The Study of Elementary Electricity and Magnetism by Experiment
by Thomas M. St. John · English · Gutenberg #48041 · 442,450 chars · public domain (US)
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Title: The Study of Elementary Electricity and Magnetism by Experiment
Author: Thomas M. St. John
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*** START OF THE PROJECT GUTENBERG EBOOK THE STUDY OF ELEMENTARY ELECTRICITY AND MAGNETISM BY EXPERIMENT ***
_BY THE SAME AUTHOR._
=FUN WITH MAGNETISM.= A book and complete outfit for _Sixty-One
Experiments_.
=FUN WITH ELECTRICITY.= A book and complete outfit for _Sixty
Experiments_.
=FUN WITH PUZZLES.= A book and complete outfit for _Four
Hundred Puzzles_.
=FUN WITH SOAP-BUBBLES.= A book and complete outfit for _Fancy
Bubbles and Films_.
=HUSTLE-BALL.= An American game. Played by means of magic wands
and polished balls of steel.
=JINGO.= The great war game, including =JINGO JUNIOR=.
=HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS.= A book
containing complete directions for making all kinds of simple
apparatus for the study of elementary electricity.
=THE STUDY OF ELEMENTARY ELECTRICITY AND MAGNETISM BY
EXPERIMENT.= This book is designed as a text-book for amateurs,
students, and others who wish to take up a systematic course of
simple experiments at home or in school.
_IN PREPARATION._
=THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY.= This book
explains, in simple, straightforward language, many things
about electricity; things in which the American boy is
intensely interested; things he wants to know; things he should
know.
_Ask Your Toy Dealer, Stationer, or Bookseller for
our Books, Games, Puzzles, Educational
Amusements, Etc._
Thomas M. St. John, 407 West 51st St., New York.
The Study of Elementary Electricity
and Magnetism by Experiment
Containing
TWO HUNDRED EXPERIMENTS
PERFORMED WITH
SIMPLE, HOME-MADE APPARATUS
BY
THOMAS M. ST. JOHN, Met. E.
Author of "Fun With Magnetism," "Fun With Electricity," "How
Two Boys Made Their Own Electrical Apparatus," Etc.
[Illustration: Logo]
NEW YORK
THOMAS M. ST. JOHN
407 West 51st Street
1900
Copyright, 1900,
By THOMAS M. ST. JOHN.
To the Student.
This book is designed as a text-book for amateurs, students, and others
who wish to take up a systematic course of elementary electrical
experiments at home or in school.
The student is advised to begin at the beginning, to perform the
experiments in the order given, and to understand each step before
proceeding. Certain principles and explanations necessarily precede the
practical and perhaps more interesting applications of those principles.
In selecting the apparatus for the experiments in this book, the author
has kept constantly in mind the fact that the average student will not
buy the expensive pieces usually described in text-books.
The two hundred experiments given can be performed with simple,
inexpensive apparatus; in fact, the student should make at least a part
of his own apparatus.
For the benefit of those who wish to make their own apparatus, the
author has given, throughout the work, explanations that will aid
in the construction of certain pieces especially adapted to these
experiments. For those who have the author's "How Two Boys Made Their
Own Electrical Apparatus," constant references have been made to it
as the "Apparatus Book," as this contains full details for making
almost all kinds of simple apparatus needed in "The Study of Elementary
Electricity and Magnetism by Experiment."
THOMAS M. ST. JOHN.
_New York, April, 1900._
The Study of Elementary Electricity and
Magnetism by Experiment
PART I--MAGNETISM
PART II--STATIC ELECTRICITY
PART III--CURRENT ELECTRICITY
The Study of Elementary Electricity and Magnetism by Experiment.
TABLE OF CONTENTS.
PART I.--MAGNETISM.
PAGE.
CHAPTER I. =Iron and Steel= 3
Introduction.--Kinds of iron and steel.--Exp. 1, To
study steel.--Discussion.--Exp. 2, To find whether a
piece of hard steel can be made softer.--Annealing.--Exp.
3, To find whether a piece of annealed steel can be
hardened.--Hardening; Tempering.--Exp. 4, To test the
hardening properties of soft iron.--Discussion.
CHAPTER II. =Magnets= 7
Kinds of magnets.--Exp. 5, To study the horseshoe magnet.--Poles;
Equator.--Exp. 6, To ascertain the nature of substances attracted
by a magnet.--Magnetic Bodies; Diamagnetic Bodies.--Practical
Uses of Magnets.--Exp. 7, To study the action of magnetism
through various substances.--Magnetic Transparency; Magnetic
Screens.--Exp. 8, To find whether a magnet can give magnetism
to a piece of steel.--Discussion; Bar Magnets.--Exp. 9, To make
small magnets.--Exp. 10, To find whether a freely-swinging bar
magnet tends to point in any particular direction.--North-seeking
Poles; South-seeking Poles; Pointing Power.--The Magnetic
Needle; The Compass.--Exp. 11, To study the action of magnets
upon each other.--Exp. 12, To study the action of a magnet upon
soft iron.--Laws of Attraction and Repulsion.--Exp. 13, To
learn how to produce a desired pole at a given end of a piece
of steel.--Rule for Poles.--Our Compass.--Review; Magnetic
Problems.--Exp. 14, To find whether the poles of a magnet
can be reversed.--Discussion; Reversal of Poles.--Exp. 15,
To find whether we can make a magnet with two N poles.--Exp.
16, To study the bar magnet with two N poles.--Discussion;
Consequent Poles.--Exp. 17, To study consequent poles. Exp.
18, To study the theory of magnetism.--Theory of Magnetism;
Magnetic Saturation.--Exp. 19, To find whether soft iron
will permanently retain magnetism.--Retentivity or Coercive
Force; Residual Magnetism.--Exp. 20, To test the retentivity
of soft steel.--Discussion.--Exp. 21, To test the retentivity
of hard steel.--Exp. 22, To test the effect of heat upon a
magnet.--Discussion.--Exp. 23, To test the effect of breaking a
magnet.--Discussion.
CHAPTER III. =Induced Magnetism= 20
Exp. 24, To find whether we can magnetize a piece of iron
without touching it with a magnet.--Temporary Magnetism;
Induced Magnetism.--Exp. 25, To find whether a piece of steel
can be permanently magnetized by induction.--Exp. 26, To
study the inductive action of a magnet upon a piece of soft
iron.--Polarization; Pole Pieces.--Exps. 27-30, To study pole
pieces.
CHAPTER IV. =The Magnetic Field= 23
Exp. 31, To study the space around the magnet, in
which pieces of iron become temporary magnets by
induction.--Discussion; The Magnetic Field.--Exp. 32, To
study the magnetic field of a bar magnet.--Magnetic Figures;
Lines of Magnetic Force.--Exps. 33-37, To study
the magnetic fields of various combinations of bar magnets.--Exps.
38-39, To study the lifting power of combinations
of bar magnets.--Discussion; Compound Magnets.--Exps.
40-42, To study the magnetic field of the
horseshoe magnet.--Discussion; Resistance to Lines of
Force.--Exp. 43, To show that lines of force are on all
sides of a magnet.--Discussion.--Exp. 44, To study a
horseshoe magnet with movable poles.--Discussion; Advantages
of Horseshoe Magnets.
CHAPTER V. =Terrestrial Magnetism= 31
The Magnetism of the Earth.--Declination.--Exp. 45, To study
the lines of force above and below a bar magnet placed
horizontally.--The Dip or Inclination of the Magnetic
Needle.--Exp. 46, To study the dip or inclination of the magnetic
needle due to the action of the earth.--Discussion; Balancing
Magnetic Needles.--Exps. 47-48, To study the inductive influence
of the earth.--Discussion.--Natural Magnets.--Exp. 49, To test
the effect of twisting a wire held north and south in the earth's
magnetic field.--Exp. 50, To test for magnetism in bars of iron,
tools, etc.
PART II.--STATIC ELECTRICITY.
CHAPTER VI. =Electrification= 39
Some Varieties of Electricity.--Exp. 51-52, To study
electrification by friction.--Discussion.--Electrified and
Neutral bodies.--Force; Resistance; Work; Potential Energy;
Electrification.--Heat and Electrification.--Exps. 53-54, To
study electrical attraction.--Discussion.--Exp. 55, To study
mutual attractions.--Mutual Attractions.--Exps. 56-58, To study
electrical repulsions.--The Carbon Electroscope.--Discussion of
Experiments 56, 57, 58.--Exp. 59, To study the electrification
of glass.--Questions.--Exp. 60, To compare the electrification
produced by ebonite and flannel with that produced by glass and
silk.--Discussion.--Laws.
CHAPTER VII. =Insulators and Conductors= 47
Exps. 61-63, To study insulators.--Conductors.--Exp.
64, To study conduction.--Discussion.--Exp. 65,
To study conduction.--Telegraph line using static
electricity.--Discussion.--Relation between conductors and
insulators.--Electrics and Non-electrics.--Exp. 66, To study the
effect of moisture upon an insulator.--Discussion.--Exp. 67, To
test the effects of moisture upon bodies to be electrified.
CHAPTER VIII. =Charging and Discharging Conductors= 52
The Electrophorus.--Exp. 68, To learn how to use the
electrophorus.--Exp. 69, To study "charging by conduction."--Exp.
70, To study potential; Electromotive force.--Pressure;
Potential; Electromotive force; Current, Spark.--Theories about
Electrifications.--Exp. 71, To study some methods of discharging
an electrified body.--Disruptive, Conductive and Convective
Discharges.--Exp. 72, To study intermittent or step-by-step
discharges.--Discussion.--Exp. 73, To ascertain the location of
the charge upon an electrified conductor.--Discussion.--Hollow
and Solid Conductors.--Exp. 74, To study the effect of points
upon a charged conductor.--Electric Density.--Electric Wind.
CHAPTER IX. =Induced Electrification= 60
Electric Fields; Lines of Force.--Exp. 75, To study
electric induction.--Electric Polarization; Theory of
Induction.--Exp. 76, To learn how to charge a body by
induction.--Free and Bound Electrifications.--Exp. 77, To
show that a neutral body is polarized before it is attracted
by a charged one.--Polarization Precedes Attraction.--Exp.
78, To find whether electric induction will act through an
insulator.--Dielectrics.--Exp. 79, To find whether a polarized
conductor can act inductively upon another conductor.--Successive
Induction.--Inductive Capacity.--Exp. 80, To study the action of
the electrophorus.--Discussion.--Details of action.--Exp. 81,
To see, hear, and feel the results of inductive influence and
polarization.--Discussion.
CHAPTER X. =Condensation of Electrification= 68
Exp. 82, To find whether a large surface will hold more
electrification than a small one.--Electrical Capacity.--Exp.
83, To find whether the capacity of a given conductor can
be increased without increasing its size.--Condensation;
Condensers.--The Leyden Jar.--Fulminating Panes.--Induction
Coil Condensers.--Submarine Cables.--Exp. 84, To study the
condensation of electrification.--Discussion.--Exp. 85, To study
the action of the condenser.--Discussion.--Exp. 86, To study the
effect of electrical discharges upon the human body.--Shocks;
Dischargers.--Exps. 87-88, To show the strong attraction between
opposite electrifications in the condenser.--Discussion.--Exp.
89, To show how the condenser may be slowly discharged.--The
Electric Chime.--Exp. 90, To ascertain the location of a charge
in a condenser.--Discussion.--Exp. 91, To find whether any
electrification remains in the condenser after it has once been
discharged.--Residual Charge.--Exp. 92, To study successive
condensation; the chime cascade.--Discussion.
CHAPTER XI. =Electroscopes= 77
Electroscopes.--Our leaf electroscope.--Exp. 93, To study the
leaf electroscope; charging by conduction.--Discussion.--Exp.
95, To learn some uses of the electroscope.--Discussion.--The
Proof-plane.
CHAPTER XII. =Miscellaneous Experiments= 81
Exp. 96, To show that friction always produces two kinds of
electrification.--Discussion.--Exp. 97, To show "successive
sparks."--Exp. 98, To show to the eye the strong attraction
between a charged and a neutral body.--Exp. 99, To feel the
strong attraction between a charged and a neutral body.--Exp.
100, The human body a frictional electric machine.--Static
Electric Machines.
CHAPTER XIII. =Atmospheric Electricity= 84
Atmospheric Electricity.--Lightning.--Thunder.--Lightning
Rods.--Causes of Atmospheric Electricity.--St. Elmo's
Fire.--Aurora Borealis.
PART III.--CURRENT ELECTRICITY.
CHAPTER XIV. CONSTRUCTION AND USE OF APPARATUS 89
Exp. 101, To study the effect of the electric current upon
the magnetic needle.--Electrical Connections.--Current
Detectors.--Exp. 102, To study the construction and use of a
simple "key."--Exp. 103, To study the construction and use of
a simple "current reverser."--Exp. 104, To study the simple
current detector.--Exp. 105, To study the construction and use
of the simple galvanoscope.--Discussion; True Readings.--Exp.
106, To study the construction and use of a simple astatic
needle.--Astatic Needles.--Exp. 107, To study the construction
and use of a simple astatic galvanoscope.--Astatic Galvanoscopes.
CHAPTER XV. GALVANIC CELLS AND BATTERIES 102
Exp. 108, To study the effect of dilute sulphuric acid
upon carbon and various metals.--To amalgamate.--Dilute
sulphuric acid.--Discussion.--Exp. 109, To study the effect
of dilute sulphuric acid upon various combinations of
metals.--Discussion.--Exp. 110, To study the construction of a
simple Voltaic or Galvanic cell.--The Electric Current.--Source
of the Electrification.--The Electric Circuit; Open and Closed
Circuits.--Plates or Elements.--Direction of Current.--Poles or
Electrodes.--Chemical Action in the Simple Galvanic Cell.--Action
in Cell Using Impure Zinc; Action Using Pure Zinc.--Exp. 111, To
see what is meant by "local currents" in the cell.--Local Action;
Local Currents.--Reasons for Amalgamating Zinc Plates.--Exp.
112, To study the "single-fluid" Galvanic Cell.--The Simple
Cell.--Polarization of Cells.--Effects of Polarization.--Remedies
for Polarization; Depolarizers.--Exp. 113, To study the
"two-fluid" Galvanic Cell.--Setting Up the Two-Fluid Cell.--Care
of Two-Fluid Cell.--Copper Sulphate Solution.--Chemical Action
in the Two-Fluid Cell.--Various Galvanic Cells; Open and Closed
Circuit Cells.--The Leclanché Cell--Dry Cells.--The Bichromate of
Potash Cell.--The Daniell Cell.--The Gravity Cell.
CHAPTER XVI. THE ELECTRIC CIRCUIT 115
Exp. 114, To see what is meant by "divided circuits" and
"shunts."--Divided Circuits; Shunts.--Exp. 115, To see what is
meant by "short circuits."
CHAPTER XVII. ELECTROMOTIVE FORCE 117
Electromotive Force.--Unit of E. M. F.; The Volt.--Exp.
116, To see whether the E. M. F. of a cell depends upon the
materials used in its construction.--Discussion.--Electromotive
Series.--Exp. 117, To see whether the E. M. F. of a cell depends
upon its size.--Discussion.
CHAPTER XVIII. ELECTRICAL RESISTANCE 120
Resistance.--Exp. 118, To study the general effect of
"resistance" upon a current.--External Resistance; Internal
Resistance; Unit of Resistance; The Ohm.--Resistance Coils;
Resistance Boxes.--Simple Resistance Coil.--Exp. 119, To
test the power of various substances to conduct galvanic
electricity.--Conductors and Nonconductors.--Exp. 120, To
find the effect of sulphuric acid upon the conductivity
of water.--Internal Resistance.--Exp. 121, To find what
effect the length of a wire has upon its electrical
resistance.--Discussion.--Exp. 122, To find what effect the
size (area of cross-section) of a wire has upon its electrical
resistance.--Discussion.--Exp. 123, To compare the resistance of
a divided circuit with the resistance of one of its branches.
Discussion.--Exp. 124, To study the effect of decreasing the
resistance in one branch of a divided circuit.--Current in
Divided Circuits.
CHAPTER XIX. MEASUREMENT OF RESISTANCE 130
Exp. 125, To study the construction and use of a simple
Wheatstone's Bridge.--The Simple Bridge.--Equipotential
Points.--Example.--Exp. 126, To measure the resistance of a wire
by means of Wheatstone's Bridge; the "bridge method."--Allowances
for Connections.--Exps. 127-137, To measure the resistances of
various wires, coils, etc., by the "bridge method."--Table.--Exp.
138, To study the effect of heat upon the resistance of
metals.--Effect of Heat upon Resistance.--Exp. 139, To measure
the resistance of a wire by the "method of substitution."--Simple
Rheostat.--Exp. 140, To measure the E. M. F. of a cell by
comparison with the two-fluid cell.--Exp. 141, To measure the
internal resistance of a cell by the "method of opposition."
CHAPTER XX. CURRENT STRENGTH 142
Strength of Current.--Unit of Current Strength; The
Ampere.--Measurement of Current Strength.--The Tangent
Galvanometer.--The Ammeter.--The Voltameter.--Unit of Quantity;
The Coulomb.--Electrical Horse-power; The Watt.--Ohm's
Law.--Internal Resistance and Current Strength.--Exp. 142,
Having a cell with large plates, to find how the strength of the
current is affected by changes in the position of the plates, the
external resistance being small.--Exp. 143, Same as Exp. 142,
but with small plates.--Exp. 144, To find whether the changes
in current strength, due to changes in internal resistance,
are as great when the external resistance is large, as they
are when the external resistance is small.--Discussion, with
examples.--Arrangement of Cells and Current Strength.--Cells in
Series.--Cells Abreast.--Exp. 145, To find the best way to join
two similar cells when the external resistance is small.--Exp.
