

[Illustration: Fig. 94]

TABLE OF RESISTANCE OF COPPER WIRES

_Nos._ _Diameter_ _Resistance_

0 .32 inch 10560 feet to the ohm
6 .16 " 2640 " " " "
12 .08 " 660 " " " "
18 .04 " 165 " " " "
24 .02 " 40 " " " "
30 .01 " 10 " " " "
36 .005 " 2.5 " " " "
42 .003 " 1 " " " "

10,560 feet equal two miles.

Number 36 is the wire used upon the spools of telegraph receivers. They
offer 75 ohms of resistance and therefore contain 30 feet of wire (30 ×
2.5 = 75). These resistances are for ordinary school room temperatures.

Since iron has six times, and German silver twelve times the resistance
of copper, divide the figures of the third column by six, and the table
will answer for iron wire, or divide those figures by twelve and the
table may be used for German silver wire, thus:

_Number Feet to the Ohm_
_Nos._ _Diameter_ _Copper_ _Iron_ _German Silver_

0 .32 inch 10560 1760 880
6 .16 " 2640 440 220
12 .08 " 660 110 55
18 .04 " 165 27 14
24 .02 " 40 6 32 inch
30 .01 " 10 1.5 8 "
36 .005 " 2.5 .45 2 "
42 .003 " 1 2 inch 1 "

These figures are not exact, but useful.

We procured a string of eight small lamps (Fig. 95), such as are used
in lighting Christmas trees. Each was marked 14 volt, 2-candle-power.
The carbon filament of each was about one inch long and apparently the
same diameter as that of the 16-candle-power lamp. When the 110-volt
current was sent through the group of eight connected in series they
seemed to give about the same light as the single 16-candle-power lamp.
It is as though the filament of the 16-candle-power lamp had been
cut into eight pieces, and distributed through eight small lamps. We
introduced an ammeter into the circuit and found that half an ampere of
electricity passed through the single 16-candle-power lamp--and half an
ampere likewise passed through the group of eight 2-candle-power lamps.

[Illustration: Fig. 95]

The 110-volt current can push an ampere of electricity through eight
inches of carbon thread seven thousandths of an inch in diameter, and
when this happens the filament gets hot enough to give out as much
light as sixteen standard candles. In the place of the 16-candle-power
lamp, we put a 32-candle-power 110-volt lamp. The ammeter indicated
one ampere. The carbon filament was larger (No. 30, diameter =
.01 inch), so as to allow more current to pass. An 8-candle-power
110-volt lamp was substituted; one quarter of an ampere passed. A
4-candle-power 110-volt lamp was used; one eighth of an ampere passed.
A 100-candle-power 110-volt lamp was substituted; three amperes of
current passed through it. In all these cases the lamps which passed
the larger current had the larger filaments. A little practice would
enable one to distinguish between these lamps without labels by
examining their filaments. Among these 110-volt lamps, it is to be
noted that the amount of light which they give is proportional to the
amount of current which they pass. And it is convenient to remember
that one ampere of electricity for one hour costs about one cent.

We introduced into the socket a "Hylo" lamp (Fig. 96). The filament,
_A_, took half an ampere of electricity, gave 16-candle-power of
light, and cost half a cent an hour. When the lamp was turned in its
socket the current was switched off of the filament _A_, and on to the
filament _a_. This took .03 of an ampere, gave one candle-power of
light, and cost .03 of a cent an hour, or at the rate of about $3.00 a
year, burning continuously day and night.

[Illustration: Fig. 96]

The uses of such a lamp are apparent in rooms which have no daylight.
However, a wall switch at the entrance of such a room, making it easy
to throw on and off the light entirely, seems to be a more satisfactory
arrangement. One of the boys connected a wattmeter in the circuit with
a hylo lamp and found that the small filament did not pass current
enough to move the armature of the wattmeter. Hence that may be burned
alone without affecting the consumer's bills.

We took a 16-candle-power 220-volt lamp, and lighted it by a 220-volt
current. The meter showed that it allowed only one quarter of an ampere
to pass. The filament was very much smaller than that in the 110-volt,
16-candle-power lamp. The pressure was twice as great as before, but
the resistance was four times as great, and hence only half as much
current passed. We find that it costs just as much to generate one
quarter of an ampere at 220-volt pressure as it does to generate half
an ampere at 110-volt pressure.

We must, of course, pay for electricity according to the cost of
producing it. To produce .5 ampere at 110-volt pressure costs the same
as one ampere at 55-volt pressure, or .25 amperes at 220 volts. It will
be noticed that the products of the two factors in each case are the
same. The product of an ampere multiplied by a volt is a watt. In each
of the above three cases the amount of electrical energy is 55 watts.
This will produce a definite quantity of light--about 16 candle-power
when the carbon filament is used, and this quantity does not vary as
either volts or amperes, but as the product of these, namely, watts.

Each of these lamps is called a 55-watt lamp, and, since they each give
16 candle-power of light, a carbon filament lamp gives one candle-power
of light for three and a half watts of electricity. Electricity for
lighting purposes usually costs _10 cents per kilowatt hour_, that is,
10 cents for 1000 watts for one hour, or one cent for 100 watts for
one hour. Hence a 55-watt lamp costs a trifle more than half a cent for
one hour, or exactly .55 cents, and a 32-candle-power lamp costs 1.1
cents per hour.

We introduced into the socket a 48-candle-power 110-volt tungsten lamp
(Fig. 97), and turned on the 110-volt current. The ammeter showed
55 ampere. Hence the lamp is a 60-watt lamp, and requires one and a
quarter watts per candle-power. That is, the metal tungsten is nearly
three times as efficient as carbon for producing light from electricity.

[Illustration: Fig. 97]

With pincers we broke off the tip of a 32-candle-power carbon filament
lamp, making a small hole in the large end of the bulb. The air rushed
in. We then put the lamp in the socket and turned on the current.
The carbon filament glowed as usual, and slowly burned up, growing
smaller as it did so. The ammeter which was in circuit showed that the
current, which was one ampere at the beginning, grew steadily less as
the filament grew smaller, until finally when it was about one quarter
of an ampere, the circuit was broken by the filament burning in two.
We removed the lamp from the socket and with a dropper tube introduced
a little lime water, and shook it to absorb any gas which might have
been formed in there. It became milky white, as it always does when
introduced where carbon has been burned. This would be a sufficient
proof that the filament was made of carbon, if we did not already
know it. The air is exhausted from these bulbs to prevent the carbon
filament from burning up.