146, To find the best way to join two similar cells when the
external resistance is large.--Best Arrangement of Cells.
CHAPTER XXI. CHEMICAL EFFECTS OF THE ELECTRIC
CURRENT 151
Chemical Action and Electricity.--Electrolysis.--Exp.
147, To study the electrolysis of water.--Composition
of Water.--Electromotive Force of Polarization.--Exp.
148, To coat iron with copper.--Exp. 149, To
study the electrolysis of a solution of copper
sulphate.--Electroplating.--Exp. 150, To study the chemistry of
electroplating.--Discussion.--Electrotyping.--Voltameters.--Exp.
151, To study the construction and action of a simple "storage"
cell.--Secondary or Storage Cells.
CHAPTER XXII. ELECTROMAGNETISM 158
Electromagnetism.--Exp. 152, To study the lines of force about
a straight wire carrying a current.--Ampere's Rule.--Lines
of Force About Parallel Wires.--Exp. 153, To study the
lines of force about a coil of wire like that upon the
galvanoscope.--Exp. 154, To study the magnetic field about a
small coil of wire.--Coils.--Polarity of Coils.--Exp. 155, To
test the attracting and "sucking" power of a magnetized coil
or helix.--Exp. 156, To find whether a piece of steel can be
permanently magnetized by an electric current.--Exp. 157, To
study the effect of a piece of iron placed inside of a magnetized
coil of wire.
CHAPTER XXIII. ELECTROMAGNETS 165
Electromagnets.--Cores of Electromagnets.--Exps. 158-163,
To study straight electromagnets; Lifting power; Residual
magnetism of core; Magnetic tick; Magnetic figures; Magnetic
field.--Horseshoe Electromagnets.--Use of Yoke.--Experimental
Magnets.--Method of Joining Coils.--Exps. 164-173, To study
horseshoe electromagnets; To test the poles; To study the
inductive action of one core upon the other; Magnetic figures;
Permanent Magnetic Figures; Lifting power; Residual magnetism
when magnetic circuit is closed.--Closed Magnetic Circuits.
CHAPTER XXIV. THERMOELECTRICITY 175
Exp. 174, To find whether electricity can be produced by
heat.--Home-made Thermopile.--Thermoelectricity.--Peltier
Effect.--Thermopiles.
CHAPTER XXV. INDUCED CURRENTS 178
Electromagnetic Induction.--Exp. 175, To find whether a current
can be generated with a bar magnet and a hollow coil of
wire.--Discussion.--Induced Currents and Work.--Exp. 176, To
find whether a current can be generated with a bar magnet and
a coil of wire having an iron core.--Exp. 177, To find whether
a current can be generated with a horseshoe magnet and a coil
of wire having an iron core.--Induced Currents and Lines of
Force.--Exp. 178, To find whether a current can be generated
with an electromagnet and a hollow coil of wire.--Exp. 179, To
find whether a current can be generated with an electromagnet
and a coil of wire having an iron core.--Discussion of Exps.
178-179.--Exp. 180, To study the effect of starting or stopping
a current near a coil of wire or other closed circuit.--Exp.
181, To study the effect of starting or stopping a current
in a coil placed inside of another coil.--Discussion of
Exps. 180-181.--Direction of Induced Current.--Laws of
Induction.--Primary and Secondary Currents.--Exp. 182, To see
what is meant by alternating currents.--Direct and Alternating
Currents.--Self-induction; Extra Currents.
CHAPTER XXVI. THE PRODUCTION OF MOTION BY CURRENTS 187
Currents and Motion.--Exp. 183, Motion produced with a hollow
coil and a piece of iron.--Exp. 184, Motion with hollow coil
and bar magnet.--Exp. 185, Motion with electromagnet and
piece of iron.--Exp. 186, Motion with electromagnet and bar
magnet.--Exp. 187, Motion with electromagnet and horseshoe
magnet.--Exp. 188, Motion with two electromagnets.--Discussion
of Exps. 183-188.--Exp. 189, Rotary motion with a hollow coil
of wire and a permanent magnet.--Exp. 190, Rotary motion with
an electromagnet and a permanent magnet.--Discussion of Exps.
189-190.
CHAPTER XXVII. APPLICATIONS OF ELECTRICITY 192
Things Electricity Can Do.--Exp. 191, To study the action of
a simple telegraph sounder.--Discussion.--Telegraph Line;
Connections.--Operation of Line.--Exp. 192, To study the action
of the "relay" on telegraph lines.--The Relay.--Exp. 193, To
study the action of a two-pole telegraph instrument.--Exp.
194, To study the action of a simple "single needle telegraph
instrument."--Exp. 195, To study the action of a simple
automatic contact breaker, or current interrupter.--Automatic
Current Interrupters.--Exp. 196, To study the action of a
simple electric bell, or a "buzzer."--Electric Bells and
Buzzers.--Exp. 197, To study the action of a simple telegraph
"recorder."--Exp. 198, To study the action of a simple
"annunciator."--Discussion.--Exp. 199, To study the shocking
effects of the "extra current." Induction Coils.--Action of
Induction Coils.--Transformers.--The Dynamo.--The Electric
Motor.--Exp. 200, To study the action of the telephone.--The
Telephone.--The Bell, or Magneto-transmitter.--The Receiver.--The
Carbon Transmitter.--Induction Coils in Telephone Work.--Electric
Lighting and Heating.--Arc Lamps.--The Incandescent Lamp.
CHAPTER XXVIII. WIRE TABLES 208
APPARATUS LIST 210
INDEX 215
MAGNETISM
A Few Dont's for Young Students.
Don't fail to make at least a part of your own apparatus; there
is a great deal of satisfaction and pleasure in home-made
apparatus.
Don't experiment in all parts of the house, if working at home.
Fit up a small room for your den, and carry the key.
Don't begin an experiment before you really know what you are
trying to do. Read the directions carefully, then begin.
Don't rush through an experiment to see what happens at the end
of it. See what happens at each step, and notice every little
thing that seems unusual.
Don't try to do all parts of an experiment at the same time.
Understand one part, then proceed.
Don't fail to ask yourself questions, and form an opinion about
the results of an experiment before you read what the author
has to say about it.
Don't fail to keep a note-book. Keep all the data and
arithmetical work for future reference.
Don't leave the apparatus around after you have finished the
day's work.
PART I.--MAGNETISM.
CHAPTER I.
IRON AND STEEL.
_=1. Introduction.=_ We should know something about iron and steel
at the start, because we are to use them in nearly every experiment.
The success with some of the experiments will depend largely upon the
quality of the iron and steel used.
When we buy a piece of iron from the blacksmith, we get more than
iron for our money. Hidden in this iron are other substances (carbon,
phosphorus, silicon, etc.), which are called "impurities" by the
chemist. If all the impurities were taken out of the iron, however,
we should have nothing but a powder left; this the chemist would call
"chemically pure iron," but it would be of no value whatever to the
blacksmith or mechanic. The impurities in iron and steel are just
what are needed to hold the particles of iron together, and to make
them valuable. By regulating the amount of carbon, phosphorus, etc.,
manufacturers can make different grades and qualities of iron or steel.
When carbon is united with the _pure_ iron, we get what is commonly
called iron.
_=2. Kinds of Iron and Steel.=_ _Cast iron_ is the most impure form of
iron. Stoves, large kettles, flatirons, etc., are made of cast iron.
_Wrought iron_ is the purest form of commercial iron. It usually comes
in bars or rods. Blacksmiths hammer these into shapes to use on wagons,
machinery, etc. _Steel_ contains more carbon than wrought iron, and
less than cast iron.
_Soft steel_ is very much like wrought iron in appearance, and it is
used like wrought iron.
_Hard steel_ has more carbon in it than soft steel. Tools, needles,
etc., are made of this.
=EXPERIMENT 1. To study steel.=
_Apparatus._ A steel sewing-needle (No. 1).[A]
[Footnote A: _=NOTE. Each piece of apparatus used in the following
experiments has a number. See "Apparatus list" at the back of this book
for details. The numbers given under "Apparatus," in each experiment,
refer to this list.=_]
=3. Directions.= (A) Bend a sewing-needle until it breaks. Is
the steel brittle?
(B) If you have a file, test the hardness of the needle.
_=4. Discussion.=_ "Needle steel" is usually of good quality. It will
be very useful in many experiments. Do you know how to make the needle
softer?
=EXPERIMENT 2. To find whether a piece of hard steel can be
made softer.=
[Illustration: Fig. 1.]
_Apparatus._ Fig. 1. A needle; a cork, Ck (No. 2); lighted
candle (No. 3). The bottom of the candle should be warmed and
stuck to a pasteboard base.
=5. Directions.= (A) Stick the point of the needle into Ck,
Fig. 1, then hold the needle in the flame until it is red-hot.
(The upper part of the flame is the hottest.)
(B) Allow the needle to cool in the air.
(C) Test the brittleness of the steel by bending it. Test its
hardness with a file (Exp. 1).
_=6. Annealing.=_ This process of softening steel by first heating it
and then allowing it to cool slowly, is called _annealing_. All pieces
of iron and steel are, of course, hard; but you have learned that some
pieces are much harder than others.
=EXPERIMENT 3. To find whether a piece of annealed steel can be
hardened.=
_Apparatus._ The needle just annealed and bent; cork, etc., of
Exp. 2; a glass of cold water.
=7. Directions.= (A) Heat the bent portion of the needle in the
candle flame (Exp. 2) until it is red-hot, then immediately
plunge the needle into the water.
(B) Test its brittleness and hardness, as in Exp. 2.
_=8. Hardening; Tempering.=_ Good steel is a very valuable material;
the same piece may be made hard or soft at will. By sudden cooling, the
steel becomes very hard. This process is called _hardening_, but it
makes the steel too brittle for many purposes. By _tempering_ is meant
the "letting down" of the steel from the very hard state to any desired
degree of hardness. This may be done by suddenly cooling the steel when
at the right temperature, it not being hot enough to produce extreme
hardness. (The approximate temperature of hot steel can be told by the
colors which form on a clean surface. These are due to oxides which
form as the steel gradually rises in temperature.)
=EXPERIMENT 4. To test the hardening properties of soft iron.=
_Apparatus._ A piece of soft iron wire about 3 in. (7.5 cm.)
long (No. 4); the candle, water, etc., of Exp. 3.
=9. Directions.= (A) Test the wire by bending and filing.
(B) Heat the wire in the candle flame as you did the needle
(Fig. 1), then cool it suddenly with the water. Study the
results.
_=10. Discussion.=_ Soft iron contains much less carbon than steel. The
hardening quality which steel has is due to the proper amount of carbon
in it. If you have performed the experiments so far, you will be much
more able to understand later ones, and you will see why we are obliged
to use soft iron for some parts of electrical apparatus, and hard steel
for other parts.
CHAPTER II.
MAGNETS.
_=11. Kinds of Magnets.=_ Among the varieties of magnets which we
shall discuss, are the natural, artificial, temporary, permanent, bar,
horseshoe, compound, and electro-magnet.
[Illustration: Fig. 2.]
_The Horseshoe Magnet_, H M (Fig. 2), is the most popular form of
small magnets. The red paint has nothing to do with the magnetism. The
piece, A, is called its _armature_, and is made of soft iron, while the
magnet itself should be made of the best steel, properly hardened. The
armature should always be in place when the magnet is not in use, and
care should be taken to thoroughly clean the ends of the magnet before
replacing the armature. The horseshoe magnet is _artificial_, and it is
called a _permanent_ magnet, because it retains its strength for a long
time, if properly cared for.
=EXPERIMENT 5. To study the horseshoe magnet.=
_Apparatus._ Fig. 2. The horseshoe magnet, H M (No. 16).
=12. Directions.= (A) Remove the armature, A, from the magnet,
then move A about upon H M to see (1) if the curved part of
H M has any attraction for A, and (2) to see if there is any
attraction for A at points between the curve and the extreme
ends of H M.
_=13. Poles; Equator.=_ The ends of a magnet are called its _poles_.
The end marked with a line, or an N, should be the _north_ pole. The
unmarked end is the _south_ pole. N and S are abbreviations for north
and south. The central part, at which there _seems_ to be no magnetism,
is called the _neutral point_ or _equator_.
=EXPERIMENT 6. To ascertain the nature of substances attracted
by a magnet.=
_Apparatus._ The horseshoe magnet, H M (Fig. 2); silver,
copper, and nickel coins; iron filings (No. 17), nails, tacks,
pins, needles; pieces of brass, lead, copper, tin, etc.
(Ordinary tin is really sheet iron covered with tin.) Use the
various battery plates for the different metals.
=14. Directions.= (A) Try the effect of H M upon the above
substances, and upon any other substances thought of.
_=15. Magnetic Bodies; Diamagnetic Bodies.=_ Substances which are
attracted by a magnet are said to be _magnetic_. A piece of soft iron
wire is magnetic, although not a magnet. Very strong magnets show
that nickel, oxygen, and a few other substances not containing iron,
are also magnetic. Some elements are actually repelled by a powerful
magnet; these are called _diamagnetic_ bodies. It is thought that all
bodies are more or less affected by a magnet.
_=16. Practical Uses of Magnets.=_ Many practical uses are made of
magnets, such as the automatic picking out of small pieces of iron
from grain before it is ground into flour, and the separation of iron
from other metals, etc. The most important uses of magnets are in the
compass and in connection with the electric current, as in machines
like dynamos and motors. (See experiments with electro-magnets.)
=EXPERIMENT 7. To study the action of magnetism through various
substances.=
_Apparatus._ Horseshoe magnet, H M; a sheet of stiff paper;
pieces of sheet glass, iron, zinc, copper, lead, thin wood,
etc.; sewing-needle. (A tin box may be used for the iron, and
battery plates for the other metals.)
=17. Directions.= (A) Place the needle upon the paper and move
H M about immediately under it.
(B) In place of the paper, try wood, glass, etc.
(C) Invent an experiment to show that magnetism will act
through your hand.
(D) Invent an experiment to show that magnetism will act
through water.
_=18. Magnetic Transparency; Magnetic Screens.=_ Substances, like
paper, are said to be _transparent_ to magnetism. Iron does not allow
magnetism to pass through it as readily as paper and glass; in fact,
thick iron may act as a _magnetic screen_.
=EXPERIMENT 8. To find whether a magnet can give magnetism to a
piece of steel.=
=19. Note.= You have seen that the horseshoe magnet can lift
nails, iron filings, etc.; you have used this lifting power to
show that the magnet was really a magnet, and not merely an
ordinary piece of iron painted red. Can we give some of its
magnetism to another piece of steel? Can we pass the magnetism
along from one piece of steel to another?
_Apparatus._ The horseshoe magnet, H M; two sewing-needles that
have never been near a magnet; iron filings.
=20. Directions.= (A) Test the needles for magnetism with the
iron filings, and be sure that they are not magnetized.
(B) Remove the armature, A, from H M, then touch the point of
one of the needles to one pole of H M.
(C) Lay H M aside, and test the point of the needle for
magnetism.
(D) If you find that the needle is magnetized, rub its point
upon the point of the other needle; then test the point of the
second needle for magnetism.
_=21. Discussion; Bar Magnets.=_ A piece of good steel will attract
iron after merely touching a magnet. To thoroughly magnetize it,
however, a mere touch is not sufficient. There are several ways of
making magnets, depending upon the size, shape, and strength desired.
For these experiments, the student needs only a good horseshoe magnet,
or, better still, the electro-magnets described later; with these any
number of small magnets may be made. Straight magnets are called _bar
magnets_.
=EXPERIMENT 9. To make small magnets.=
_Apparatus._ Fig. 3. The horseshoe magnet, H M; sewing-needles;
iron filings. (See Apparatus Book, Pg. 140, for various kinds
of steel suitable for small magnets.)
=22. Directions.= (A) Hold H M (Fig. 3) in the left hand, its
poles being uppermost. Grasp the point of the needle with the
right hand, and place its point upon the N or marked pole of H
M.
(B) Pull the needle along in the direction of its length (see
the arrow), continuing the motion until its head is at least an
inch from the pole.
(C) Raise the needle at least an inch above H M, lower it to
its former position (Fig. 3), and repeat the operation 3 or 4
times. Do not slide the needle back and forth upon the pole,
and be careful not to let it accidentally touch the S pole of H
M.
(D) Test the needle for magnetism with iron filings, and save
it for the next experiment.
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
=EXPERIMENT 10. To find whether a freely-swinging bar magnet
tends to point in any particular direction.=
_Apparatus._ Fig. 4. A magnetized sewing-needle (Exp. 9); the
flat cork, Ck (No. 2); a dish of water. (You can use a tumbler,
but a larger dish is better.)
=23. Note.= An oily sewing-needle may be floated without the
cork by carefully lowering it to the surface of the water. All
magnets, pieces of iron and steel, knives, etc., should be
removed from the table when trying such experiments. Why?
=24. Directions.= (A) Place the little bar magnet (the needle)
upon the floating cork, turn it in various positions, and note
the result.
_=25. North-seeking Poles; South-seeking Poles; Pointing Power.=_ It
should be noted that the _point_ swings to the north, provided the
needle is magnetized as directed in Exp. 9. This is called the north,
or north-seeking pole. The N-seeking pole is sometimes called the
marked pole. For convenience, we shall hereafter speak of the N-seeking
pole as the N pole, and of the S-seeking pole as the S pole. We shall
hereafter speak of the tendency which a bar magnet has to point N and
S, as its _pointing power_. An unmagnetized needle has no pointing
power.
_=26. The Magnetic Needle; The Compass.=_ A small bar magnet, supported
upon a pivot, or suspended so that it may freely turn, is called
a _magnetic needle_. When balanced upon a pivot having under it a
graduated circle marked N, E, S, W, etc., it is called a _compass_.
These have been used for centuries. (See Apparatus Book for Home-made
Magnetic Needles.)
=EXPERIMENT 11. To study the action of magnets upon each other.=
_Apparatus._ Two magnetized sewing-needles (magnetized as in
Exp. 9); the cork, etc., of Exp. 10.
=27. Directions.= (A) Float each little bar magnet (needles)
separately to locate the N poles.
(B) Leave one magnet upon the cork, and with the hand bring the
N pole of the other magnet immediately over the N pole of the
floating one. Note the result.
(C) Try the effect of two S poles upon each other.
(D) What is the result when a N pole of one is brought near a S
pole of the other?
=EXPERIMENT 12. To study the action of a magnet upon soft iron.=
_Apparatus._ A magnetized sewing-needle; cork, etc., of Exp.
10; a piece of soft iron wire, 3 in. long; iron filings.
=28. Directions.= (A) Test the wire for magnetism with
filings. Be sure that it is not magnetized. If it shows any
magnetism, twist it thoroughly before using. (Exp. 19.)
(B) Float the magnetized needle (Exp. 10), then bring the end
of the wire near one pole of the needle and then near the other
pole.
(C) Place the wire upon the cork, hold the needle in the hand
and experiment.
_=29. Laws of Attraction and Repulsion.=_ From experiments 11 and 12
are derived these laws:
(_=1=_) _=Like poles repel each other=_; (_=2=_) _=Unlike poles attract
each other=_; (_=3=_) _=Either pole attracts and is attracted by
unmagnetized iron or steel.=_
The attraction between a magnet and a piece of iron or steel is mutual.
Attraction, alone, simply indicates that at least one of the bodies is
magnetized; repulsion proves that both are magnetized.
=EXPERIMENT 13. To learn how to produce a desired pole at a
given end of a piece of steel.=
_Apparatus._ Same as in Exp. 9.
=30. Directions.= (A) Magnetize a sewing-needle (Exp. 9) by
rubbing it upon the N pole of H M from _point to head_. Float
it and locate its N pole.
(B) Take another needle that has not been magnetized, and rub
it on the same pole (N) from _head to point_. Locate its N pole.
(C) Magnetize another needle by rubbing it from _point to
head_ upon the S pole of H M; locate its N pole. Can you now
determine, beforehand, how the poles of the needle magnet will
be arranged?
_=31. Rule for Poles.=_ The end of a piece of steel which last touches
a N pole of a magnet, for example, becomes a S pole.
_=32. Our Compass=_ (No. 18). While the floating magnetic needle
described in Exp. 10, and shown in Fig. 4, does very well, it will
be found more convenient to use a compass whenever poles of pieces
of steel are to be tested. Fig. 5 shows merely the cover of the box
which serves as a base for the magnetic needle furnished. We shall
hereafter speak of this apparatus as _our compass_, O C. (See Apparatus
Book, Chap. VII, for various forms of home-made magnetic needles and
compasses.)
=33. Review; Magnetic Problems.= To be sure that you understand
and remember what was learned in Exp. 11, do these problems:
1. Using the S pole of the horseshoe magnet, magnetize a needle
so that its head will become a N pole. Test with floating cork,
as in Exp. 11.
2. Using the N pole of the horseshoe magnet, magnetize a needle
so that its head shall be a S pole. Test.
3. Magnetize two needles, one on the N and one on the S pole of
the horseshoe magnet, in such a way that the two points will
repel each other. Test.
If the student cannot do these little problems at once, and
test the results satisfactorily to himself, he should study the
previous experiments again before proceeding.
[Illustration: Fig. 5.]
[Illustration: Fig. 6.]
=EXPERIMENT 14. To find whether the poles of a magnet can be
reversed.=
_Apparatus._ Fig. 6. The horseshoe magnet, H M; a thin wire
nail, W N, 2 in. (5 cm.) long; a piece of stiff paper, cut as
shown, to hold W N; thread with which to suspend the paper;
compass, O C (No. 18).
=34. Directions.= (A) Magnetize W N so that its point shall be
a S pole. Test with O C to make sure that you are right.
(B) Swing W N in the paper (Fig. 6), then _slowly_ bring the S
pole of H M near its point. Note result.
(C) _Quickly_ bring the S pole of H M near the point. Is W N
still repelled? Has its S pole been reversed?
_=35. Discussion; Reversal of Poles.=_ The poles of weak magnets may
be easily reversed. This often occurs when the apparatus is mixed
together. It is always best, before beginning an experiment, to
remagnetize the pieces of steel which have already served as magnets.
The same may be shown by magnetizing a needle, rubbing it first in one
direction, and then in another upon the magnet, testing, in each case,
the poles produced.
=EXPERIMENT 15. To find whether we can make a magnet with two N
poles.=
_Apparatus._ The horseshoe magnet, H M; an unmagnetized
sewing-needle; compass, O C (No. 18).
=36. Note.= You have already learned that the polarity of a
weak magnet can be changed (Exp. 14). Can you think of any
method by which _two N poles_ can be made in one piece of steel?
=37. Directions.= (A) Place the needle upon H M, as in Fig. 7.
(B) Keeping the part, C, in contact with the N pole of H M, and
using the N pole of H M as a pivot, turn the needle end for end
so that its head will be in contact with the S pole of H M.
(C) Pull the needle straight from H M, being careful not to
slide it in either direction.
(D) Test the polarity of the ends with O C (Fig. 5), and save
it for the next experiment.
[Illustration: Fig. 7.]
[Illustration: Fig. 8.]
=EXPERIMENT 16. To study the bar magnet with two N poles.=
_Apparatus._ The strange magnet just made (Exp. 15); iron
filings; compass, O C (No. 18).
=38. Directions.= (A) Sprinkle filings over the whole length of
the needle and then raise it (Fig. 8).
(B) Break the needle at its center, and test, with O C, the
two new ends produced at that point. Remember that repulsion
is the test for polarity.
_=39. Discussion; Consequent Poles.=_ Iron filings cling to a magnet
where poles are located. In this case, two small magnets were made
in one piece of steel; they had a common S pole at the center. The
pointing power (§ 25) of such a magnet is very slight; would it have
_any_ pointing power if we could make the end poles of equal strength?
Intermediate poles, like those in the needle just discussed, are called
_consequent poles_. Practical uses are made of consequent poles in the
construction of motors and dynamos.
=EXPERIMENT 17. To study consequent poles.=
_Apparatus._ An unmagnetized sewing-needle; horseshoe magnet, H
M (No. 16); iron filings (No. 17); compass (No. 18).
=40. Directions.= (A) Let _w_, _x_, _y_, and _z_ stand for four
places along the body of the needle, _w_ being at its point and
_z_ at its head.
(B) Touch _w_ with the N pole of H M, _x_ with the S pole, _y_
with the N pole, and _z_ with the S pole. Do not slide H M
along on the needle, just _touch_ the needle as directed.
(C) Cover the needle with filings, then lift it.
=EXPERIMENT 18. To study the theory of magnetism.=
_Apparatus._ A thin bar magnet, B M (No. 21); iron filings; a
sheet of paper. Fig. 9 shows simply the edge of B M and the
paper. B M should be magnetized as directed in Exp. 9.
[Illustration: Fig. 9.]
=41. Directions.= (A) Sprinkle some iron filings upon a sheet
of paper.
(B) Bring one pole of B M in contact with the filings (Fig. 9),
and lightly sweep it through them several times, always in the
same direction. Are the filings _simply_ pushed about?
(C) Do the same with a stick, and compare the result with that
produced with B M.
_=42. Theory of Magnetism; Magnetic Saturation.=_ This bringing into
line the particles of iron indicates that each particle became a
magnet. This experiment should aid in understanding what is thought to
take place when steel is magnetized. The pile of filings represents the
body to be magnetized, and each little filing stands for a particle of
that body. A bar of steel is composed of extremely small particles,
called _molecules_. They are very close together and do not move from
place to place as easily as the pieces of filings. A magnet, however,
when properly rubbed upon the steel, seems to have power to make the
molecules point in the same direction. This produces an effect upon the
whole bar.
Each molecule of the steel is supposed to be a magnet. When these
little magnets pull together, the whole bar becomes a strong magnet.
When a magnet is jarred, and the little magnetized molecules are mixed
again, they pull in all sorts of directions upon each other. This
lessens the attraction for outside bodies.
Steel is said to be _saturated_, when it contains as much magnetism as
possible. A piece of steel becomes slightly longer when magnetized.
It is thought, by many, that there is a current of electricity around
each molecule, making a little magnet of it. (See electro-magnets.)
=EXPERIMENT 19. To find whether soft iron will permanently
retain magnetism.=
_Apparatus._ A piece of soft iron wire, 3 or 4 in. (7.5 to 10
cm.) long (No. 4); the horseshoe magnet, H M; iron filings;
flat cork, F C (No. 2), and the dish of water used in Exp. 10
(Fig. 4).
=43. Directions.= (A) Magnetize the wire (Exp. 9). Notice that
the wire clings strongly to H M.
(B) Test the lifting power of the little wire magnet by seeing
about how many iron filings its poles will raise.
(C) Test the pointing power (§ 25) of the wire by floating it
on F C (Fig. 4).
(D) Holding one end of the wire in the hand, thoroughly jar it
by striking the other end several times against a hard surface.
(E) Test the lifting and pointing powers, as in B and C.
_=44. Retentivity or Coercive Force; Residual Magnetism.=_ Soft iron
loses _most_ of its magnetism when simply removed beyond the action of
a magnet. We say that it does not retain magnetism; that is, it has
very little _retentivity or coercive force_. This is an important fact,
the action of many electric machines and instruments depending upon it.
A slight amount of magnetism remains, however, in the softest iron,
after removing it from a magnet. This is called _residual magnetism_. A
piece of iron may show poles, when tested with the compass, although it
may have almost no pointing power.
=EXPERIMENT 20. To test the retentivity of soft steel.=
_Apparatus._ A wire nail, W N (No. 19); horseshoe magnet, H M;
iron filings; flat cork, F C; the dish of water (Exp. 10, Fig.
4).
=45. Directions.= (A) With H M magnetize the nail; this is made
of soft steel.
(B) Test the lifting and pointing powers of W N (Exp. 19).
(C) Strike W N several times with a hammer to jar it.
(D) Again test its lifting and pointing powers.
_=46. Discussion.=_ Soft steel has a greater retentivity than soft
iron. It contains less carbon than cast or tool steel, and is called
mild steel or machinery steel. You do not want soft steel for permanent
magnets.
=EXPERIMENT 21. To test the retentivity of hard steel.=
_Apparatus._ A hard steel sewing-needle (No. 1); other articles
used in Exp. 20.
=47. Directions.= (A) Magnetize the needle with H M.
(B) Test its lifting and pointing powers (Exp. 19).
(C) Hammer the needle and test again as in (B).
=EXPERIMENT 22. To test the effect of heat upon a magnet.=
_Apparatus._ A magnetized sewing-needle; the candle, cork,
etc., of Exp. 2. (See Fig. 1.)
=48. Directions.= (A) Test the needle for magnetism.
(B) Stick the needle into the cork (Fig. 1), and heat it until
it is red-hot.
(C) Test the needle again for magnetism.
(D) See if you can again magnetize the needle.
_=49. Discussion.=_ Heating a body is supposed to thoroughly stir up
its molecules. Jarring or twisting a magnet tends to weaken it. (See
Exp. 19.)
The molecules of steel do not move about or change their relative
positions as readily as those of soft iron. When the molecules of
hard steel are once arranged, by magnetizing them, for example, they
strongly resist any outside influences which tend to mix them up again.
A magnet does not attract a piece of red-hot iron. The particles of the
hot iron are supposed to vibrate too rapidly to be brought into line;
that is, the iron cannot become polarized by induction. (See Exp. 24.)
=EXPERIMENT 23. To test the effect of breaking a magnet.=
_Apparatus._ A magnetized sewing-needle; iron filings; compass,
O C (No. 18).
[Illustration: Fig. 10.]
=50. Directions.= (A) Break the little bar magnet (needle), and
test the two new ends produced for magnetism, with the iron
filings. (Fig. 10).
(B) Touch the two new poles together to see whether they are
like or unlike.
(C) Test the nature of the poles with O C (Fig. 5)
(D) Break one of the halves and test its parts.
_=51. Discussion.=_ The above results agree with the theory that each
molecule is a magnet (Exp. 18). No matter into how many pieces a magnet
is broken, each part becomes a magnet. (Fig. 10). This shows that those
molecules near the equator of the magnet really have magnetism. Their
energy, however, is all used upon the adjoining molecules; hence no
external bodies are attracted at that point.
CHAPTER III.
INDUCED MAGNETISM.
[Illustration: Fig. 11.]
=EXPERIMENT 24. To find whether we can magnetize a piece of
iron without touching it with a magnet.=
_Apparatus._ Horseshoe magnet, H M; iron filings, I F (Fig. 11).
=52. Directions.= (A) Hold the armature of the magnet in a
vertical position (Fig. 11), its lower end being directly in a
little pile of iron filings.
(B) Bring the N pole of H M near the upper end of A, but do not
let them touch each other.
(C) Keeping A and the pole of H M the same distance apart, lift
them. Do any filings cling to A?
(D) Without moving or jarring A, take H M away from it and note
result upon the filings.
_=53. Temporary Magnetism; Induced Magnetism.=_ The armature, A, was
induced to become a magnet without even touching H M. Its magnetism was
_temporary_, however, as the filings dropped as soon as the _inductive
action_ of H M was removed. A small amount of residual magnetism (44)
remained in A. Soft iron is exceedingly valuable, because it has very
little retentivity (44), and because it can be easily _magnetized
by induction_. The armature was made of soft iron. It had _induced
magnetism_. It was a _temporary magnet_.
=EXPERIMENT 25. To find whether a piece of steel can be
permanently magnetized by induction.=
_Apparatus._ An unmagnetized sewing-needle; horseshoe magnet, H
M; iron filings; sheet of stiff paper.
=54. Directions.= (A) Test the needle for magnetism.
(B) Place the unmagnetized needle upon the paper, then move
H M about immediately under it, so that the needle will be
attracted.
(C) Test the needle again for permanent magnetism.
[Illustration: Fig. 12.]
=EXPERIMENT 26. To study the inductive action of a magnet upon
a piece of soft iron.=
_Apparatus._ Horseshoe magnet, H M; iron filings, I F; a piece
of soft iron wire about an inch long, I W (Fig. 12), placed
upon the N pole of H M; compass, O C (No. 18), (§ 32).
=55 Directions.= (A) Test the lower end of I W for magnetism
with I F.
(B) Leaving I W upon the N pole of H M, test the pole at the
lower end of I W with O C, to determine whether it is N or S.
(C) Jar I W (Exp. 19), then place it upon the S pole of H M,
and again test the polarity of the lower end.
_=56. Polarization; Pole Pieces.=_ The wire, I W (Fig. 12), was acted
upon by induction (Exp. 24) and behaved like a magnet. Poles were
produced in it, so we say that the wire was _polarized_. Pieces of
iron, placed upon the poles of a magnet, are called _pole pieces_. It
should be noted that the lower end of the wire has a pole _like_ the
pole of H M, to which it is attached.
=EXPERIMENTS 27-30. To study pole pieces.=
_Apparatus for Experiments 27-30._ Horseshoe magnet, H M; soft
iron wires; iron filings, I F.
=57. Directions.= (A) Suspend two wires, each about an inch
long (Fig. 13) from one pole of H M. Do their lower ends
attract or repel each other?
[Illustration: Fig. 13.]
[Illustration: Fig. 14.]
=EXPERIMENT 28.=
=58. Directions.= (A) Place the two wires just used so that one
shall cling to the N pole of H M, and the other to the S pole
of H M (Fig. 14).
(B) Bring the lower ends of the wires near each other. Do they
attract or repel each other?
=EXPERIMENT 29.=
=59. Directions.= (A) Bend a 2-inch iron wire, as in Fig. 15,
and place it upon the poles of H M.
(B) See if its central part, marked X, will strongly attract
filings.
[Illustration: Fig. 15.]
[Illustration: Fig. 16.]
=EXPERIMENT 30.=
=60. Directions.= (A) Bend the wire just used a little more,
and place its ends upon _one_ pole of H M (Fig. 16).
(B) See if the iron filings and small wires will cling to its
central part.
CHAPTER IV.
THE MAGNETIC FIELD.
=EXPERIMENT 31. To study the space around a magnet, in which
pieces of iron become temporary magnets by induction.=
_Apparatus._ A bar magnet, B M (No. 21); a compass (No. 18); a
sheet of stiff paper about 1 ft. (30 cm.) square, with a center
line, C L, drawn parallel to one of its sides (Fig. 16-1/2),
and with another line, E W, drawn perpendicular to C L. (See
Apparatus Book, Chap. VI., for various ways of making home-made
permanent magnets.)
=61. Directions.= (A) Lay the paper upon the table, and place
the compass over the center of the line, C L, previously drawn.
(B) Place the eye directly over the compass-needle, then turn
the paper until the line is N and S; that is, until the line
is parallel to the length of the needle. Pin the paper to the
table to hold its center line N and S.
(C) Place B M upon the paper, as shown (Fig. 16-1/2), its N
pole to the north, and its center at the cross line, E W.
[Illustration: Fig. 16-1/2.]
(D) Slowly move the compass entirely around and near B M, and
note the various positions taken by the needle. Note especially
the way in which its N pole points. This is to get a general
idea of the action of the needle.