[Illustration: Fig. 98]

The carbon filament lamps were, as has been said, the invention of
Mr. Thomas A. Edison in 1879. Such a statement must, however, be
qualified by the assertion that this, like nearly all invention, was
but the consummation of a long line of researches made by many men for
many years. The early filaments were made of bamboo thread, charred,
but now they are drawn like spider's web out of a sticky liquid and
carbonized at a high temperature. They are attached in the lamp to
short pieces of platinum wire which are sealed through the glass walls
of the bulb. One wire connects with the brass collar of the bulb, and
the other with the central piece of brass at the base of the bulb.
We dissected a socket and found that when the lamp is placed in the
socket, the collar of the lamp is screwed into the collar of the
socket, and the base of the lamp comes in contact with a brass spring
in the bottom of the socket (Fig. 98). The spring is connected with
one copper wire bringing electricity from the dynamo. The collar is
connected with the other wire from the dynamo. This connection is made
and broken by turning the key of the socket. The wires are made of
copper since copper is a particularly good conductor of electricity.
No electricity can flow unless this circuit is complete. Socket keys
and wall switches make or close gaps in this circuit. No copper wires
for carrying electric-lighting current are smaller than No. 12, which
has a diameter of .08 or about one twelfth of an inch. The intention is
to have as little resistance to the current as possible, except in the
filament of the lamp itself. There resistance is purposely introduced
in order to convert electricity into light, light without heat if that
were possible, but since that has not yet been found possible, heat for
the sake of the accompanying light. Unhappily only 4 per cent. of the
electrical energy goes into light and 96 per cent. goes into useless,
or even harmful, heat. The tungsten lamps, which are now coming into
use, are nearly three times as efficient in the production of light as
are the carbon filament lamps. The dynamo exerts its entire pressure
upon the lamp and furnishes current as follows:

A dynamo of 110-volt pressure gives:

1 ampere = 110 watts, through a 32-candle-power lamp, cost one cent an
hour, or

.5 ampere = 55 watts, through a 16-candle-power lamp, cost half a cent
an hour, or

.25 ampere = 27-1/2 watts, through an 8-candle-power lamp, cost a
quarter of a cent an hour.

A dynamo of 220-volt pressure gives:

.5 ampere = 110 watts, through a 32-candle-power lamp, cost one cent an
hour, or

.25 ampere = 55 watts, through a 16-candle-power lamp, cost half a cent
an hour, or

.125 ampere = 27-1/2 watts, through an 8-candle-power lamp, cost a
quarter of a cent an hour.

The carbon filament lamps, barring accidents, have a natural life
varying from 600 to 1000 hours of actual incandescence. At the end of
that period the filament has become so thin that it will fall apart by
ordinary usage. It is never profitable, however, to use them for their
whole lifetime. The lamp gradually volatilizes carbon and deposits
it upon the inner walls of the bulb, producing a smoky appearance and
shutting off light. As the filament grows thinner by this process, it
offers greater resistance to the current, and as the amount of current
grows less the proportion of light to current grows rapidly less,
so that at last instead of paying for 3.5 watts of electricity per
candle-power of light one must pay for perhaps seven or eight watts per
candle-power. We pay fifteen cents apiece for 16-candle-power lamps,
and it is economy to renew them about twice a year, if they are burned,
say three hours a day, or a little over five hundred hours. It is
interesting to note that when a direct current is used the evaporation
from the carbon filament always takes place at the negative end alone,
that is, the end from which the current is leaving the lamp. If an
alternating current is used the evaporation goes on from all parts of
the filament alike. This is a case of evaporation from the solid state.
Carbon does not boil below 6,000 degrees, and the filament reaches
about 2,450 degrees.

Tantalum, tungsten, and osmium lamps have metal filaments. These metals
are better conductors than carbon but unlike carbon their resistance
increases as their temperature rises, and their special virtue is that
they are capable of enduring an extremely high temperature without
melting. The wire used in some of these filaments is as small as .002
of an inch, or No. 44. In order to furnish sufficient resistance to
prevent the 110-volt current from melting, they often have a length
exceeding two feet. This is laced back and forth within the small bulb.
At the temperature of bright incandescence their resistance may be
increased as much as fivefold and sometimes becomes about ten ohms to
the inch. Like all metals they are more brittle when cold than hot.
Hence when cleaning such lamps it is advisable to turn on the current
to avoid breaking the filament by jarring. Filaments which are too
fragile to endure the jar of ordinary railway travel, when cold, have
gone through railway wrecks safely when lighted.

It is a general rule that good conductors of electricity grow more
resistant as the temperature rises while non-conductors resist less as
the temperature rises. Hence the insulating material which is used to
cover copper wires fails to protect if highly heated.

If a 110-volt lamp is put into a 220-volt circuit, one might expect
that the lamp would burn out without doing further damage to the
circuit, but this is not the case. As the filament approaches its
melting point, 6000 degrees, it becomes so good a conductor that it
carries current enough to melt a fifteen ampere fuse. It is, therefore,
the fuse that protects the circuit and not the burning out of the lamp.
The bulb containing the highly heated carbon vapour would conduct the
current as an arc lamp does.

[Illustration: Fig. 99]

23. _Arc Lamp._--We fastened two electric light carbons to the ends of
copper wires connected for the 110-volt current. A rheostat, _R_ (Fig.
99), in circuit, was set at 6.5 ohms. One lower carbon was fastened
into a clamp, and the other was touched to it, and then drawn away
about three-eighths of an inch. A very brilliant light was produced.
Probably about 1800 candle-power. The ammeter _A_ showed 10 amperes,
and the volt meter _V_ showed 45 volts. 45 volts × 10 amperes = 450
watts, 1800 candle-power, 25 watts per candle-power.