(E) Place the compass in the position marked 1, which is on E
W, about 1 in. from the line, C L. Press the wooden support
down firmly upon the paper to show, by the dent made in the
paper by the pin-head, the exact place on the paper that is
under the center of the compass-needle. Before removing the
compass from this position, look down upon it again, and make a
dot on the paper with a pencil directly under each end of the
needle. Remove the compass, and draw a line through the dent
and the two dots just made. This will show a plan of the exact
position of the needle.
(F) Repeat this for the various points marked 2, 4, 6 in. from
C L, always marking on the plan the position of the N pole of
the needle. Do the same with the other points marked on Fig.
16-1/2 by dots, and study the resulting diagram.
_=62. Discussion; The Magnetic Field.=_ The compass-needle was
decidedly affected all around B M (Fig. 17), showing that induction
can take place in a considerable space around a magnet; this space
is called the _magnetic field_ of the magnet. Let us consider _one_
position taken by the compass-needle in the field of B M (Fig. 17),
as, for example, the one in which the needle has been made black. The
S pole of the black needle is attracted by the N pole of B M, and is
repelled by the S pole of B M. The N pole of the compass-needle is
attracted by the S pole of B M, and is repelled by B M's N pole. The
position which it takes, therefore, is due to the action of these 4
forces, together with its tendency to point N and S.
[Illustration: Fig. 17.]
Every magnet has a certain magnetic field, with its lines of force
passing through the surrounding air in certain definite positions. As
soon, however, as a piece of iron or another magnet is brought within
the field, the original position of the lines of force is changed. This
has to be considered in the construction of electrical machinery.
=EXPERIMENT 32. To study the magnetic field of a bar magnet.=
_Apparatus._ A sheet of stiff paper; iron filings, I F; bar
magnet, B M (No. 21); a sifter for the filings (No. 24); (See
Apparatus Book, §48, 49, 50, for home-made sifters.)
=63. Directions.= (A) Place B M upon the table, and lay the
paper over it.
(B) With the sifter sprinkle some filings upon the paper
directly over B M, then tap the paper gently, to assist the
particles to take final positions. Study the results.
_=64. Magnetic Figures; Lines of Magnetic Force.=_ The filings clearly
indicated the extent and nature of the magnetic field of B M. You
should notice how the filings radiate from the poles, and how they
form curves from one pole to the other. They make upon the paper a
_magnetic figure_. Each particle of the filings becomes a little
magnet, by induction (Exp. 24), and takes a position which depends upon
attractions and repulsions, as discussed in Exp. 31.
Magnetism seems to reach out in lines from the poles of a magnet. The
position and direction of some of the lines are shown by the lines of
filings. They are very distinct near the poles, and are considered, for
convenience, to start from the N pole of a magnet, where they separate.
They then pass through the air on all sides of the magnet, and finally
enter it again at the S pole. These lines are called _lines of force_
or _lines of magnetic induction_.
The poles must not be considered mere points at the ends of a magnet.
As shown by magnetic figures, the lines of magnetic induction flow from
a considerable portion of the magnet's ends.
=EXPERIMENTS 33-37. To study the magnetic fields of various
combinations of bar magnets.=
_Apparatus for Exps. 33-37._ Two bar magnets, B M (Nos. 21,
22); an iron ring, I R (No. 23); iron filings, I F; a sheet of
stiff paper; the sifter (No. 24).
=65. Note.= The student will find it very helpful to make
the magnetic figures of the combinations given. Thoroughly
magnetize the bar magnets upon an electro-magnet, or upon a
strong horseshoe magnet, and mark their N poles in some way.
The N poles may be marked by sticking a small piece of paper to
them.
=66. Directions.= (A) Arrange the two magnets, B M, as in
Fig. 18, with their unlike poles about an inch apart. (The
dotted circle indicates the iron ring to be used in the _next_
experiment. About a quarter, only, of the magnets are shown.)
(B) Place the paper over the magnets, and sift filings upon it
immediately over the unlike poles. Note particularly the lines
of filings between N and S.
(C) Make a sketch of the result. (See experiments with
electromagnets, and the illustrations of magnetic figures with
them.)
=EXPERIMENT 34.=
=67. Directions.= (A) Leaving the opposite poles an inch apart,
as in Exp. 33, place the iron ring, I R (No. 23), between them
(Fig. 18, dotted circles).
(B) Place the paper over it all, and sprinkle filings upon it
to get the magnetic figure.
(C) Make a sketch of the resulting figure, and compare it with
the figure made in Exp. 33. Why do the lines of force appear
indistinct in the center of the ring and around it? (See §74.)
[Illustration: Fig. 18.]
[Illustration: Fig. 19.]
=EXPERIMENT 35.=
=68. Directions.= (A) Arrange the two bar magnets, as in Exp.
33, but with their two N poles an inch apart.
(B) Make the magnetic figure of the combination. Do the lines
of force flow from one N pole directly to the N pole of the
other? Do the particles of filings reaching out from one B M
attract or repel those from the other B M?
=EXPERIMENT 36.=
=69. Directions.= (A) Place the two bar magnets side by side,
so that their unlike poles shall be arranged as in Fig. 19.
(B) Make the magnetic figure.
=EXPERIMENT 37.=
=70. Directions.= (A) Turn one B M end for end, so that their
like poles shall be near each other, but otherwise arranged as
in Fig. 19.
(B) Make and study the magnetic figure.
=EXPERIMENTS 38-39. To study the lifting power of combinations
of bar magnets.=
_Apparatus for Exps. 38-39._ Two bar magnets, B M (No. 21, 22),
of about equal strength; iron filings, I F.
=71. Directions.= (A) Find out about how many filings you can
lift with the N pole of one magnet.
(B) Place the two magnets together (Fig. 20), their _like_
poles being in contact; then see whether the two N poles will
lift more or less filings than one pole.
[Illustration: Fig. 20.]
=EXPERIMENT 39.=
=72. Directions.= (A) Remove all filings from the two magnets
just used, and hold them tightly together (Fig. 20), with their
_unlike_ poles in contact.
(B) Compare the amount of filings you can lift at one end of
this combination with that lifted in Exp. 38 (A) and (B).
_=73. Discussion; Compound Magnets.=_ Many lines of force pass into
the air from two like poles. Such a combination is called a _compound
magnet_. A piece of thin steel can be magnetized more strongly in
proportion to its weight than a thick piece, because the magnetism
does not seem to penetrate beyond a certain distance into the steel.
Thin steel may be magnetized practically through and through. A thick
magnet has but a crust of magnetized molecules; in fact, a thick magnet
may be greatly weakened by eating the outside crust away with acid.
By riveting several thin bar or horseshoe magnets together, thick
permanent magnets of considerable strength are made.
_=74.=_ Lines of force, in passing from the N to the S pole of a magnet,
meet a resistance in the air, which does not carry or conduct them as
easily as iron or steel. In the arrangement of Exp. 39 the lines of
force are not obliged to push their way through the air, as each magnet
serves as a return conductor for the lines of force of the other.
Either magnet may be considered an armature for the other.
To show in another way that few lines of force pass into the air,
the student may lay the above combination upon the table and make a
magnetic figure. (See Apparatus Book, p. 38, for method of making
home-made compound magnets.)
In the case where a ring was placed between the poles of two bar
magnets (Exp. 34), the lines of force from the N pole jumped across the
first air-space. They then disappeared in the body of the ring, until
they were obliged to jump across the second air-space, to get to the S
pole. The weakness of the field in the central space was clearly shown
by the filings. There were no stray lines of force passing through the
air, because it was easier for them to go through the iron ring. This
will be discussed again under "Dynamos and Motors." (See also § 78.)
=EXPERIMENTS 40-42. To study the magnetic field of the
horseshoe magnet.=
_Apparatus for Exps. 40-42._ Horseshoe magnet, H M; iron
filings, I F; sheet of stiff paper.
=75. Directions.= (A) Place H M, with its armature removed,
flat upon the table, and cover it with the paper; then make the
magnetic figure. (Exp. 32.)
(B) Compare the number of well-defined curves at the poles with
the number at the equator.
=EXPERIMENT 41.=
=76. Directions.= (A) Make the magnetic figure of H M with its
armature in place.
(B) Is the attraction for outside bodies increased or decreased
by placing the armature on H M?
=EXPERIMENT 42.=
=77. Directions.= (A) Lay H M flat upon the table, and place
one or two matches between its poles and the armature; cover
with paper as before, and make the magnetic figure. Do lines of
force still pass through the armature?
_=78. Discussion; Resistance to lines of Force.=_ It is evident, from
the last 3 experiments, that lines of force will pass through iron
whenever possible, on their way from the N to the S pole of a magnet.
When the armature of a horseshoe magnet is in place, most of the lines
of magnetic induction crowd together and pass through it rather than
push their way through the air. Air is not a good conductor of lines of
force; and the magnet has to do work to overcome the resistance of the
air, when the armature is removed, in order to complete the magnetic
circuit. This work causes a magnet to become gradually weaker. The soft
iron armature is an excellent conductor of lines of force; it completes
the magnetic circuit so perfectly that very little work is left for the
magnet to do.
=EXPERIMENT 43. To show that lines of force are on all sides of
a magnet.=
_Apparatus._ Our compass, O C (No. 18); horseshoe magnet, H
M; glass tumbler, G T; sheet of stiff paper; iron filings, I
F. Arrange as in Fig. 21. H M may be supported in a vertical
position by placing paper, or a handkerchief, under it. The
poles should just touch the stiff paper placed over the tumbler.
[Illustration: Fig. 21.]
=79. Directions.= (A) Sprinkle iron filings upon the paper, and
study the resulting magnetic figure.
(B) Place O C upon the paper in different positions. Does the
magnetic needle always come to rest about parallel to the lines
of filings?
_=80. Discussion.=_ The student should keep in mind the fact that the
filings in the magnetic figure show the approximate extent and form of
the magnetic field simply in one plane. If the paper were held in some
other position near the magnet (in a tilted position, for example,)
the lines of filings would not be the same as those produced in Exp.
40-42. The lines of force come out of every side of the N pole. When
a magnetic needle is placed in any magnetic field, its N pole points
in the direction in which the lines of force are passing; that is, it
points towards the S pole of the magnet producing the field.
=EXPERIMENT 44. To study a horseshoe magnet with movable poles.=
_Apparatus._ A narrow strip of spring steel, S S (No. 25); iron
filings, I F.
=81. Directions.= (A) Magnetize the spring steel, S S.
(B) Bend S S until its poles are about 1/4 in. apart, then
using it as a horseshoe magnet, and keeping its poles the same
distance apart, see about how many filings you can lift.
(C) Clean the poles of S S, press them tightly together, then
again test its lifting power with filings.
[Illustration: Fig. 22.]
_=82. Discussion; Advantages of Horseshoe Magnets.=_ When the opposite
poles of the flexible magnet are pressed together, the lines of force
do not have to pass through the air; there is very little attraction
for outside bodies. The same effect is produced with the armature (Exp.
41). A horseshoe magnet has a strong attraction for its armature,
because it has a _double power to induce and to attract_. Suppose the N
pole of a bar magnet, B M (Fig. 22), be placed near one end of a piece
of iron, as, for example, the armature, A. A will become a temporary
magnet by induction (Exp. 24). The S pole of A, polarized by induction,
will be attracted by B M, while its N pole will be repelled by B M; so,
you see, that a bar magnet does not pull to advantage.
CHAPTER V.
TERRESTRIAL MAGNETISM.
_=83. The Magnetism of the Earth.=_ The student must have guessed,
before this, that the earth acts like a magnet. It causes the magnetic
needle to take a certain position at every place upon its surface, and
this position depends upon the earth's attractions and repulsions for
it. The earth has lines of force which flow from its N magnetic pole,
and these lines, before they can get to the earth's S magnetic pole,
must spread out through the air on all sides of the earth.
As the magnetic needle points to the earth's N magnetic pole (which
is more than 1,000 miles from its _real_ N pole), it is evident that
the compass-needle does not show the _true_ north for all places upon
the earth's surface. In fact, the N pole of the needle may point E, W,
or even S. This effect would be seen by carrying a compass around the
earth's N magnetic pole.
[Illustration: Fig. 23.]
_=84. Declination.=_ For convenience, we shall represent the true N
and S, at the place where you are experimenting, by the full line,
N S, in Fig. 23. The dotted line shows the direction taken by the
compass-needle. The angle, A, between them, is called the _angle of
variation_ or the _declination_. This angle is not the same for all
places; and, in fact, it changes slowly at any given place; so it
becomes necessary to construct _magnetic maps_ for the use of mariners
and others.
=EXPERIMENT 45. To study the lines of force above and below a
bar magnet placed horizontally.=
_Apparatus._ A bar magnet, B M (No. 21); compass, O C (No. 18).
=85. Directions.= (A) Lay B M upon the table and place O C upon
its center. Note the position of the compass-needle.
(B) Slide O C along from one end of B M to the other, and study
the effect upon its needle. Do lines of force curve _over_ B M
as well as around its sides, as shown in Exp. 31?
(C) Place O C upon the table. Hold B M horizontally above O C,
and move O C back and forth under B M. Does the needle remain
horizontal, or does it show that lines of force pass _under_ B
M on their way from its N to its S pole?
[Illustration: Fig. 24.]
_=86. The Dip or Inclination of the Magnetic Needle.=_ The needle
is said to dip when it takes positions like those in Fig. 24.
Compass-needles should be horizontal, when properly balanced, and
entirely free from all effects other than those of the earth. The
excessive dip shown (Fig. 24) is due, of course, to the efforts of the
magnetic needle to place itself in the direction in which the lines of
force of B M pass.
=EXPERIMENT 46. To study the dip or inclination of the magnetic
needle, due to the action of the earth.=
_Apparatus._ Fig. 25. Our compass, O C (No. 18); horseshoe
magnet, H M (No. 16); piece of paper.
=87. Directions.= (A) Place O C upon the table, and mark upon
a piece of paper the height of the N pole of its needle above
the table. (Fig. 25.) The paper should be held in a vertical
position, and near the pole.
[Illustration: Fig. 25.]
(B) With H M reverse the poles of the compass-needle (Exp. 13),
so that its former N pole shall become a S pole.
(C) Place the needle upon its pivot again, and mark upon the
paper, as before, the height of its new N pole above the table.
Does the needle remain horizontal?
(D) Remagnetize the needle, and reverse its poles so that it
will again balance.
[Illustration: Fig. 26.]
_=88. Discussion; Balancing Magnetic Needles.=_ If a piece of
unmagnetized steel be balanced and then magnetized, it will no longer
remain horizontal; it will dip. Try this. Compass-needles are balanced
after they are magnetized. Can you now see why the needle did not
remain horizontal after its poles were changed? A piece of steel first
balanced and then magnetized, has to have its S pole slightly weighted,
as suggested by the line at S (Fig. 26 x), to make it horizontal.
The magnetic needle does not tend to dip at the earth's equator,
because the lines of force of the earth are nearly horizontal at the
equator. As we pass toward the north or south on the earth, the lines
of force slant more and more as they come from or enter the earth's
magnetic poles. What position would the needle take if we should hold
it directly over the earth's N magnetic pole? Fig. 24 shows what the
needle does when held near the poles of a bar magnet.
=EXPERIMENTS 47-48. To study the inductive influence of the
earth.=
_Apparatus for Exps. 47-48._ Compass, O C, (No. 18); an iron
stove poker, or other rod of iron; a hammer. (The iron and
hammer are not furnished.)
=89. Note.= You have seen (Exp. 24), that iron becomes
magnetized by induction when placed near a magnet. As the earth
acts like a huge magnet, having poles, lines of force, etc.,
will it magnetize pieces of iron which are in the air or upon
its surface?
=90. Directions.= (A) Test the poker for poles with O C,
remembering that _repulsion_ is necessary to prove that it is
polarized. If the poker has very weak poles, proceed; but if
it shows some strength, hold it in an east and west direction,
and hit it several sharp blows on the end with the hammer. Test
for polarity again.
(B) With one hand hold the poker in the N and S line, give it
a dip toward the north, and strike it several times with the
hammer to thoroughly stir up its molecules.
(C) Test again for poles with O C, and note especially whether
the lower end (of the poker) became a N or a S pole.
=EXPERIMENT 48.=
=91. Directions.= (A) Turn the poker end for end (See Exp. 47);
repeat the striking, and test again the pole produced at the
lower and north end of it.
(B) Now hold the poker horizontally in the east and west line,
and pound it.
(C) Test for poles. Has this strengthened or weakened the poker
magnet?
_=92. Discussion.=_ Dipping the poker places it nearly in the same
direction as that taken by the earth's lines of force. The magnetic
influence of the earth acts to advantage upon the poker, by induction,
only when the poker is properly held.
It no doubt occurs to the student that the end of a magnetic needle
which points to the north is really opposite in nature to the north
magnetic pole of the earth. The N pole of a needle, then, must be in
reality a S pole to be attracted by the earth's N pole. It has been
agreed, for convenience, to call the N-seeking pole of a magnet its N
pole.
_=93. Natural Magnets.=_ Nearly all pieces of iron become more or less
magnetized by the inductive action of the earth's magnetism. Your poker
was slightly magnetized at the start, perhaps, from standing in a
dipping position.
Induction takes place along lines of force. In northern latitudes the
earth's lines of force have a dip to the north. You should now see why
the greatest effect was produced upon the poker when it, also, was made
to dip.
Parts of machinery, steel frames of bridges and buildings, tools in the
shop, and even certain iron ores, become polarized by this inductive
action. These might all be called natural magnets. Magnetic iron ore,
called lodestone, is referred to, however, when speaking of _natural
magnets_. Lodestone was used thousands of years ago to indicate N and
S, and it was discovered, later, that it could impart its power to
pieces of steel when the two were rubbed together.