The arc light is the cheapest of all lights but is too dazzlingly
bright for household purposes. It is used for outdoor lighting
chiefly, and particularly for large search-lights. The temperature is
over 6000 degrees, which boils the carbon and fills the gap between the
two pencils with a stream of carbon vapour. This conducts the current
like the filament in an incandescent lamp. The air gap between the
carbon pencils would have a resistance of many thousand ohms if it
were not for the presence of the carbon vapour. The hot carbon vapour
reduces the resistance of this space to 4.5 ohms.

(45 volts)/(4.5 ohms) = 10 amperes.

(110 volts)/(6.5 + 4.5 ohms) = 10 amperes.

The carbon pencils account for part of this resistance--not more than a
third of an ohm however.

It is evident that arc lamps in use must have an automatic mechanism
which shall permit the carbons to touch whenever the current is not
passing, but which shall draw them apart to the proper distance after
the carbon vapour has been formed, or, as we say, after the arc has
been established. This mechanism is nothing else than electro-magnets
which are operated by the lighting circuit itself. It may require
thoughtful examination to recognize these as electro-magnets, in every
case, but that is what they are. Sometimes they are coils of wire,
which do not have iron cores and armatures separate to be sure--but
nevertheless they have both of these united in one movable rod, and
they produce magnetic fields.

Suppose I pass an electric current around this coil _A_ (Fig. 100).
The region about the coil becomes a magnetic field with its north pole
situated at a point in space, say _N_. The influence of this field
causes the iron rod to become a magnet with its south pole uppermost,
and if the current is strong enough, and the field which it produces is
strong enough, it will lift the iron rod up into the coil. By varying
the strength of the current you see I may make this rod dance up and
down in space touching nothing--a veritable ghost dance.

[Illustration: Fig. 100]

It may be pettifogging to say that the upper portion of this iron
rod is the core of the magnetic field, and its lower portion is the
armature. Yet this is right, and pettifogging may be right when it is
the only way to bring out the fact.

Our great study now is to produce light without heat, or at least to
come as near to it as the firefly does. The firefly gives 98 per cent.
light and two per cent. heat. The arc lamp gives 12 per cent. light
and 88 per cent. heat. The carbon filament gives 4 per cent. light and
96 per cent. heat. When we have made considerable progress in that
direction we shall take electric lamps out of the chapter on electric
heating and form a new chapter on electric lighting.

One might expect that a rod made of carbon would quickly burn up,
particularly when raised to the exceeding high temperature of the
electric arc. While it is true that carbon in the form of charcoal
burns so readily that it is used instead of kindlings for lighting a
fire, carbon in the form of graphite in our so-called "lead" pencils
and carbon as it is prepared for electric light pencils burns only very
slowly even at exceedingly high temperatures. The carbon rods used in
arc lamps endure a temperature of over 6000 degrees, without losing
more than one inch an hour, and half of that is simply volatilized--not
burned.

One of the most interesting improvements ever made in the arc light
is that of enclosing the arc in an inner glass globe. This globe
is closed airtight below with a small opening above. When the arc
is formed the oxygen of the air in the inner globe is soon consumed
and then combustion is no longer possible. We illustrated this by
an experiment. An ordinary cork was chosen to fit the large end of
an argand lamp chimney and through a hole in this was passed one of
the carbon rods (Fig. 101). A metal clamp made connections between
this carbon and the negative wire from the dynamo. The other carbon,
attached by a clamp to the positive wire, was thrust down into the
upper end of the chimney until it touched the negative carbon, and then
drawn upward a short distance, drawing an arc, as we say. This soon
makes an atmosphere within the chimney where combustion cannot go on
for want of oxygen. The arc, however, continues to glow as in the open
air, and the carbons may be drawn further apart than in the open air
without breaking the arc, hence more of the external resistance may be
cut out and a higher voltage put upon the lamp.

[Illustration: Fig. 101]

Carbons which burn out in a single night if used in open arc lamps last
two weeks in enclosed arc lamps.

The lower carbon, when removed from the lamp chimney of the last
experiment, served as a lead pencil to write on paper. The positive
carbon would not make a mark on paper. In all arc lamps carbon is
distilled from the positive pencil, condensing upon the negative pencil
as graphite, which is the material used in making "lead" pencils. They
are called "lead" pencils because they were originally made of lead,
but now they are made of graphite which is mined from the earth.

As soon as the arc is broken it becomes evident that the positive
carbon has been heated much the hotter of the two, a fact that could
not be detected while it was lighted because of the dazzling brightness
of the arc. The negative carbon turns black almost immediately, while
the positive carbon remains at a bright red heat for some time.

This fact needs to be borne in mind when adjusting arc light carbons in
search-lights, stereopticons, and all like apparatus in which the light
must be placed at the focus of a lens. That is, it is necessary to know
from what point the light really comes and it is necessary to have some
adjusting device to keep this point continually at the focus of the
lens.

24. _Search-Light._--(Fig. 102). This is simply an arc lamp with
reflectors behind it and lenses in front of it. The whole apparatus is
pivoted so as to be easily made to shine in any direction. The function
of the lenses and the reflectors is to collect stray rays of light and
send them all out in the same direction. This is shown in Fig. 103
where for simplicity the lens is represented as a single piece. _L_
represents a point of light which will naturally send its rays out in
all directions as the radii of a sphere; _m_, _m_, _m_ represents a
bright reflecting surface which is given that peculiar curve called
a parabola. It has the unique faculty of reflecting in a parallel
direction all the rays which may fall upon it from _L_, so long as _L_
is kept at that particular point called the focus, _a b_ is a lens of
glass which has that peculiar curve that enables it to bend all rays
which fall upon it from _L_, so that they may pass out parallel.

[Illustration: Fig. 102]

25. _Stereopticon._--This also has the necessary devices to gather the
rays of the arc lamp and send them forth parallel, and in addition it
has a series of lenses which produce upon a distant screen an enlarged
picture of any transparent object held in these parallel rays.