=EXPERIMENT 49. To test the effect of twisting a wire held
north and south in the earth's magnetic field.=
_Apparatus._ Compass, O C (No. 18); a piece of soft iron wire,
6 in. (15 cm.) long (No. 15). Bend up about an inch of the wire
at each end so that it may be firmly held when twisting it.
=Note.= You have seen that we can _pound_ magnetism into or out
of a piece of iron at will. Can we _twist_ it into a wire and
out again without the use of magnets?
=94. Directions.= (A) Test the wire for poles with O C.
(B) Hold the wire in a N and S direction, dipping it at the
same time, as directed in Exp. 47 for the poker, and twist it
back and forth.
(C) Test again for poles with O C. As the poles of the wire may
be very weak, bring them _slowly_ toward the compass-needle
(see Exp. 14), and note the _first_ motions produced upon the
needle.
(D) Hold the wire horizontally east and west, twist and test
again. Has its magnetism become weaker or stronger than before?
=EXPERIMENT 50. To test for magnetism in bars of iron, tools,
etc.=
_Apparatus._ Steel drills; files; chisels; bars or rods of
iron that have been standing in an upright position; stove-lid
lifters; stove pokers, etc., etc.; a compass.
=95. Directions.= (A) With the compass test the ends of the
above for magnetism, and note which ends are S.
Notes.
STATIC ELECTRICITY
PART II.--STATIC ELECTRICITY
CHAPTER VI.
ELECTRIFICATION.
_=100. Some Varieties of Electricity.=_ _Static electricity_ does not
seem to "flow in currents" as readily as some other varieties; its
tendency is to stand still, hence the name, static. The simplest way to
produce it is by friction. _Thermo electricity_ is produced by changes
in temperature. When certain combinations of metals become hotter or
colder, a current is produced. _Voltaic_ or _Galvanic electricity_ is
produced by chemical action. Batteries give this variety. _Induced
electricity_ is produced by other currents, and by combinations of
magnets and moving coils of wire, as in the dynamo. This is, by far,
the most important variety of electricity, and the dynamo is the most
important producer of it.
Each of the above varieties of electricity will be studied
experimentally with simple apparatus.
=EXPERIMENTS 51-52.= To study electrification by friction.
_Apparatus._ Ebonite sheet, E S (No. 26); flannel cloth, F C
(No. 30). See what is said in preface about static electricity.
=101. Directions.= (A) Examine E S. Note that its surface is
not smooth, like that of ordinary hard-rubber combs. Can you
think of any reason for this?
(B) Hold its flat surface near your face, then near the back of
your hand. Do you feel anything unusual?
(C) Lay E S upon a flat board, or uncovered wooden table, and
slide it about. Can you easily pick it up?
(D) Place E S flat upon the table again; keep it from sliding
about with your left hand, and rub it _vigorously_ for a
_minute_ with F C. Does E S act exactly as it did before in (B)
and (C)?
(E) Repeat the experiment in a dark room.
(F) Thoroughly electrify E S, and see if it will cling to the
wall strongly enough to support its own weight.
_=102. Discussion; Electrified and Neutral Bodies.=_ The ebonite
sheet became _electrified_ or _charged_; and as the _electrification_
was produced by friction, we may say that the action of the ebonite
indicated the presence of _frictional electricity_. No one can tell
_just_ why the ebonite acted so queerly, but we can learn a great
deal by experimenting. Bodies which are not charged are said to be
_neutral_. The table, chairs, earth, etc., are neutral. We may consider
that a neutral body has been _discharged_.
_=103. Force; Resistance; Work; Potential Energy; Electrification.=_ It
takes _force_ to raise water into a tank placed on the roof. In raising
the water, _work_ has to be done, and _we_ have to do the work; but
when we once have the water in the tank we have accomplished something.
The water has _potential energy_; that is, on account of its high
_position_, we can make it do some work by simply turning a stop-cock
so that the water can run out and turn a water-wheel, for example.
It takes _force_ to vigorously rub a piece of ebonite with a flannel
cloth, for _resistance_ has to be _overcome_; that is, _work_ has to be
done. Several things are accomplished by this work; heat is produced,
for we can _feel_ that the ebonite gets warm; we can _hear_ sounds and
_see_ sparks. The simple muscular exertion on our part has been changed
to heat, light, and sound. The most wonderful part of it all, however,
is that we have electrified or charged the ebonite. _We_ did the work
at first, and now the ebonite has the power to do something, as you
will soon see. _Electrification_ is, then, a sort of potential energy.
_=104. Heat and Electrification.=_ We say that heat passes to or from a
body to make it hot or cold. Heat _produces_ the sensation of warmth,
but heat isn't warmth. We can force a cold body to become hot; in
other words, we can get it into a hot condition in various ways, such
as rubbing it, hammering it, or by placing it near or in contact with
another hot body.
Electrification is, also, a condition or state into which we can force
a body; but electrification isn't electricity. We know whether a body
is hot or cold by its effects upon us, upon thermometers, and upon
other bodies. We can tell, also, whether a body is electrified or not
by the way it acts, and, in certain cases, by the sound, heat, and
light which accompany the electrification.
Do not get the idea that an electrified body is covered with a layer of
electricity just as a board is covered with a layer of paint.
[Illustration: Fig. 28.]
=EXPERIMENT 52.=
=105. Directions.= Repeat Exp. 51, but in place of the ebonite,
use hot tissue-paper, hot brown paper, hot newspaper, or a hot
silk handkerchief. Rub your hand vigorously over them. Do these
become charged?
=EXPERIMENTS 53-54. To study electrical attractions.=
_Apparatus._ The ebonite sheet, E S (No. 26); flannel cloth,
F C (No. 30); small pieces of dry tissue-paper, T P (No. 31);
thread (No. 32).
=106. Directions.= (A) Thoroughly electrify E S as before, then
lift and hold it in the air. (Fig. 28.)
(B) See what the paper and thread will do when held loosely
near E S.
_=107. Discussion.=_ Exp. 53 shows that _an electrified body attracts
neutral ones_. This much was known about electricity over 2,000 years
ago. They didn't have ebonite then, but some of the educated men of
Greece knew that amber would attract light bodies after being rubbed.
The Greek word for amber is _elektron_, and from this has been made the
word _electricity_.
=EXPERIMENT 54.=
=108. Directions.= Charge a sheet of hot paper by friction;
lift it, by its opposite ends, and lower it over small pieces
of tissue-paper placed on the table. What happens to the little
pieces?
=EXPERIMENT 55. To study mutual attractions.=
_Apparatus._ The support and its attachments (See § 109);
support wire, S W (No. 36); silk thread, S T (No. 33), or a
rubber band, R B (No. 45); ebonite rod, E R (No. 28); flannel
cloth, F C (No. 30); wire swing, W S (No. 37).
Tie one end of S T to W S, Fig. 29; tie the other end of S T
to S W; adjust W S by bending, if necessary, so that it will
securely hold E R. It will be found convenient to use a rubber
band instead of S T; if you do, let W S straddle one end of R B
(Fig. 33), and hang the other end of R B upon S W.
=109. The support= consists of a support base (S B, Fig. 56), a
support rod (S R, Fig. 56), and a support wire (S W, Fig. 29).
There is a small hole in one end of S R to receive the wire, S
W, and a large hole in the other end to take the short ebonite
which holds the insulating table (Fig. 32). A little paper
should be wound around the lower end of S R, so that it will
stand solidly in the spool which forms a part of the base.
=110. Directions.= (A) Electrify E R with F C, and place E R in
the swing, W S (Fig. 29).
[Illustration: Fig. 29.]
(B) Bring your finger near one side of the rubbed end of E R,
then near the unrubbed end, and compare the results.
=111. Mutual Attractions.= _A neutral body_, like the hand, for
example, _attracts electrified ones_. From Exp. 53, 54, 55, it is
seen that the attraction between a neutral and an electrified body is
mutual; each attracts the other.
=EXPERIMENT 56. To study electrical repulsions.=
_Apparatus._ Same as for Exp. 55; ebonite sheet, E S (No. 26).
=112. Directions.= (A) Charge E R, and place it in W S, Fig. 29.
(B) Charge E S, and bring it slowly near one side of the
charged end of E R.
=EXPERIMENT 57. To study electrical repulsions.=
_Apparatus._ A sheet of tissue-paper, T P (No. 31); shears or a
knife. Cut T P, as in Fig. 30. Each leg should be about 1/4 in.
wide and 3 or 4 in. long.
=113. Directions.= (A) Heat the paper, then place it flat upon
the table and electrify it by rubbing it with your hand. You
must rub away from the uncut part, or you will break the legs.
(B) Raise T P, holding it by the uncut part. Note the action of
legs, and make a sketch of them.
[Illustration: Fig. 30.]
[Illustration: Fig. 31.]
=EXPERIMENT 58. To study electrical repulsions.=
_Apparatus._ Ebonite rod, E R (No. 28); a carbon electroscope,
C E, Fig. 31 (see § 114); the support complete (see § 109);
small piece of damp tissue-paper.
_=114. The Carbon Electroscope.=_ Light an ordinary match, and let
it burn until it is charred through and through. The black substance
remaining is _carbon_. This is very light; it has, also, another
important property which you will soon understand. Tie a small piece
of the carbon to one end of a dry _silk_ thread, and fasten the other
end of the thread to the support wire, S W, which is fastened to the
support (Fig. 31). We shall call this piece of apparatus the _carbon
e-lec-tro-scope_. (See Electroscopes, Chapter XVIII., Apparatus Book.)
=115. Directions.= (A) Electrify E R, then hold it near the
carbon of the electroscope.
(B) Bring the charged rod near little pieces of _damp_
tissue-paper.
_=116. Discussion of Experiments 56, 57, 58.=_ In 56 the two pieces
of ebonite were made of the same material, and both were rubbed with
flannel. They must have been similarly electrified. In 57, different
parts of the same piece of paper were similarly electrified. In 58,
the little piece of carbon took some of the electrification from the
charged rod, just as it would take molasses from your finger should
your sticky finger touch it. The electrification on the carbon must
have been of the same kind as that on the rod. The carbon was _charged
by contact_. We learn, then, that _two bodies repel each other when
they have the same kind of electrification_. Do two charged bodies
_always_ repel each other? Is it possible that there are different
kinds of electrifications?
=EXPERIMENT 59. To study the electrification of glass.=
_Apparatus._ The sheet of glass, G (No. 38), heated (a hot
bottle or lamp chimney will do); a piece of silk large enough
to rub G. (A silk handkerchief is just the thing, but in case
you have no silk, use the flannel cloth, F C, No. 30.)
=117. Directions.= (A) Vigorously rub the hot glass with the
silk (or flannel), also heated.
(B) Test G for electrification by means of little pieces of
tissue-paper and the carbon electroscope, Exp. 58.
_=118. Questions.=_ Will two pieces of electrified glass repel each
other? Arrange an experiment to show whether you are right or not. Is
the charge on the glass exactly like that on the ebonite? Do you know
how to find out?
=EXPERIMENT 60. To compare the electrification produced by
ebonite and flannel with that produced by glass and silk.=
_Apparatus._ The support (see § 109); wire swing, W S (No. 37);
ebonite rod, etc., of Exp. 55 (Fig. 29); the glass, G, and silk
of Exp. 59.
=119. Directions.= (A) Electrify E R, and place it in W S, Fig.
29.
(B) Bring the uncharged glass near E R, noting the action of E
R.
(C) Heat and electrify G; bring it near E R, and carefully note
whether the attraction between them is stronger or weaker than
before, or whether they repel each other.
_=120. Discussion.=_ We know that the glass was electrified, because it
lifted tissue-paper; hence, its charge was not of the same kind as that
on the ebonite. Had the electrifications been exactly alike, we should
have had a repulsion (Exps. 56, 57, 58).
The exact difference between these two kinds of electrifications is not
known. It has been agreed, for convenience, to call that produced by
glass and silk a _positive_ electrification. With ebonite and flannel a
_negative_ electrification is produced. The sign + is generally written
for the word positive, and - for negative. These signs indicate _kind_,
and not more or less, as in arithmetic.
_=121. Laws.=_ We have learned from the experiments these facts, which
are called _laws_:
(1) Charges of the same kind repel each other; (2) charges of unlike
kinds attract each other; (3) either kind of a charge attracts, and is
attracted by a neutral body.
CHAPTER VII.
INSULATORS AND CONDUCTORS.
=EXPERIMENT 61. To study insulators.=
_Apparatus._ Ebonite rod, E R (No. 28); flannel cloth, F C (No.
30); tissue-paper, T P (No. 31).
=122. Directions.= (A) Holding one end of E R in the hand,
charge the other end by rubbing it with F C.
(B) With bits of the T P test each end of E R for a charge, and
compare the results.
=EXPERIMENT 62. To study insulators.=
_Apparatus._ The ebonite sheet, E S (No. 26); flannel cloth, F
C (No. 30).
=123. Directions.= (A) Thoroughly electrify E S (Exp. 51, D),
then lift and hold it in the air, as in Fig. 28.
(B) By moving your rounded knuckle about near the surface of E
S, see if you can get more than one spark from it.
=EXPERIMENT 63. To study insulators.=
_Apparatus._ A hard-rubber comb (not furnished); flannel cloth,
F C (No. 30); dull pointed nail (No. 19).
=124. Directions.= (A) Electrify the comb with F C.
(B) Move the nail along near the teeth of the comb, and listen
carefully.
_=125. Discussion of Experiments 61, 62, 63; Insulators.=_ In 61 the
electrification remained at one end of the rod. In 62 and 63 the sparks
showed that all parts of the ebonite were not discharged at the same
time. A substance, like ebonite, which will not allow electrification
to pass from one part of it to another, is called an _insulator_. Silk
and glass are also insulators. Do you now see why a silk thread was
used to make the carbon electroscope?
Why do they fasten telegraph wires to glass insulators?
_=126. Conductors.=_ It has already been stated that water in an
elevated tank has potential energy. We can allow the water to flow
through a conducting pipe to another tank a little lower than the
first, and it will still retain much of the potential energy, but not
all.
Can we conduct from one place to another this peculiar state of things,
this queer form of potential energy which we call electrification? It
is clear, from the last experiments, that in order to do it we need
something besides ebonite, which really acts like a closed stop-cock to
the flow of electrification.
To keep electrification in one place we need an insulator; to get it
from one place to another we need a _conductor_. Insulators are as
important as conductors.
You saw that sparks went to the finger from the ebonite, so we call the
finger a conductor. You have learned that attractions and repulsions
show the presence of electrification. Can we have our charged body in
one place and get attractions or repulsions at some other place?
[Illustration: Fig. 32.]
=EXPERIMENT 64. To study conduction.=
_Apparatus._ Fig. 32; the support (see § 109); a bent hairpin,
H P (No. 39); ebonite sheet, E S; flannel cloth, F C; tin disk,
B F B (No. 40), which is the bottom of the flat-box, F B; the
insulating table, I T (see § 127).
=127. The Insulating Table= consists of a tin box (exactly like
that used for the electrophorus cover), and an ebonite rod
about 1-3/4 in. long. See § 139 for full details about fitting
the rod into the box, etc. The lower end of the short rod fits
into the large hole in one end of the support rod, S R. Arrange
as in Fig. 32. B F B should swing about very easily.
=128. Directions.= (A) Charge E S, then rub it upon I T, as
shown, noting the action of B F B.
_=129. Discussion.=_ Ebonite being an insulator (§ 125), we say that I
T, H P and B F B were _insulated_. You can see that the electrification
must have passed through I T and H P to get to the disk, B F B. H P was
the _conductor_, allowing the disk, also, to become charged. The wood,
S R, is a conductor, and, as it was not insulated from the earth, S R
was neutral. Account for the attraction. (See § 121.)
[Illustration: Fig. 33.]
=EXPERIMENT 65. To study conduction.=
_Apparatus._ A copper wire, C W (No. 44); insulating rubber
band, R B (No. 45, Fig. 33); wire swing, W S (No. 37); the
other half of the flat box, T F B (No. 41); apparatus of Exp.
69.
=130. Telegraph Line.= To have our telegraph line using
frictional electricity complete, we must have: (1) Some way of
generating or making the electricity; (2) Some means of getting
it or its effects to the other end of the line; (3) Some way of
showing that it has been taken there.
The charged E S will be the source of the electrification. New
York will represent the end at which we _send_ the message,
so at N. Y. we must have a _sending instrument_. See Fig. 33,
which explains itself. R B or a silk thread must be used to
_insulate_ the sender. Around one leg of W S is twisted one
bare end of the _conductor_, C W.
Boston will represent the end of the line at which the message
is received, and there we need a _receiving instrument_. This
is similar to the apparatus described in Exp. 69, Fig. 37. In
addition to this, tie the middle of a moist cotton thread that
is 6 in. long, to B C (Fig. 37), and let its two free ends lie
over the top and reach down against the bottom of the tin; that
is, on the left-hand side. Fig. 42 will give you an idea in
regard to the looks of the thread; at first, however, it should
be close to the bottom of the tin. Twist the other bare end of
the copper wire around B C.
When the line is properly constructed and ready for use, both
instruments and C W are entirely insulated. Do not let any part
of C W touch the table or your clothing.
=131. Directions.= (A) Touch the insulated sending instrument
with the charged ebonite sheet, and watch for any motion in the
receiving instrument.
=Note.= Better results will be obtained by using the charged
electrophorus cover as the source of electrification, instead
of E S. (Exp. 68.)
_=132. Discussion.=_ The action here was like that in the previous
experiment, the difference being that a longer _conductor_ was used.
Electrification is always looking for some place to get to the earth,
just as water will run from a roof to the ground. You will understand
more about it a little later. In our apparatus just described, the only
way that the earth could be reached was through the wooden rod S R. Do
not get the idea that real messages are sent in any such way, or that
electricity flows through a wire as water flows through a pipe.
_=133. Relation between Conductors and Insulators.=_ The above terms
are merely relative. Static electricity is easily conducted by dry
wood, while Galvanic electricity is practically insulated by it. A
substance may be an insulator for currents of low potential, while at
the same time it will conduct high potential currents. (See Potential §
144.)
_=134. Electrics and Non-electrics.=_ Bodies like glass, sealing-wax,
amber, etc., were called electrics by the first students of
electricity, because it was upon these substances that they could
easily produce electrification. They called iron and other metals
non-electrics, because they could detect no electrification
after rubbing them. Can you explain why they did not detect any
electrification on metals? Can you devise an experiment to prove that
metals may be charged? Do you see any relation between a non-electric
and a conductor?
=EXPERIMENT 66. To study the effect of moisture upon an
insulator.=
_Apparatus._ Same as for Exp. 65, with the exception of the
copper wire; this is to be replaced by a dry silk thread about
2 feet (60 cm.) long (No. 33).
=135. Directions.= (A) See if a charge can be sent through the
thread, in the same manner as it was through the copper. Is dry
silk a conductor?
(B) Thoroughly wet the thread, being careful not to wet the
rubber band insulator (Fig. 33); see if wet silk is a conductor.
_=136. Discussion.=_ Dry silk is an insulator, while wet silk is a good
conductor of _static_ electricity. It is the water, however, which
really does the conducting. Even small amounts of moisture on glass, or
other insulators, will allow the charge to escape. Glass collects much
moisture from the air. Do you now see why it is necessary, to get good
results, to have the paper, glass, etc., hot before electrifying them?
=EXPERIMENT 67. To test the effects of moisture upon bodies to
be electrified.=
_Apparatus._ Two pieces of newspaper, each about 4 in. (10 cm.)
square.
=137. Directions.= (A) Heat one piece to make it thoroughly
dry, and leave the other cold.
(B) Stroke each, say 10 times, with your hand, pressing them
upon the table; then place them upon the wall at the same time,
being careful not to let them touch your clothing. See which
will cling to the wall the longer.
CHAPTER VIII.
CHARGING AND DISCHARGING CONDUCTORS.
_=138. The Electrophorus.=_ While the ebonite sheet alone, or a good
hard-rubber comb, may be used for many experiments in frictional
electricity, the sparks produced are small, and the ebonite has to be
electrified as often as it is discharged. To obtain real good sparks,
and to avoid this continual rubbing, the student should be provided
with an _e-lec-troph'-o-rus_. This is, really, a simple, cheap, and
efficient frictional electric machine. An electrophorus consists of 2
insulators and 1 conductor--that is, of 3 parts: (1) insulating handle,
(2) cover, and (3) a plate or base of insulating material.
[Illustration: Fig. 34.]
=139. Our Electrophorus= is shown in Fig. 34. For the insulating
_handle_ use the ebonite rod, E R (No. 28); for the _plate_, use the
ebonite sheet, E S (No. 26). The _electrophorus cover_, E C (No. 42),
furnished, is a tin box with a fancy top. A hole has been punched in
the center of its top, and into the hole has been riveted a short tube,
so that the handle, E R, can be firmly held. The hole has been made
a little larger than E R for convenience. To make E R fit tightly in
the hole, so that you can lift E C, wrap a small piece of paper around
the end of E R before pushing it into the hole. You can easily find
out how much paper to use to make a good fit. With a knife cut away
all loose points of paper that stick out of the hole around E R; this
is _important_. The top and bottom of E C should be pressed firmly
together.
First learn how to use the electrophorus. With the large amount of
electrification produced we can then find out how it works.
=EXPERIMENT 68. To learn how to use the electrophorus.=
_Apparatus._ Shown in Figs. 34, 35. _Do not fail to read_ § 139.
=140. Directions.= (A) Place E S upon a _flat_, uncovered,
wooden table, and rub it _vigorously_ for a _minute_ with the
_warm_ flannel, F C, to thoroughly charge it. Do not let E S
slide about, and do not lift it from the table.
(B) With the right hand grasp E R at its extreme end, and place
E C upon E S.
(C) Touch E C for an instant with a finger of your left hand
(Fig. 35).
(D) Remove your finger entirely from E C, then lift E C by its
insulating handle, E R, at the same time holding E S down to
the table, if it tries to follow E C.
[Illustration: Fig. 35.]
[Illustration: Fig. 36.]
(E) Bring your left hand near E C (Fig. 36). You should get a
good spark from E C.
(F) It is not necessary to immediately rub E S again. You have
discharged E C by taking a spark from it. To _recharge_ it,
simply place it upon E S again; let it remain there while you
count 5; touch it as before, and then lift by E R.
=141. Extra Notes.= You may repeat the above operation many
times. As soon as the sparks begin to get small, electrify E S
again. The charge on E C is +, although that on E S is -. You
will understand, later, why this is so.
=If you do not get a good spark= from the electrophorus, read
the directions again. The ebonite must be well electrified;
the cover must be lifted by the _end_ of its handle; you must
_touch_ the cover and _withdraw your finger_ from it _before_
lifting. You must allow the cover to remain upon the ebonite
3 or 4 seconds each time. The board, or table, upon which E S
rests, must be _flat_, and not warped, so that E C will fit
down perfectly upon E S.
=EXPERIMENT 69. To study "charging by conduction."=
_Apparatus._ Fig. 37. To one end of a _silk_ thread, S T, is
tied a little bent clamp, B C (No. 46); the other end of S T is
tied to the support wire, S W (No. 36); the bottom of the flat
box, B F B (No. 40), is supported by B C, and thus _insulated_
from the table and earth; the electrophorus (Exp. 68) is also
necessary.
=142. Directions.= (A) Charge E C (Exp. 68), and bring it near
B F B (Fig. 37). Note the spark.
(B) Repeat (A) twice, noting the relative sizes of the sparks.
Does B F B continue to be attracted by E C?
(C) Bring your knuckle slowly towards the charged disk, B F B.
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
=EXPERIMENT 70. To study potential; electro-motive force.=
_Apparatus._ The insulating table, I T, Fig. 38. (For details
see Exp. 64; the electrophorus Exp. 68).
=143. Directions.= (A) Pass a spark from the thoroughly charged
E C (Exp. 68) to I T.
(B) Recharge E C, and see how many times I T will take good
sparks from it, and note the relative sizes of the sparks.
(C) As soon as I T refuses to take more sparks from E C, touch
E C to see if it is completely discharged.
(D) Touch I T.
_=144. Pressure; Potential; Electro-motive Force.=_ Water runs down
hill. It always tries to run from a high place to a lower one.
Electrification acts very much like water in this respect. We say that
water has a _pressure_, or a _head_ of so many feet. In speaking of a
charge, we say that it has a _potential, or an electro-motive force_.
Water may have a high or low pressure, and a charge may have a high or
low potential. The greater the pressure of water, the harder it tries
to break away and get somewhere; the greater the potential of a charge,
the farther it will jump to your hand.
_=144a. Current; Spark.=_ Electrification will easily pass from a place
of high potential to one of low potential through a conductor, and
when it _passes_ we say we have an _electric current_, or a _current
of electricity_. Water has no desire to flow on a dead level, and the
electric current does not care to flow between two places of equal
potential. The potential of the earth and of all neutral bodies is
zero; that is, they have no charge, no potential; so it is very easy
for a charge to escape into the earth.
Dry air is a pretty good insulator, but when the attraction between
a charged and a neutral body gets great enough, the spark rips
right through the air. Benjamin Franklin proved by experiment that
lightning is caused by the electrification in the clouds and air. (See
Atmospheric Electricity.)
=145. Theories about Electrifications.= _The "One-Fluid" Theory_
suggests that neutral bodies have a certain amount of electrification,
and that they have a certain potential called zero potential. If the
potential of a body becomes greater than that of the earth, the body
is said to be positively electrified; if the potential of the body is
less than that of the earth, it is said to be negatively electrified.
If we fill a bottle with sea water, we have a great deal of water
when we compare it with the bottle, but a very little water when we
compare it with the sea. The earth is so large that small amounts
of electrification taken from it or added to it do not affect its
potential to any extent.
=146.= _The "Two-Fluid" Theory_ suggests that there are two absolutely
different kinds of electrification, one called positive (+), and the
other negative (-). When these two are equal in quantity, the body is
said to be neutral. If the body contains more + than -, the body is
said to be charged positively.
It is evident then, if the two-fluid theory be accepted, that no matter
how strongly a body is charged positively there must be in it _some_
negative electrification; that is, we may charge a neutral body + by
adding + electrification to it, or by taking - electrification from it.
There must always be, then, some + and - electrifications in a body.
These theories do not require much consideration by the student of
elementary electricity. The best thing he can do is to learn what
electricity can do, and how it can be used.
[Illustration: Fig. 39.]
=EXPERIMENT 71. To study some methods of discharging an
electrified body.=
_Apparatus._ The electrophorus (Exp. 68); an ordinary pin (Fig.
39).
=147. Note.= You have seen sparks pass from E C to your rounded
knuckle, and to other conductors. In all of these cases the
discharge was _sudden_, one spark doing the work. Can we
_slowly_ discharge E C, or discharge it without sounds?
=148. Directions.= (A) Thoroughly charge E C, and test it with
your knuckle to be sure that it is working properly.
(B) Charge E C again; hold the pin in your left hand (Fig. 39),
and _slowly_ bring its _head_ toward E C; listen for sparks.
(C) Recharge E C, and bring the _point_ of the pin slowly
toward it. Touch E C to see whether it has been discharged or
not.
_=149. Disruptive, Conductive, and Convective Discharges.=_ Sudden
discharges, accompanied by bright sparks, are said to be _disruptive_.
When the electrification is continuously carried away by a conductor,
there is a _conductive_ discharge. There is a _convective_ discharge
when the electrification escapes from points into the air. (See § 155.)
The nature of the discharge depends upon the potential of the charge,
upon the nature of the charged conductor, and upon the nature of the
surrounding air and objects. Convective discharges are often _silent_,
as in Exp. 71 (C). In this case, electrification passed from the earth
through the pin-point to the cover to neutralize it. (See Induced
Electricity.)
[Illustration: Fig. 40.]
=EXPERIMENT 72. To study intermittent or step-by-step
discharges.=
_Apparatus._ Electrophorus (Exp. 68); carbon electroscope (§
114), (Exp. 58).
=150. Directions.= (A) Charge E C, then hold your hand on one
side of the carbon (Fig. 40), and hold E C upon the opposite
side. What should the carbon do?
_=151. Discussion.=_ The carbon and E C were insulated, while the hand
was "grounded"--that is, it was connected with the earth. Carbon is a
good conductor; it may be quickly charged and discharged.
=EXPERIMENT 73. To ascertain the location of the charge upon an
electrified conductor.=
_Apparatus._ The electrophorus (Exp. 68); the insulating table,
I T (Exp. 64); the tin box, T B (No. 47), Fig. 41; a piece of
moist cotton thread, C T, 5 or 6 in. long, bent double, and
hung over the edge of the open box, T B. One-half of C T should
be inside of T B, which, in turn, should stand on I T.
[Illustration: Fig. 41.]
=152. Directions.= (A) Charge E C; pass a spark to T B, and
note the action of both parts of C T.
_=153. Hollow and Solid Conductors.=_ The moist thread, being a
conductor, became charged as well as the box. The electrification
seemed to be entirely on the outside of T B. A hollow conductor will
hold as large a charge as a solid one having the same amount of
surface. This refers to charges of static electricity, not to currents.
An electric current passes through the whole substance of a conductor.
=EXPERIMENT 74. To study the effect of points upon a charged
conductor.=
_Apparatus._ The electrophorus (Fig. 34); a pin, bent slightly
to keep it from rolling.
=154. Directions.= (A) Charge E C; test its charge with your
knuckle. Be sure that you get a good spark.
(B) Charge E C again, and hold it by its insulating handle,
E R, long enough to count 10 before discharging it with your
knuckle. Be sure that it holds its charge during this time.
(C) While E C is upon E S (Fig. 34), lay the bent pin upon E C,
so that its point will project into the air. The point should
stick out about 1/4 in. from the edge of E C.
(D) Touch E C; raise it by E R; count 10 as before; then test
with your knuckle to see if E C is still charged.
_=155. Electric Density; Electric Wind.=_ A charge resides upon the
outside of a conductor (Exp. 73), and it continually tries to escape.
It seems to pile up at points and corners, and we say that it is denser
at such places than at well-rounded parts of a charged conductor. All
points and sharp places should be removed from a conductor, if it is
desired to keep a charge for any length of time.
Electrification may escape from a point so rapidly that currents
are produced in the surrounding air. As the particles of air become
charged, they repel each other. The movement of the air particles may
be so great that a lighted candle will be affected when placed near the
point. This current of air is called _electric wind_.
Electrification easily passes from points, and the electrophorus may be
easily and silently discharged by holding a pointed pin near it (Exp.
71, C). Thorns, leaves with sharp edges, etc., have a great effect upon
atmospheric electricity. They allow a silent escape of electrification
from the earth to neutralize that in the clouds which is opposite in
nature. (See Atmospheric Electricity.)
CHAPTER IX.
INDUCED ELECTRIFICATION.
_=156. Electric Field; Lines of Force.=_ In our study of magnetism
you learned that a magnet can act through the air, and induce a piece
of iron to become a magnet. You saw how the iron filings arranged
themselves around the magnet, showing that the lines of force reached
out from the poles in a very peculiar manner. There is an _electric
field_ all around a charged conductor, just as there is a magnetic
field about a magnet. The lines of force in the electric field pass
from the positively charged body to the negatively charged one, or to
some neutral one, which, you will soon see, is practically the same
thing. When the positively charged electrophorus cover is held above
the negatively charged ebonite sheet, a very strong electric field
exists between them.
=157. Note.= You have seen that we can _charge_ an insulated conductor
by _touching_ it with the charged cover, or by allowing a spark to
pass to the conductor. What effect, if any, has a charged body upon
an insulated conductor _before_ they touch each other, and before any
spark passes to the conductor?
[Illustration: Fig. 42.]
=EXPERIMENT 75. To study electric induction.=
_Apparatus._ Fig. 42. The insulating table, I T (for details
see Exp. 64); tin box, T B (No. 47, Fig. 42); moist cotton
thread, C T; the electrophorus (Exp. 68); tie C T around one
end of the closed T B, and leave the ends of C T long enough to
hang down over the end. Place a match on each side of T B to
keep it from rolling.
=158. Directions.= _Part 1._--(A) Pass a spark from the charged
E C to T B, and note the action of the thread, which will be
our electroscope. Remove E C.
(B) Touch the charged T B with the finger, watching C T.
_Part 2._ (C) Bring the re-charged E C near the neutral T B,
and parallel to its end surface; but keep them at least an inch
apart, so that a spark cannot pass. Watch C T.
(D) Withdraw E C, and try to explain the action of C T.
_=159. Electric Polarization; Theory of Induction.=_ This experiment
should remind the student of Exp. 24, in magnetism, in which a piece of
soft iron was magnetized by the inductive action of a magnet. The soft
iron was in a magnetic field; it became polarized. Is it possible that
the box, T B, was polarized, being in the electric field of E C?
We know, by the action of C T (Fig. 42), that the top end of T B was
charged while E C was in place. The charge was not conducted.
You know, from previous experiments, that + and - electrifications rush
together whenever possible. Why can we not suppose that a neutral body,
like the box at the start, contains an equal amount of both kinds, and
that these different electrifications have already rushed together?
If you imagine a small army of positive soldiers struggling, "man to
man," with the same number of equally strong negative soldiers, you can
readily see that one-half of them can hold the other half from running
away. A body remains neutral, then, according to this idea, as long as
it has an equal quantity of the two opposite kinds of electrification.
(See Theories, § 145, 146.)
As soon as the positively charged E C was brought near T B, it
destroyed the neutrality of T B, by pulling at its - electrification,
and by pushing back its + electrification to the top end and into C
T. We say that the charged E C produced a separation of the combined
electrifications of T B by _induction_, and not by contact. As soon as
the inductive action of E C was removed, T B became neutral again.
[Illustration: Figs. 43-44.]
=160. Note.= Figs. 43 and 44 may aid the student. In Fig. 43, T B
is supposed to be neutral. The "double sign" means that the + and -
electrifications are united; and, as there are an equal number of both
kinds, none are left free to tell the tale. Fig. 44 shows what happens
when the + E C is near.
What would happen if we could cut into T B at the middle with an
insulated knife while it is polarized by E C?
=EXPERIMENT 76. To learn how to charge a body by induction.=
_Apparatus._ Fig. 42, same as in Exp. 75.
=161. Directions.= (A) Bring the charged E C within an inch of
the bottom of T B, and as soon as C T is repelled, showing that
T B is polarized (Exp. 75), touch T B with your finger; then
remove your finger while you still hold E C in place.
(B) Withdraw E C and its inductive action. Explain the motions
of C T during the experiment. Is it still repelled by T B after
E C is removed?
_=162. Free and Bound Electrifications.=_ As explained in Exp. 75, and
as shown in Fig. 44, T B became polarized. The - electrification was
drawn towards E C; it was held or _bound_ there as long as E C was
near. The + was actually repelled by E C, and it was _free_ to escape
through your arm as soon as T B was touched, leaving the top end of T B
neutral. As soon as E C was removed, the - electrification, no longer
held by E C, spread all over T B and on to C T. T B was _charged by
induction_. It was charged negatively by driving out + electrification.