[Illustration: Fig. 103]

[Illustration: Fig. 104]

26. _Burglar's Flash-Light._--There are many forms of this. The one we
examined is represented in Fig. 104. We unscrewed a metal ring at the
left-hand end and found, first a glass lens and behind that a miniature
electric light, requiring three volts and half an ampere. We knew,
therefore, that it must be supplied with two cells, since one cell
may give not more than 1.5 volts. We also knew that it would only be
used to _flash_ a light, since if dry cells are required to furnish
half an ampere continuously they soon run down. Behind the lamp there
was a bright metal reflector--the lens and reflector are fairly well
represented in Fig. 103. The filament of the lamp is connected with two
small battery cells in the handle. These may be removed and replaced
by new ones by unscrewing a cap at the right-hand end. The circuit is
closed by a metal spring on the side of the tube, which acts as a push
button. It is situated where it may be conveniently pressed by the
thumb. The small batteries necessarily have a short life and must be
replaced quite frequently. Being a special thing they cost nearly twice
what the regular dry cell does.

[Illustration: Fig. 105]

27. _Mercury Vapour Lamp._--This is an interesting variety of arc
light in which the vapour of mercury takes the place of the vapour of
carbon. _G_, in Fig. 105, represents a glass tube from which the air
has been exhausted. The wires of the lighting circuit are fused into
the ends of the tube. At one end, and in contact with one of these
wires, is a small pool of mercury. By pulling the cord _c_ the tube is
tilted on the pivot _p_, so that a stream of mercury flows along the
whole length of the tube and closes the electric circuit. When the tube
falls back into its normal position, as represented in the figure, the
electric arc persists upon the mercury vapour. Incandescent mercury
vapour gives light strong in green, blue, and violet, but deficient in
red and yellow. It, therefore, gives nothing its natural appearance but
casts a ghastly hue over everything.

This lamp was invented in 1902, by Peter Cooper-Hewitt, grandson of the
founder of Cooper Union in New York City.

It gives a very suitable light for making photographic prints, and is
much used for that. This lamp operates upon the 110-volt circuit. It
is the longest step yet taken toward getting light without heat, but
perhaps shows what we must expect when we reach that goal, namely,
unsatisfactory colour values in the light. Probably such is the case
with the firefly.

28. _The Moore Light._--In 1896 Prof. D. McFarland Moore brought out
his vacuum tube light (Fig. 106). We visited an ordinary dry goods
store which had been equipped with this. Glass tubing is put together
very much as one would put up a stove pipe or a job of plumbing. The
joints are fused and made air-tight by playing a flame upon them after
the pipe is up in place. This pipe is led around into all nooks and
corners where there would be dark places. The air is pumped out of
this tube and a trifling amount of some vapour is introduced, the kind
varying according to the tint of colour which is desired.

[Illustration: Fig. 106]

Metal terminals are fused into the ends of this tube. The tube we saw
was seventy-five feet long. A 1000-volt alternating current is applied
to the terminals and the vapour becomes incandescent, filling the whole
tube full of light. The first thing that the boys remarked was that
although the room was brilliantly lighted no object cast a shadow. It
seemed as though light was everywhere and there was no chance to screen
it off.

29. _The Nernst Lamp._--In 1897 the Nernst lamp appeared in Germany. It
is a good illustration of an insulating substance becoming a conductor
when heated to a high temperature. The "glower," as it is called, is
composed of one or several short rods of clay-like material. This is
first heated by sending the electric current through resistance wire
placed directly underneath it and connected in shunt with it. When it
gets hot, current begins to pass through it, and is automatically cut
off from the resistance coil. The glower produces an intensely bright
and white light although it does not itself exceed the temperature of
1742 degrees.

Electric installations are now so carefully constructed that fires from
poor insulation are very rare. Less than one fire in three hundred
appears to be traceable to that cause.

30. _Electric Welding._--Nothing is more common in electrical matters
than heat produced by poor contacts. In this laboratory are two
chandeliers, each controlled by a wall switch. After the current has
been on the chandeliers for half an hour you will always find one of
those wall switches warm, while the other is not perceptibly warmer
than other objects in the room. The explanation is that there is poor
contact in one of them. When two metal conductors touch one another
at a mere point the electric current, in passing from one of these
conductors to the other across such a narrow bridge, meets resistance
and develops heat--sometimes heat enough to fuse the point, and either
break the contact, or, what is more likely, start a minute arc at that
point. In some cases this makes the apparatus dangerously hot, and in
other cases it bridges the gap with a broader and better contact--a
true electric weld. Electric welding is applied to everything, from
chicken fence to railway rails. Enormously large currents are used for
the purpose, in some cases as high as 50,000 amperes being employed.
The rails of railroads are welded end to end by a current of several
thousand amperes sent through the joint by perhaps two or three volts.
The joint heats and fuses together merely because the poor contact
offers resistance to this enormous current.

LIGHTING A SUMMER CAMP BY ELECTRICITY

Summer had arrived. The Science Club had held its last meeting for the
season. Harold had engaged three other boys to spend the summer at the
farm. I had the roof of an old mill reshingled and gave it to them
for a camp. They were to make it over inside. I sent the boys to the
country as early as it was possible for them to get away. It would be
six weeks later before I could follow them.

[Illustration: Fig. 107]

When I did arrive I found they had elaborate schemes indeed. The first
floor of the mill had been partitioned off into rooms, as shown in
diagram (Fig. 107), _a_, _b_, _c_ and _d_ being bedrooms; _e_ was a
wash room, the like of which has never been seen before. It had not
occurred to me that the mill pond _m_, which came to the very corner
of the building, would furnish the boys a complete system of city
water-works. At _g_, in the corner of this room, they had cut a hole
in the floor and nailed slats across upon the under side of the
timbers, making a depressed floor for a shower bath. This was directly
over a stream of water which issued from the mill pond. Hanging from
the ceiling over this spot was the nozzle of a garden hose. The other
end of this hose ran into the mill pond. The nozzle was capable of
delivering either a stream or a shower, according to which way it was
twisted in its socket. It was also capable of shutting off entirely the
flow of water. The boys asked me to hold my hand in the shower, and to
my astonishment it was warm. "What, pray, is your heating system?" I
inquired. They invited me to go and see. Moored outside in the mill
pond at the corner of the building was our motor boat, which the boys
were allowed to use freely and which they understood as well as any one.

[Illustration: Fig. 108]

They said that ordinarily they used for the shower the cool water of
the lake, which they much preferred, and which ran of its own accord,
the lake being a trifle higher than the nozzle of the shower, but
knowing my antipathy for the cold bath they had slipped the end of the
rubber hose over the outlet pipe of the pump which served to cool the
gasolene engine in the boat. The engine uncoupled from the propeller
was heating and pumping water for my shower bath, and I immediately
accepted the invitation to enjoy it.

Certainly no bath was ever more delightful than that one, coming, as it
did, at the close of a hot, dirty ride from the city.

I had hastened the bath, because it was already dusk and I had no
candle at the mill, but suddenly the room lighted up as if by magic.
I saw then what had before escaped my notice, a miniature electric
lamp, six-volt, two-candle-power, tungsten, such as are used for tail
lights on automobiles. Since tungsten requires about 1.25 watts per
candle-power it was a 2.5-watts lamp, and since it was adapted to six
volts it would take about four tenths of an ampere.

6 volts × .4 ampere = 2.4 watts. The little wire filament looked to be
about 1.5 inches long. Its resistance must have been 15 ohms.

6 volts/15 ohms = .4 ampere.

A battery of five cells was used to furnish electric current for the
lamp. Lamps were installed in the bedrooms also and were not intended
to be used more than half an hour at a time. Dry battery cells are
excellent for this purpose, and for so small a current the cheapest
dry cells are as good as the more expensive ones. These cost fifteen
cents a cell. They were connected by short pieces of bare copper wire;
No. 18 "in series," as shown in Fig. 109. A wire ran from the central
(carbon) binding post of one cell to the marginal (zinc) binding post
of the next cell. This battery was placed on a shelf in a convenient
place. A bare copper wire, No. 18, was attached to the carbon post at
one end of the battery and another to the zinc post at the other end
of the battery, and these two wires ran to all the rooms where lamps
were placed. The wires were fastened up on the walls by staples, taking
care that they should nowhere come in contact with each other and
"short circuit" the battery. Whenever it was necessary for one wire to
cross another, small pieces of pasteboard were tacked up to prevent
their touching each other. The lamps _L_ (Fig. 109) were connected
to these wires "in parallel." They cost forty cents apiece, and the
miniature sockets, into which they were screwed, cost five cents each.
One of these sockets was screwed to the side of the door casing in each
bedroom. Wires were attached to the line wires, simply by twisting them
together. One of these came down to one side of the socket and the
other came to the other side of the socket through a switch, _s_, made
of a strip of sheet zinc. The cost of the entire installation was as
follows:

5 dry cells at 15c .75
5.2 cp., 6-volt tungsten lamps at 40c 2.00
5 miniature wall sockets at 5c .25
Wire, etc. .20
----
$3.20

[Illustration: Fig. 109]

Suppose each lamp is used thirty minutes a day for 100 days, making a
total of fifty hours. There are five lamps, making a total of 250 lamp
hours. Each lamp takes .4 of an ampere, making a total of 100 ampere
hours. The lamps are operated at six volts, making a total of 600 watt
hours.

100 days
.5 an hour each day
---
50 hours
5 lamps
---
250 lamp hours
.4 ampere for each lamp
---
100 ampere hours
6 volts
---
600 watt hours

This amount of electrical energy would cost six cents if generated
by a dynamo. It is generally stated that electricity costs fifty
times as much if generated by battery as by dynamo. In this case the
battery actually did serve for the whole season of 100 days and was not
exhausted at the end of the season.

Indeed, since that season, the boys have found that battery cells
which had been too much exhausted for use on the engine served very
well on the lamps. By use the cells lose, not much in voltage, but in
the ability to furnish sufficient quantity in amperes to make the hot
spark required for igniting the mixture of gasolene and air in an
engine cylinder. When they have been discarded for use with the engine
they may still furnish the small amount of current required for the
lamps--provided not too many lamps are used at one time.

The dynamo current is always surprisingly cheap when compared with
that produced by a battery, but, on the other hand, we are never as
economical in the use of the dynamo current as we are with that of the
battery.

If all five of the lamps in the above equipment were lighted at the
same time and kept burning for half an hour, the battery would run down
rather badly and would not fully recover. But if one only is used at a
time and for not more than thirty minutes, or if more than one is used
at a time and for a proportionately shorter period, the battery will
receive no damage.

Dry battery cells may be purchased for either twenty-five cents or
fifteen cents each. The chief difference is that the former are
capable of giving larger current than the latter, when working against
very small resistance. For example, the former may give twenty to
twenty-five amperes on a short circuit, that is, connected directly
with the ammeter without other resistance, while the latter may give
not more than six to ten amperes under similar conditions. For most
purposes, other than igniting gasolene engines, in which dry cells
are used, an exceedingly small current is required. The electric
bell, for example, may not require more than .2 of an ampere and that
intermittently. Now it is found by experience that the dry cells which
are only capable of furnishing on short circuit six to ten amperes will
last quite as long in bell work as one which may give on short circuit
twenty to twenty-five amperes. Hence it is good economy to buy them.

"What a fine sitting room you have here! (Fig. 107, _f._) When do you
expect to fit it up?" said I. Instantly reminding myself, however, that
boys do not want a sitting room, I inquired what they intended to use
this fine, large room for. They told me that they had plans for making
a machine shop out of that. The idea had been suggested by a counter
shaft which still hung from the ceiling, and they had discovered that
the old mill wheel would still roll over if the penstock were repaired.
I replied that I would see what could be done about that sometime.

On the next day matters concerning the motor boat engaged our
attention.

HOW ELECTRICITY FEELS

What is more fickle and yet more fascinating than a motor boat? On the
morning after my arrival at Millville the boys wanted me to go out with
them in the motor boat on the mill pond, as our beautiful little lake
is called.

Each one took a hand at trying to start the boat, but although she had
acted perfectly well the day before, on this morning no one could get
a single explosion. The switch was closed. The gasolene was turned on.
The carburetor valves were set at the mark. The spark coils responded
with their familiar buzz. She had been primed and, when she had refused
to respond to this treatment, the pet valves were opened and the wheel
rolled over several times to sweep out the cylinders. But absolutely
nothing moved her--neither coaxing nor gibes. Suddenly some one rolled
the wheel over for the five-hundredth time and she started and behaved
well all day.

All this would not have given us the slightest aggravation if we could
only have found out what was the matter and what it was we finally did
to correct it. But this we shall probably never know, and hence we are
worshippers of the motor boat while we continue to distrust it and
complain of it.