=EXPERIMENT 77. To show that a neutral body is polarized before
it is attracted by a charged one.=
_Apparatus._ The electrophorus (Exp. 68); dry tissue-paper, T
P. Cut out 2 pieces of T P, each about 1/4 inch square.
=163. Directions.= (A) Place the bits of dry T P upon a board
or table, and convince yourself that they are attracted equally
by the charged E C.
(B) Slightly moisten one piece of T P only. See if one is
attracted by E C more readily than the other.
_=164. Polarization Precedes Attraction.=_ Dry tissue-paper is not a
good conductor; you have seen (Exp. 52) that it can be electrified,
which indicates that it is at least a partial insulator. Insulators
are not easily polarized. (Why?) Even if the pieces of T P were
polarized, the opposite electrifications were so near each other that
the attraction of E C for the - was nearly overcome by the repulsion
for the +; the result being that T P was not strongly attracted by E C
until the + had a chance to escape. The moist tissue-paper allowed its
+ to escape more quickly than the dry piece. A conductor is attracted
by a charged body more strongly than an insulator, because the latter
is not easily polarized. A neutral body, then, is really no longer
neutral when it is in the electric field. _Polarization precedes
attraction._
=EXPERIMENT 78. To find whether electric induction will act
through an insulator.=
_Apparatus._ Small bits of carbon (Exp. 58); bits of moist
tissue-paper, T P; one-half of the flat box, T F B (No. 41);
sheet of glass, G (No. 38); electrophorus (Exp. 68). Place the
carbon and T P into T F B (Fig. 45), and cover with the glass.
=165. Directions.= (A) Charge the electrophorus cover, E C
(Exp. 68), move it about a little above the glass, and see if
the carbon, etc., are attracted.
_=166. Dielectrics.=_ The carbon must have been polarized and attracted
_through_ the glass. You saw, Exp. 7, that the lines of magnetic force
could penetrate and act through paper, glass, etc.; it is now evident
that the electric field is not easily fenced in, even by an insulator.
Substances, like the glass, which allow this inductive influence to act
through them, are called _dielectrics_.
[Illustration: Fig. 45.]
[Illustration: Fig. 46.]
=EXPERIMENT 79. To find whether a polarized conductor can act
inductively upon another conductor.=
_Apparatus._ Fig. 46. Insulating table, I T (for details see
Exp. 64); ebonite sheet, E S (No. 27); flat box complete F
B (Nos. 40, 41); sheet of glass, G (No. 38); small piece of
slightly moist tissue-paper, T P; charged electrophorus cover,
E C. Arrange as shown.
=167. Directions.= (A) Hold E C, charged, near and under I T,
then bring your finger, F, near T P. Explain the action of T P.
_=168. Successive Induction.=_ The inductive influence of E C first
polarized I T; this acted through the dielectric, E S, and polarized
F B, which, in turn, polarized T P through the second dielectric, G.
This induction after induction is called _successive induction_.
_=169. Inductive Capacity.=_ Dielectrics are insulators. Two substances
may be equally good insulators, that is, they may equally well resist
the _spread_ of electrification _over_ their surfaces, or the _flow_ of
the electric current _through_ them, while one may be, nevertheless,
a better _dielectric_ than the other. The better the dielectric,
the easier it is for the electric field to polarize a conductor
placed beyond the dielectric. A good dielectric is said to have a
high _inductive power or capacity_. Glass is about 3 times as good a
dielectric as dry air; and as the latter (under certain conditions)
is taken as the standard, or as unity, we may say that the _specific
inductive capacity_ of glass is about 3.
=EXPERIMENT 80. To study the action of the electrophorus.=
_Apparatus._ The electrophorus (Exp. 68); small bits of moist
tissue-paper, T P.
=170. Directions.= (A) Thoroughly electrify E S, Fig. 34, and
place E C upon it by its handle, E R.
(B) Touch E C, as directed in Exp. 68, and listen for a small
spark which should pass from E C to your finger.
(C) Again, place a little piece of T P upon E C before lowering
it upon E S. Do not touch E C, but bring your finger near T P.
What does T P do? Now, touch E C and see, when you bring your
finger near it, if T P acts as it did before.
(D) Again, place several pieces of T P upon E C (E S being
thoroughly charged); touch E C, then lift it by its handle.
Note action of T P, which should be slightly moist.
_=171. Discussion.=_ The electrification upon the ebonite is negative
(Exp. 60). Although E S and E C (Fig. 34) seem quite smooth, there are
many little hills, valleys, and air-spaces between them, which keep
them from touching each other perfectly. The ebonite has the electric
field at the start, and it really acts across these minute air-spaces
_by induction_ (Exp. 75), and polarizes E C. The air-spaces form the
dielectric (Exp. 78). The - electrification of E C being repelled by
the - of E S, it is driven to the top of E C, while the + is drawn to
the bottom. This + is kept from rushing to the - of E S by the air
dielectric, and because E S is a non-conductor. By touching E C the
free - escapes to the earth, leaving E C _positively_ charged when it
is lifted.
[Illustration: Fig. 47.]
[Illustration: Fig. 48.]
[Illustration: Fig. 49.]
[Illustration: Fig. 50.]
[Illustration: Fig. 51.]
=172. Details of Action.= The different steps in the action of the
electrophorus are shown graphically in Figs. 47 to 51. Fig. 47 shows
E S negatively charged. E C is neutral at first, Fig. 48; that is, it
is supposed to contain both + and -, as shown by the "double sign" (§
160). Fig. 49 shows that E C has been polarized by the inductive action
of E S. The repelled - escapes to the finger (this escaping is what
gave the small spark to the finger and charged the T P in the last
experiment), leaving the top uncharged, while the + is _bound_ (Fig.
50). As soon as E C is lifted (Fig. 51) the + spreads all over E C,
which is then charged. The +, upon going to the top, charged the pieces
of T P (Exp. 80, D), causing them to be repelled. The charge of - upon
E S has not been removed, so the operation may be repeated many times
before E S must be again electrified.
The - electrification on the ebonite acts inductively through E S,
drawing up + electrification from the earth. To make this action easier
a "sole," or metal conductor, is often placed under the ebonite.
=EXPERIMENT 81. To see, hear, and feel the results of inductive
influence and polarization.=
_Apparatus._ Ebonite sheet, E S (No. 26); insulating table, I
T; flannel cloth, F C.
=173. Directions.= (A) Thoroughly charge E S with F C. With the
right hand bring E S near and parallel to the top surface of I
T, but do not let them touch each other.
(B) Remove E S, then touch I T to see if it is charged.
(C) Repeat (A), and while you hold E S about 1/2 inch from I T,
their flat surfaces being parallel, touch I T. Watch for any
sparks, and note any peculiar actions of E S.
(D) Remove your finger from I T, then withdraw E S; finally
touch I T with your knuckle.
_=174. Discussion.=_ This apparatus is really the electrophorus upside
down. It shows very clearly (1) the escape of the - electrification
from I T, by the spark; (2) that the attraction between I T and E S is
much greater than before, when this - is removed; and (3) it shows the
different steps of the inducing and charging process, as described in
Exp. 75, and as shown in Figs. 43 and 44.
CHAPTER X.
CONDENSATION OF ELECTRIFICATION.
=EXPERIMENT 82. To find whether a large surface will hold more
electrification than a small one.=
_Apparatus._ The insulating table (for details, see Exp. 64); a
large tin basin or pan (not furnished); the electrophorus (Exp.
68).
=175. Directions.= (A) Test the electrophorus and be sure that
it is working properly.
(B) As in Exp. 70, see how many good sparks I T will take
from E C (which should be recharged at each trial) before the
potential of I T is raised so that it equals the potential of E
C.
(C) Carefully set the basin or pan upon I T, then count the
number of good sparks you can pass to it from E C (recharged at
each trial). Compare the number of sparks necessary to raise
the potential of the large surface until it equals that of E C,
with the number found in part (B).
_=176. Electrical Capacity.=_ It takes more heat to raise the
temperature of a gallon of ice-water to the boiling point, than it
takes for a quart of ice-water. You have just seen that a large
insulated surface will take more sparks from a charged body than a
small one, before its potential is raised to that of the small one, and
to that of the charging body. We say that a large surface has a greater
_capacity_ than a small one, the shape and other conditions being the
same.
=EXPERIMENT 83. To find whether the capacity of a given
conductor can be increased without increasing its size.=
_Apparatus._ Fig. 52. Insulating table. I T (Exp. 64); the
extra ebonite sheet, E S (No. 27); the complete flat box, F B
(No. 40, 41); the charged electrophorus cover, E C (Exp. 68).
Arrange, as shown, I T being insulated from the earth by E S. F
B should rest upon a wooden table or other large conductor.
=177. Directions.= (A) See how many good sparks I T will take
from E C. Re-charge E C at each count, and note the relative
sizes of the sparks.
(B) Discharge I T by touching it with your knuckle.
[Illustration: Fig. 52.]
_=178. Condensation; Condensers.=_ As I T easily held more sparks
than it would take before (Exp. 70), we say that its _capacity_ has
been increased. Its potential didn't increase, because that could not
get greater than the potential of E C, the charging body. To describe
this state of affairs, we say that the electrification was denser than
before, and that it was _condensed_. The _capacity of I T was greatly
increased by the presence of another conductor, F B, insulated from I
T, but "grounded_." Such a combination, 2 conductors, with a dielectric
between them, is called a condenser.
A condenser can hold much more electrification at a certain potential
than an equal amount of surface can hold when not properly arranged.
We might call a condenser a storage battery for static electricity.
The capacity of a condenser depends, among other things, upon the area
of the conducting surfaces, and upon the thickness and nature of the
dielectric. Among the various forms of condensers may be mentioned the
Leyden jar, and the fulminating pane.
=179. The Leyden Jar= consists of a wide-mouthed glass jar,
with tin-foil pasted upon the inside and outside to within
2 or 3 inches of the top. The inner coat or conductor is
connected to a knob or ball at the top by means of a chain. To
charge the jar, the outer coat is connected with the earth by
holding it in the hand, or by resting it upon a table while
the electrification is passed to the knob. A _Leyden Battery_
consists of 2 or more connected jars, the object being to
increase the area of the surface. The jar is discharged by
touching one end of a _discharger_ (§ 188) to the outer coat,
and swinging its other end over to the knob, when a bright
spark will pass between the knob and discharger. (See Exp. 86.)
=180. Fulminating Panes=, or Franklin's Plates, are practically
the same as a Leyden Jar. The tin-foil, however, is pasted upon
the opposite sides of a pane of glass, a margin of about an
inch being left all around. One side of the pane is charged,
and takes the place of the inside coat of the jar. The other
side is grounded. The pane is discharged by connecting the two
sheets of foil.
=181. Induction Coil Condensers= consist of sheets of tin-foil
separated by sheets of paraffined paper, which act as the
dielectric. (See Induction Coils.)
=182. Submarine Cables=, with the surrounding water, act like
condensers, the result being that the condensing effect slows
up the electric current and retards the signals. These make
a condenser of enormous capacity. The wires inside form one
conductor, and the water the other, while the insulation around
the wires forms the dielectric.
=EXPERIMENT 84. To study the condensation of electrification.=
_Apparatus._ Same as in last experiment, but arrange so that
F B and I T shall be near each other at one side; that is, so
that the edge of E S shall be even with the edges of the two
tins.
=183. Directions.= (A) Pass good sparks to I T from the charged
E C until something happens. Watch the side where I T and F B
are near each other.
_=184. Discussion.=_ We may say that the electrification was condensed,
in this experiment, until the charge became so great that the
_condenser_ suddenly discharged itself. Condensers may be made in many
ways, but they all consist of 2 conductors, with a dielectric between
them. One conductor is insulated, and receives the charge; the other
conductor is grounded.
[Illustration: Fig. 53.]
=EXPERIMENT 85. To study the action of the condenser.=
_Apparatus._ Fig. 53. The insulating table, I T; ebonite
sheet, E S (No. 27); flat box, F B, complete (Nos. 40, 41);
the electrophorus (Exp. 68). Note that this is really the same
apparatus as that just used; both conductors of this condenser,
however, are insulated and reversed in position.
=185. Directions.= (A) See that your electrophorus works
properly, then find out how many good sparks you can pass from
E C to F B, recharging E C each time. Note the relative sizes
of the sparks, and compare the result with the number taken by
the condenser in the last experiment.
(B) When F B seems to be fully charged, touch I T with your
knuckle. (From your study of induction what should be the
result?)
(C) Now see if F B will again take good sparks from the charged
E C. Pass sparks to F B until it seems fully charged.
(D) Again touch I T, then repeat (A) and (B) several times,
until a bright spark passes from F B over the edge of E S to I
T.
_=186. Discussion.=_ The action of the condenser, as clearly shown,
depends upon induction. You should now be able to explain and show by
diagram the different steps.
E C was positively charged (Exp. 80). This also charged F
B positively by contact. F B acted inductively through the
dielectric, E S, drawing up _some_ of the - in I T, and
repelling _some_ of the +. As I T was insulated, this free +
electrification could not escape. Before we touched I T, its
+ and - electrifications, although partially separated, were
struggling against this inductive action; and, on account of
their strong attraction for each other, our efforts to charge
the condenser were retarded. Upon touching I T the free +
escaped to the earth. (This was the cause of the spark.) This
left _some_ - electrification bound on the underside of E S,
and some + bound on the upperside of E S. The capacity of F
B was increased by this process, as the + already put into
it was very much occupied by the attractions of the induced -
just under E S. As more + was given to F B, more - was drawn up
under E S and more + was pushed out of I T. This action went on
until the two conductors were strongly and oppositely charged.
This action goes on continuously when the lower conductor is
grounded. The spark between the tins was due to the rushing
together of the + and - electrifications; it showed that there
was a _momentary current of electricity_.
=EXPERIMENT 86. To study the effect of electrical discharges
upon the human body.=
_Apparatus._ The condenser (Fig. 52), with E S centrally placed
so that the apparatus cannot discharge itself; the hairpin
discharger, H P D (No. 48); the electrophorus.
=187. Directions.= (A) Charge the condenser (Exp. 83) with 10
good sparks from E C, then touch I T (Fig. 52).
(B) Recharge the condenser with 10 sparks, then touch F B.
Discharge it by again touching I T as in (A).
(C) Recharge with 10 sparks; then place your thumb against F B,
and quickly swing the first finger of the same hand over to I
T, and get a slight shock.
(D) Recharge with as many sparks as you think you can stand.
(E) Instead of using your hand to discharge the condenser, try
the bent hairpin. Keeping one end against F B, swing the other
end over near I T.
_=188. Shocks; Dischargers.=_ The two conductors being oppositely
charged in the condenser (Exp. 85), it is only necessary to place
some conductor between them to allow the charges to rush together.
Any conductor so used is called a _discharger_. The hand carried the
whole current which caused the _shock_. When I T was touched first,
the current was obliged to pass through your body, through the floor,
and up the table-legs into F B. Always touch the "grounded" conductor
first with the discharger, so that you will get a good spark and _not_
a shock.
=EXPERIMENTS 87-88. To show the strong attraction between the
opposite electrifications in the condenser.=
_Apparatus._ Flat box, F B (Nos. 40, 41); sheet of glass, G
(No. 38); electrophorus (Exp. 68). The two parts of F B are
used for the conductors of the condenser (Fig. 54) for the sake
of lightness. The bottoms should be next to the glass, which is
used for the dielectric on account of its stiffness. The lower
tin should rest upon the table. The glass should be perfectly
clean and dry (hot).
=189. Directions.= (A) Charge the condenser with 15 or 20 good
sparks from E C.
(B) Lift the condenser by one corner of G (Fig. 54), being
careful not to discharge it. Explain why the lower conductor
follows the glass.
[Illustration: Fig. 54.]
=EXPERIMENT 88.=
=190. Directions.= (A) Charge and lift the condenser as just
explained (Exp. 87). Fig. 54.
(B) With your right hand touch the upper tin alone, then the
lower tin alone.
(C) Touch both tins at the same time, and note the action of
the lower one.
_=191. Discussion.=_ This clearly shows how strongly the two
electrifications are _bound_ in the condenser. Each refuses to
escape to the earth, but they instantly rush together at the first
opportunity. The dielectric may be shattered in a very heavily-charged
condenser by this strong attraction.
[Illustration: Fig. 55.]
=EXPERIMENT 89. To show how the condenser maybe slowly
discharged.=
_Apparatus._ Fig. 55. The condenser (Exp. 83); the carbon
electroscope with support (Exp. 58); the electrophorus (Exp.
68).
=192. Directions.= (A) Charge the condenser by means of the
electrophorus; then hang the carbon so that it can swing
between the upper conductor and E C placed as shown.
_=193. The Electric Chime.=_ The charging and discharging of the carbon
being rapid, it acts like a _chime_ as it taps against the tins.
=EXPERIMENT 90. To ascertain the location of the charge in the
condenser.=
_Apparatus._ The condenser, consisting of flat box, F B (Nos.
40, 41); ebonite sheet, E S (No. 27); insulating table, I
T (Exp. 64); (when charging, arrange as in Fig. 52.); the
electrophorus; hairpin discharger, H P D (No. 48).
=194. Directions.= (A) Charge the condenser with 15 or 20 good
sparks from E C.
(B) Lift I T away from E S by its insulating handle, and set it
upon the table. (It may be necessary to hold E S down.)
(C) Lift E S directly up and away from F B. (Lift by 2 corners;
do not scrape E S along on F B; do not allow E S to touch your
clothing.)
(D) Replace E S and then I T by its handle quickly, making the
condenser complete again.
(E) With H P D see if the condenser still holds a charge. Touch
F B first (Exp. 86).