While the boat was running one of the boys noticed that a binding
post at the end of one of the spark plugs seemed to be loose. He
inadvertently put out his hand to tighten it and received a terrific
shock. This raised the question among the boys, why one gets a shock
from some of the binding posts in the electrical equipment but not from
others. I suggested that we run in and call at the house to get my
portable measuring instrument (Fig. 110) and a little lunch, and then
go up to the upper end of the lake and take our time in examining the
electrical equipment of the boat.

[Illustration: Fig. 110]

The engine had two cylinders. There were two batteries--one for each
cylinder. Each battery consisted of five dry cells like the one
represented in Fig. 111.

"Now, why don't I feel the electricity when I touch the binding posts
of this dry cell?" inquired one of the boys as he handled one of the
cells which we had taken out. "Well, I'll give you two reasons why do
you not feel it," said I. "First, because you were touching only one
binding post at a time. You must touch both of the binding posts of
the battery cell at the same time, so that the electric current may
pass from one post to the other through your body. Second, even when
you do touch both binding posts at the same time you feel no current,
simply because you offered probably about 100,000 ohms of resistance
to the passage of the current and inasmuch as the one cell exerts only
1.5 volts of pressure, it could send only about .0000015 of an ampere
through you. This you cannot feel.

[Illustration: Fig. 111]

(1.5 volts)/(100,000 ohms) = .0000015 amperes.

"I now connect my instrument as a volt meter between the binding posts
of the cell and you see it indicates 1.5 volts, and when I connect it
for an instant as an ammeter you see it indicates twenty amperes. That
is twice as much as they use for executing criminals by electricity.
So you see if you could reduce your resistance sufficiently this one
battery cell might kill you. Some people have less resistance than
others. The resistance of the body is chiefly in the outer skin. If
one's hands are dry and his skin has been made tough and horny by hard
work, he has many times the resistance of one whose hands are moist and
whose skin is thin and tender.

"Suppose we select the tip of the tongue as the portion of the body
which will offer the least resistance and will be most sensitive to
slight electric currents. Let us then connect one dry cell with the
ammeter and place the tip of the tongue between the bare ends of the
wire at _T_ (Fig. 112).

[Illustration: Fig. 112]

"I have connected the ammeter so that it will indicate thousandths of
an ampere, and you see that the needle moves only slightly. We cannot
call it more than .001 ampere." Each boy in turn tried sending the
current through his tongue and each tried to tell how it felt. One
said it tingled, another said it felt warm, another said it tasted
sour and the other said he did not feel or taste anything. "Well," I
said, "whether you feel anything or not one-thousandth of an ampere is
passing through your tongue and you are offering fifteen hundred ohms
of resistance.

(1.5 volts)/(1500 ohms) = .001 ampere

"Your hand offers nearly seventy times as much resistance as your
tongue. Suppose we try increasing the voltage, or pressure, of our
electric current. We will connect in series the ten cells, making
a battery which you see by the volt meter gives fifteen volts of
pressure. We now find that having ten times the pressure it sends ten
times as much current as formerly through the tongue."

(15 volts)/(1500 ohms) = .01 ampere

Each one now testified that the battery sent all the current he cared
to take through his tongue. If they send one thousand times as much as
that through a criminal no wonder it kills him. It produces a twitch
when the contact is first made, afterward a decided sensation of warmth
and acid taste.

If we should increase the voltage tenfold more, say the 110-volt dynamo
current (direct current), and touch the bare conductors with our hands,
the ammeter would indicate about .001 ampere. That is, although this
current has about seventy times as much push, or voltage, as a dry
cell, no more electricity passes through the fingers than did through
the tongue in the preceding experiment with one cell. The fingers offer
so much greater resistance.

By wetting the fingers and pressing them firmly upon the bare wires,
we may make the ammeter read .01, that is, we may increase the current
tenfold by reducing the resistance to one tenth. But there is nothing
disagreeable about the feeling. If the same experiment is tried with
the 110-volt alternating current, although the quantity of current
which passes through the fingers is the same as before, the tingling
is more perceptible than in the case of the direct current. If we
join together seventy-five dry cells, giving a voltage of 112, and
press the bare wires with our wet fingers, the ammeter will indicate
.01, but there is no tingling sensation, merely a slight warmth. The
battery current, being continuous, causes no twitching of the muscles
while the contact is closed. The direct current dynamo furnishes a
slightly pulsating current. Hence, one may tell by the feeling whether
an electric current comes from a battery or a direct current dynamo.
The alternating-current dynamo gives a surging of electricity back
and forth in the wires, and this may be distinguished from the direct
current by its feeling; when, however, the number of alternations per
second is increased very greatly, one may receive through the body
considerable quantities of electricity without feeling it. With a very
high frequency current one may put himself in circuit and light a
16-candle-power lamp without any disagreeable sensation.

The outer skin is our chief insulation. If it is dry and well toughened
by work it offers a resistance of over 100,000 ohms upon gentle
contact. A wounded spot, or places like the tongue with moist, thin
skin, may offer a resistance as low as 500 ohms. If one has a pin prick
or a splinter in his hand which he cannot locate, he may hold one bare
wire of a 110-volt alternating circuit in one hand and move the other
bare wire about on the suspected region, and know when it reaches the
spot by a tingling sensation.

[Illustration: Photograph by Helen W. Cooke. Feeling Electricity]

One may touch lightly the 220-volt direct current and scarcely note any
difference between this and the 110-volt direct current, because one is
not very sensitive to the difference between .001 ampere and .002
ampere passing through his body.

(100 volts)/(100,000 ohms) = .001 ampere,
and
(200 volts)/(100,000 ohms) = .002 amperes

Physicians treat certain ailments by the use of the electric current.
For this purpose they invariably use a pulsating or alternating current
and reduce the resistance by using metal handles and wet sponges for
contact with the skin, but even so a very small amount of current
passes. The moderate twitching of the muscles seems to be the end
sought.

Men who are supposed to be killed by electric shocks often die from
other causes. A man perching upon an electric light pole, repairing
wires, may come in contact with a wire charged, say, to 2000 volts. He
may receive a shock which throws him in an unconscious condition across
another live wire which burns its way into his flesh, or he may fall to
the ground and be killed by the fall. A workman may hold a tool so as
to short circuit a current through it, making it red hot in his hands.
So many men who have been shocked into unconsciousness by high voltage
currents have recovered consciousness later that we cannot say how much
current is required to kill a man. For the execution of criminals 1800
to 2000 volts are used, and by special metal contacts ten to fourteen
amperes are forced through the body.

The first execution of a criminal by electricity was performed in Sing
Sing Prison, New York State, in 1890. There was at that time a hot
controversy among experts over the question whether death, or merely
unconsciousness, could be produced by electricity. To be on the safe
side the legislature passed a law requiring that the electrocution of a
criminal should be followed immediately by the dissection of his body.
Only six states out of forty-nine have thus far adopted that method
of capital punishment, five have abolished capital punishment, and
thirty-eight still prefer hanging to electrocution. But it should be
remembered that it is amperes, not volts, that kill. One often hears
the meaningless expression, "he received 2000 volts into his body."
The volts indicate the pressure, analogous to pounds per square inch
of water pressure. Amperes of electricity are analogous to gallons of
water. It is possible to have exceedingly high voltage of electricity
without amperes enough to do damage. When one holds his finger near to
a rapidly moving leather belt and a stream of sparks passes between the
finger and the belt, the voltage may be 50,000 or even 100,000, but
the quantity in amperes is too small to do any damage or even produce
much sensation. A similar thing is true when one produces sparks by
rubbing a cat's back, or lights the gas by a spark produced by rubbing
the feet upon a carpet. Such sparks are miniature lightning discharges.
The real lightning does damage because it furnishes quantity,
measurable in amperes, as well as extremely high volts of pressure.

At this point I was reminded by the boy who had received a shock from
the engine that morning that he had touched only one binding post.
How then had he closed a circuit through his body, and how could he
receive such a terrible shock when there were only a few battery cells
to produce the electric current. I replied that he had the distinction
of having encountered about a 5000-volt current. In the language of the
newspapers he might say, _Took 5000 volts and still live._ We must next
proceed to show how he really did close the circuit and how the spark
coil enables a battery of a few dry cells to produce exceedingly high
voltages.

THE ELECTRICAL SPARKING EQUIPMENT FOR A GASOLENE ENGINE

Under the shade of a great sugar maple, with Millville Lake spread
before us, we took apart and examined the entire equipment for
producing the electric sparks to explode the mixture of gasolene and
air in the cylinders of our motor boat. The engine has two cylinders.
For each cylinder there is a separate battery and spark coil. Inasmuch
as the electrical outfit is duplicated for each cylinder it will be
necessary for us to consider the case of one cylinder only.

When this engine is running, 700 explosions per minute are produced in
each cylinder. In one-twelfth of a second the following four events
take place:

1. The cylinder is swept clear of the products of combustion formed by
the last explosion.

2. Four drops of gasolene are vaporized and mixed with one quart of air
and pushed into the cylinder by the pressure of the atmosphere.

3. This mixture is compressed by the piston in the cylinder to about
one-fifth its original volume.

4. The mixture is heated to its kindling temperature, which is above
2000 degrees. It then burns with a sudden expansion, which drives
the piston before it and pushes the crank which is concealed in the
lower end of the cylinder half-way around. The crank is attached to
the shaft, which carries the fly-wheel upon one end and the propeller
wheel upon the other end. The momentum of the moving parts--chiefly
that of the fly-wheel--suffices to accomplish the remaining half of the
revolution.

That any machine could be devised which could repeat these four events
700 times a minute was unthinkable a few years ago.

The first men who thought that a gasolene engine could be a practical
thing were considered visionaries, but now they are found to be more
practicable than steam engines. They are so efficient that they compete
with the steam engine upon its own ground, and, in addition, they
have opened up regions of usefulness which the steam engine can never
exploit. So far as we can see, they have a permanent monopoly of the
navigation of the air.

It is with the fourth event mentioned above, viz., kindling the
explosive mixture, that we are now concerned. The high temperature
required for this is obtained by forcing an electrical current against
resistance.

Five dry battery cells would very readily heat a short piece of fine
wire to a sufficiently high temperature to explode the mixture, but
it is impossible to alternately heat and cool a wire twelve times a
second. It is too slow an operation. The only other method known at
present is to imitate the lightning and force an electric current
against the resistance of the air with sufficient power to produce the
required heat. This, however, requires an extremely high voltage--at
least 5000 volts, and our battery of five cells has not more than seven
and a half volts of pressure. The interesting question then is, how
does the spark coil enable us to raise the voltage from 7 to 5000.

To help toward an understanding of the matter I took seven small wire
nails which I found in the boat--they were sixpenny finishing nails. I
then took two or three yards of No. 24 insulated magnet wire, such as
is used upon electric bells, etc. I use it more often than any other
wire, and always have some about the boat. I fastened one end of this
wire to one of the binding posts of a dry cell (Fig. 113), _a_, and
attached branches _c_ and _d_ to it. The other end, _b_, was left
free to act as a switch for closing the circuit by touching it to the
remaining binding post.

[Illustration: Fig. 113]

[Illustration: Fig. 114]

[Illustration: Fig. 115]

One boy then touched the bare ends _c_ and _d_ to the tip of his
tongue, while I touched repeatedly the binding post with _b_. There
was, of course, no sensation. We now wound a portion of the wire upon
the bundle of nails, laying on about fifty turns. (See Fig. 114.) The
tongue was now placed at _T_ and _b_ was touched a few times to the
free binding post. A very decided shock was felt, not while the end
of the wire was resting upon _b_, but at the instant of touching and
again at breaking the connection. The shock was noticeably stronger at
the instant of breaking than of making the connection. There was also
a spark formed when the connection was broken, which did not appear
before the coil was made. We next wound on more of the wire--about
fifty more turns (Fig. 115). When now connections were made and broken
at _b_ the tongue at _T_ felt a much more decided shock, and a larger
spark occurred at _b_ when the circuit was broken. Both the tongue
and the spark indicate that the voltage is creeping up very rapidly
in this series of experiments. We next connected two cells in series,
then three, four, and finally five cells in place of the one. The spark
grew larger and "fatter," as the boatmen say, with each addition of a
cell. It was not pleasant to use the tongue in the experiment after
the number of cells exceeded two. I removed the branch _d_ from the
wire _b_ and connected it to the binding post, as shown in Fig. 116.
I then removed the crystal from my watch and poured into it a little
gasolene. I rubbed the ends of _b_ and _d_ together over this, and
when they separated the spark which was produced would not light the
gasolene. We had made a coil which produced a spark that looked like a
miniature flame, but still was not hot enough to set fire to gasolene
vapour. It simply needs more iron in the core and more turns of wire
about it. Bringing the ends of the wires together and separating them
is somewhat like drawing an arc with the arc light carbons. It requires
a vastly higher voltage to make a spark jump across an air gap than it
does to lead it across thus.

[Illustration: Fig. 116]

The kind of coil we have made (only larger) is very much used in houses
as a gas-lighting coil (to be described later). It is very much used
also for exploding gasolene engines. It generally passes under the name
of the "make and break" coil. The revolving shaft of the engine is made
to push together the ends of the wire and separate them at the right
instant to make the spark for explosion. Of course this is done inside
of the engine cylinder.

That type of coil does not offer resistance enough to protect the
battery, and dry cells soon run down if used with it. The coils that
we have in this boat are somewhat different from that, the details of
which we cannot now entirely explain.

They offer enough resistance to cut the current required of the battery
down to one third what the "make and break" coil would take and at the
same time they raise the voltage so much higher that the spark will
jump across an air gap without being led across as an arc. Hence they
are called "jump spark" coils.

[Illustration: Fig. 117]

It will be remembered that when we were studying the dynamo we produced
an electric current by moving a magnet. We may now add that an electric
current may be produced by simply changing the strength of a magnetic
field. The coil that we have just made creates a magnetic field in
the region about itself whenever a current is passing through it. The
tongue at _T_ (Fig. 117) detects an extra current while the magnetic
field is being produced, or while it is dying away, or it will detect
any slight variations in the strength of the current which produces
the magnetic field. It is customary to distinguish between these two
currents. The battery current which produced the magnetic field is
called the primary current and the current which is detected by the
tongue is called the secondary current. The primary current in our
experiments had only a few volts of pressure, from one to seven. The
secondary current had many volts, as indicated by the spark. If we
rub the end of the wire _c_ across the binding post under _b_ (Fig.
117) no spark occurs. The current does not in this case go through
the coil, and no secondary current is produced. Whenever we touch the
wire _b_ to that post we have, in addition to the primary current
which has not voltage enough to produce a spark, a secondary current
flowing in the same wire at the same time and having voltage enough to
produce a spark. The primary current is continuous while the contact
is closed; the secondary current is momentary, as the tongue detects,
and is produced only while changes are being made in the strength of
the magnetic field. We will now take another piece of wire and wind
upon the coil about two hundred more turns, leaving this outer coil
wholly disconnected from the inner one, (Fig. 118). I connect _c_ and
_d_, the terminals of what we may call the secondary coil, with my
measuring instrument and I connect _a_, one of the terminals of the
primary coil, with the battery. I then rub _b_, the other primary
terminal across the free binding post of the battery. At the instant
of closing the primary circuit--that is, of building up the magnetic
field--a secondary current is induced in the secondary coil, which
lasts for only an instant, too brief a time for the needle to measure
it, although its motion indicates both the presence and the direction
of the induced current. While the primary circuit remains closed--that
is, while no change is occurring in the strength of the magnetic
field--the needle returns to zero, indicating no secondary current. But
when now the primary circuit is broken and the magnetic field loses its
strength, the needle indicates a momentary current in the secondary
coil and _in the opposite direction from what it had been at first_.

[Illustration: Fig. 118]

If, therefore, I rapidly make and break the current at _b_ I produce an
alternating current in the secondary coil. I will connect _c_ and _d_
with a miniature lamp and, resting a coarse file upon the free binding
post, I will rake the end of the wire _b_ up and down upon this file so
that, as it dances along upon the file, it will rapidly make and break
the primary circuit, and therefore rapidly change the strength of the
magnetic field. You notice that the lamp lights up moderately well. It
is being lighted by an alternating current. I move the wire a little
more slowly and you see the flicker of the alternations. According to
the label upon the lamp it requires ten volts, and our battery could
not give that. We have therefore "stepped up" the voltage as we say and
we have a veritable step-up transformer.

In this case the primary and secondary circuits are entirely separate.
It is a familiar fact that different electric currents may pass through
the same wire at the same time without apparent conflict. We send
numerous telegraph despatches through the same wire at the same time.
It is quite as easy for several pairs of persons to telephone over the
same wire at the same time as it is for those same several pairs to
carry on separate conversations in the same room at the same time, at,
say, an "afternoon tea." We may use the same wire at the same time to
carry direct and alternating currents. This fact was first discovered
in 1902 by Bedell of Cornell University.

Primary and secondary currents do not require separate primary and
secondary coils to convey them. They may or may not be connected into
one continuous coil. It is quite immaterial whether they are connected
or not so long as they are in the same magnetic field. Indeed, it seems
that the field outside of the wire may be quite as important as the
wire itself.

[Illustration: Fig. 119]

We have now 100 turns in the primary and 200 turns in the secondary
coils. Let us connect _b_ with _c_ so as to make one continuous circuit
of 300 turns. Let us then put a branch upon _b_ to connect with the
battery, thus having 100 turns for the primary circuit, and put a
branch upon _a_ to connect with the lamp, thus having 300 turns upon
the lamp, (Fig. 119). When now we rub _b_ upon the file, as before,
the lamp lights up more brightly than before, indicating that we have
stepped up the voltage still higher. Varying the strength of the
magnetic field induces a secondary current and the voltage of the
induced current is determined, in part, by the number of turns in the
secondary circuit. If what we have been saying is true we ought to be
able to get these same results from an electric bell. To test this we
connected wires with _a_ and _c_, (Fig. 120), and since I knew that
the secondary current at _S_ would be too severe for the tongue we
decided to feel it with the hands. For this purpose we want a larger
surface than the wires themselves offer for contact with the hands,
and so I twisted the bare end of each w

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