_=195. Discussion.=_ As the _conductors_ were completely discharged,
being left for a few moments upon the table, it is evident that the
opposite electrifications must reside in and upon the _dielectric_.
The conductors allow an even and _rapid_ discharge from all parts
of the dielectric at the same time. The dielectric is considerably
strained when a condenser is heavily charged. This strain, caused by
the attraction of the opposite electrifications, may be great enough to
break or puncture the dielectric.
=EXPERIMENT 91. To find whether any electrification remains in
the condenser after it has once been discharged.=
_Apparatus._ The condenser (Fig. 52); the electrophorus (Exp.
68); hairpin discharger, H P D.
=196. Directions.= (A) Thoroughly charge the condenser.
(B) Discharge it with H P D, being sure to touch F B first, and
to touch I T for an instant while H P D is against F B.
(C) After a few moments use H P D again, and see if you get a
slight spark.
_=197. Residual Charge.=_ The two electrifications on the opposite
sides of the dielectric have such an attraction for each other,
when the condenser is charged, that they seem to penetrate, or soak
into, the dielectric. These do not completely soak out again at the
discharge. The small amount left is called a _residual charge_.
[Illustration: Fig. 56.]
=EXPERIMENT 92. To study successive condensation; the chime
cascade.=
_Apparatus._ Fig. 56. This really consists of two condensers,
joined by a wire. The upper condenser consists of T F B (No.
41), E S (No. 27), and the insulating table, I T. (See Exp.
64.) The lower condenser consists of the cover of the tin box,
C T B (No. 47), the sheet of glass G (No. 38), and B F B (No.
40). The tin box, T B (No. 47), is placed under this to raise
it, simply. A wire or hairpin, H P, is hung upon the edge of T
F B, its lower end being inside of C T B and not quite touching
it. This acts like a pendulum, which is to swing to C T B at
the proper time. The source of electrification is E C.
=Note.= You have learned that in charging the condenser with
the positively charged E C, + electrification is driven from F
B into the earth. Can we use this to charge a second condenser?
=198. Directions.= (A) Pass 15 or 20 good sparks from E C to
the under side of I T (Fig. 56), noting the action of H P.
(B) Hold E C in the hand, and, with its insulating handle, poke
H P away from the condensers. Do not discharge them.
(C) With H P D test the lower condenser for a charge, touching
T B first.
(D) With H P D touch T F B first (why?), and discharge the
upper condenser.
_=199. Discussion.=_ A long row of condensers may be charged in this
way. There is no advantage in it, as the electrification is merely
divided between them. How can two condensers be joined to get the
advantages of a large surface?
CHAPTER XI.
ELECTROSCOPES.
_=200. Electroscopes=_ are instruments to show the presence, relative
amount, or kind of electrification on a body. (See Apparatus Book,
Chap. XVIII, for Home-Made Electroscopes.) The _carbon electroscope_
has been described (Exp. 58). The _pith-ball electroscope_ is made
by using pith from elder, corn-stalk, or milk-weed, in place of the
carbon. The _gold-leaf electroscope_ is a very delicate instrument. The
gold-leaf is supported, as suggested in Fig. 57, at the lower end of a
wire conductor which sticks through and hangs from the cork of a glass
jar or flask. To the top end of the wire is soldered a ball or disk.
The glass jar insulates the gold-leaf, and keeps it dry and free from
dust.
[Illustration: Fig. 57.]
=201. Our Leaf Electroscope= (Fig. 57) is made with
aluminum-leaf. Gold-leaf is too delicate for unskilful
handling, and aluminum will do for all ordinary experiments. To
cut it into any desired shape, place it between two sheets of
paper, then cut through paper and all.
=202. Construction.= Bend one leg of a hairpin, H P, as in Fig.
57, and slide it onto I T. Hang a wire, W, or another hair pin
straightened, then bent, from the horizontal leg of H P. This
is to support the "leaves," L, which are made from a strip of
aluminum-leaf about 4 in. long and 3/4 in. wide. Moisten the
under side of the horizontal part of W with paste or mucilage;
press it upon the middle of the strip laid flat upon the table,
and then lift W. The leaves should cling to W. Each leaf should
be, then, 2 in. long. They should hang close together when not
in use. A large chimney, or fruit-jar, may be used to surround
the leaves, and to keep currents of air from them. The leaves
should not touch the side of the jar when spread.
=EXPERIMENT 93. To study the leaf electroscope; charging by
conduction.=
_Apparatus._ The leaf electroscope (Fig. 57, § 201, 202);
ebonite rod, E R (No. 28); flannel cloth, F C (No. 30).
=203. Directions.= (A) Thoroughly charge E R, then scrape it
along upon I T, noting the action of the leaves, L.
(B) See if the leaves will remain spread for some time.
(C) Touch I T to discharge it, and note the action of L.
_=204. Discussion.=_ No explanation should be necessary for this. Are
the leaves charged alike? As they were charged by contact, is the
electrification on them + or -?
=EXPERIMENT 94. To charge the leaf electroscope by induction.=
_Apparatus._ Our electroscope (Fig. 57, § 202); ebonite sheet,
E S (No. 27); flannel cloth, F C (No. 30).
=205. Directions.= (A) Charge E S with F C, then hold E S above
I T (Fig. 57), their surfaces being kept parallel and about 2
or 3 inches apart. Watch the leaves.
(B) Withdraw E S. Do the leaves remain spread?
(C) Repeat (A), and before removing E S, touch I T.
(D) Remove your finger from I T, then withdraw E S. Do the
leaves now remain spread?
_=206. Discussion.=_ The permanent divergence of L was due to a charge
given by induction. (Exp. 76.) As E S was -, what was the kind of a
charge in L? Did any electrification go to the electroscope from E S?
In (C) what became of the charge in L? Explain why the leaves again
diverged in (D). The electroscope was charged with + electrification by
taking - out of it.
=EXPERIMENT 95. To learn some uses of the electroscope.=
_Apparatus._ Our electroscope (Fig. 57, § 202); ebonite rod,
E R (No. 28); ebonite sheet, E S (No. 27); glass, G (No. 38);
flannel cloth, F C (No. 30).
=207. Directions.= (A) With the charged E R charge the
electroscope negatively by conduction (Exp. 93). Note the
amount of permanent divergence of the leaves.
(B) Electrify the glass, which will be +, (or use the + E C),
and _slowly_ lower it over I T, noting the effect upon L. Raise
and lower G or E C several times. Does G, which has an opposite
charge to the electroscope, make L diverge more or less?
(C) Discharge the electroscope and recharge as in (A).
(D) Slowly lower the charged E S over I T.
(E) Slowly lower the palm of your hand over I T.
=Note.= If the + G is brought too near the -ly charged
electroscope, L will first collapse and then instantly diverge
again with a + charge by contact. The _first_ motions should be
observed.
_=208. Discussion.=_ As a neutral body causes a slight _collapse_ of
the leaves, as well as a body charged positively (when the charge in
the leaves is -), an increase of divergence is really the only sure
test to tell how a body is charged. The - leaves collapse when a + body
is brought near I T, because the - in them is drawn up towards the
body. The leaves diverge more when a - body is brought near, because
the - in I T is repelled into them.
_=209. The Proof-plane.=_ Since charges of static electricity reside
upon the outside of conductors, it is an easy matter to take samples of
the electrification. This may be done with a little instrument called
a carrier, or proof-plane. It consists of a small conductor with an
insulating handle. A ring or coin may be used for the conductor, and
a silk thread for the handle. By touching the carrier to any charged
body, it, also, becomes charged; and the nature of the charge may be
determined by the use of a previously charged leaf electroscope (Exp.
95). A delicate gold-leaf electroscope would be ruined by coming in
contact with a heavily charged body. The carrier allows a small sample
to be tested.
CHAPTER XII.
MISCELLANEOUS EXPERIMENTS.
[Illustration: Fig. 58.]
=EXPERIMENT 96. To show that friction always produces two kinds
of electrifications.=
_Apparatus._ Fig. 58. The carbon electroscope (Exp. 58);
flannel cloth, F C, doubled twice to make 4 thicknesses (see
Fig. 58); ebonite sheet, E S (No. 26); ebonite rod, E R (No.
28); charged electrophorus cover, E C.
=210. Directions.= (A) Vigorously rub E S with F C (folded as
in Fig. 58). See if you can discover any attraction between
them.
(B) Rub E S again, but do not lift F C from it with the hand
alone. Slip E R under the top fold in F C (Fig. 58), and lift F
C straight up from E S. Do not let F C touch the table or your
hand.
(C) See if F C is charged, using 2 or 3 different tests.
(D) Charge the electroscope with F C until the carbon is
strongly repelled.
(E) Bring the positively charged E C slowly near the carbon,
and note the result.
(F) Slowly bring the negatively charged E S near the carbon
that has been charged by contact with F C.
_=211. Discussion.=_ This experiment showed that while the ebonite was
negatively charged, the flannel was positively charged. One kind of
electrification is never produced without the other. It can also be
shown that the two kinds are equal in amount.
=EXPERIMENT 97. To show "successive sparks."=
_Apparatus._ Fig. 59. The electrophorus (Exp. 68); the extra
ebonite sheet, E S (No. 27); three coins (marked A, B, C, in
Fig. 59). The coins should nearly touch each other, and rest
upon E S. A part, only, of the electrophorus cover is shown.
=212. Directions.= (A) Thoroughly charge the electrophorus
cover.
(B) Place your finger upon the coin marked A, to "ground" it,
then quickly touch the coin C with the charged cover, at the
same time watching for sparks between the coins. If you cannot
see the sparks, darken the room a little, and look at the
center coin, B, while doing the experiment.
[Illustration: Fig. 59.]
[Illustration: Fig. 60.]
=EXPERIMENT 98. To show to the eye the strong attraction
between a charged and a neutral body.=
_Apparatus._ The flat box, F B (Nos. 40, 41); the electrophorus
(see Exp. 68).
=213. Directions.= (A) Stand F B upon its edge upon a level
table, then bring the charged electrophorus cover near it.
(B) Instead of the above, use light hoops made of paper,
eggshells, feathers, sawdust, etc.
=EXPERIMENT 99. To feel the strong attraction between a charged
and a neutral body.=
_Apparatus._ Fig. 60. The flat box F B (Nos. 40, 41); the
electrophorus (Exp. 68).
=214. Directions.= (A) Hold F B in the left hand, as shown,
then _slowly_ bring near it the charged cover, at the same
time looking between them so that you can keep them the same
distance apart all the way round.
(B) Bring them near enough to allow a spark to pass from E C to
F B.
=EXPERIMENT 100. The human body a frictional electric machine.=
_Apparatus._ Yourself; a carpet; a room with dry air, easily
had on a cold winter's day.
=215. Directions.= (A) Scrape your feet along upon the carpet,
then quickly touch your finger to some conductor, as, for
example, a friend's nose.
(B) It is possible to light the gas by the above process. Have
a friend turn on the gas just before you bring your finger to
the jet, and be sure that the spark from your finger passes
through the gas on its way to the conductor, the jet.
(C) Bring your finger quickly near a small piece of tissue
paper after you have scraped your feet along to charge your
body.
_=216. Static Electric Machines=_ are used to produce large quantities
of static electricity. In the early forms, the electrification was
produced by friction. Modern machines depend upon the principle of
induction. The electrophorus (Exp. 68) is really a very simple form
of induction machine. The potential of these machines is very great,
as the spark may jump many inches. Thousands of Galvanic cells would
be needed to make a spark an inch long. When the spark passes through
the air it meets with an extremely high resistance, as air practically
insulates ordinary electricity. This high resistance in the circuit
reduces the strength of the current. While the potential is very high,
the strength of the current is very low. (See Ohm's Law.)
CHAPTER XIII.
ATMOSPHERIC ELECTRICITY.
_=217. Atmospheric Electricity.=_ The air is generally electrified,
even in clear weather. Its charge is usually +. Clouds are sometimes +,
and sometimes -.
The cause of atmospheric electricity is not thoroughly understood. It
is thought, by some, to be due to the friction of the particles of
vapor upon each other. It is also thought that the evaporation of sea
water, and the friction of winds, produce it.
_=218. Lightning.=_ Benjamin Franklin, in 1752, proved by his famous
kite experiment that atmospheric and frictional electricities were of
the same nature. By means of a kite, the string being wet by the rain,
he succeeded, during a thunder-storm, in drawing sparks, charging
condensers, etc.
Lightning may be produced by the passage of electricity between clouds,
or between the cloud and the earth, which, with the intervening
air, have the effect of a condenser. If one cloud is charged, it
acts inductively upon another, producing in it the opposite kind of
electrification. When the attraction between the two electrifications
becomes great enough, it overcomes the resistance of the air, and
lightning is produced.
The flash is practically instantaneous. The direction taken seems to
depend upon the conditions of the surrounding air. It has generally a
zigzag motion, and is then called _chain lightning_. The air in the
path of the electricity becomes intensely heated; it is this effect,
and not the electricity which we see. In hot weather flashes are
often seen which light up whole clouds, no thunder being heard. This
is called _heat lightning_, and is generally considered to be due to
distant discharges, the light of which is reflected by the clouds.
The _potential_ of the lightning spark is beyond all calculation. The
spark jumps through miles of air, which is, really, an insulator. This
spark represents billions and billions of volts.
_=219. Thunder=_ is caused by the violent disturbances produced in the
atmosphere by lightning. The nature of the sound depends, among other
things, upon the distance of the observer from the discharge, and
upon the length and shape of the path taken. Clouds, hills, and other
objects produce echoes, which also modify the original sound. It takes
nearly five seconds for the sound to travel one mile.
_=220. Lightning-Rods=_, when well constructed, often prevent violent
discharges of atmospheric electricity. They have pointed prongs at the
top, which allow the negative of the earth (which is being attracted by
the cloud when it is positively charged) to pass quietly into the air
above; this neutralizes the cloud. In case of a discharge, the rods aid
in conducting the electricity to the earth.
_=221. Causes of Atmospheric Electricity.=_ There are several theories
about this. Some think that it is due to the rotation of the earth,
different parts being acted upon differently by the heat of the sun.
Some claim that the evaporation of the water in the ocean produces it,
while others say that the electrification is produced in the clouds by
the friction of their particles upon each other. The matter has not
been settled.
_=222. St. Elmo's Fire=_ Electrification from the earth is often drawn
up through the masts of ships to neutralize that in the clouds (see
§ 220), and, as it escapes from the points of the masts, light is
produced. This may be clearly shown by repeating Exp. 71 in the dark;
the head of the pin (Fig. 39) will represent the end of a mast, and the
charged electrophorus cover will be the charged cloud.
_=223. Aurora Borealis=_, also called Northern Lights, are luminous
effects often seen in the north. They often occur at the same time
with magnetic storms, at which times telegraph and telephone work may
be disturbed. The exact cause of this light is not known, but it is
thought by many to be due to disturbances in the earth's magnetism,
caused by the action of the sun.
CURRENT ELECTRICITY.
PART III.--CURRENT ELECTRICITY.
CHAPTER XIV.
CONSTRUCTION AND USE OF APPARATUS.
=Note.=--Before taking up the study of cells and the electric
current, let us perform a few experiments in order to
understand the construction and use of some of the apparatus
needed for such study. A dry cell will be used as the source
of the electricity for these first experiments, because it is
convenient. You will understand its action later. Use this cell
only as directed; improper use of it might spoil it.
[Illustration: Fig. 61.]
=EXPERIMENT 101. To study the effect of the electric current
upon the magnetic needle.=
_Apparatus._ A compass (No. 18); a dry cell, D C (No. 51);
wires with spring connectors attached (§ 226) for making
connections. Fig. 66 shows a plan or top view of the
arrangement. Any other form of cell will do in place of D C.
=226. Electrical Connections.= One must constantly join wires,
connect wires with apparatus, or connect one piece of apparatus
to another, to make the proper electrical connections. A very
simple method of connections has been used in all the apparatus
described in these experiments.
A little arrangement which we shall call a spring connector, S
C, (Fig. 61), gives us a means of quickly making connections;
that is, it does away with expensive binding-posts. It is made
of brass, nickel plated, and may be used anywhere without
affecting the magnetic needle.
Six or eight wires, about No. 24 gauge, each about 1-1/2 ft.
long, should be prepared with a connector at each end. You
may use wire furnished (No. 53). Scrape the insulation from
the ends of the wires for about 1-1/2 inches, then twist the
bare ends around the connectors as shown in Fig. 61. The wire
should pass around tightly at least 4 or 5 times and then be
twisted a little, as shown, to help tighten it. Do not put it
on so poorly or in such quantity that the part, B, will spread.
=227.= Fig. 62 shows how the connector should be slipped upon
a thin piece of metal, M, like that on the galvanoscope, for
example. The wire, W, from the apparatus itself is permanently
fastened under the head of the screw, S, while the wire from
any other apparatus is one of those kept on hand as above
mentioned and connected with S C.
[Illustration: Fig. 62.]
[Illustration: Fig. 63.]
[Illustration: Fig. 64.]
=228.= Fig. 63 shows how several wires may be quickly joined,
electrically, by slipping the connectors at their ends
upon a thin metal plate, M P, which may be a piece of tin,
zinc, copper, etc. M P should not be too thick. In case the
connectors become too much spread to pinch the plate, squeeze
the part, A, a little more together.
=229.= Fig. 64 shows the method of connecting with the special
form of binding-post used, for example, upon the resistance
coil, R C. The end, C, of S C, is pressed down into the tube,
T, until you feel, by moving it, that it springs firmly against
the sides of the tube. In case you wish S C to fit very tightly
in T, one of the legs may be slightly bent outwards.
[Illustration: Fig. 65.]
=230.= The connector may be used in still another way; that
is, by pushing the part, A, into the hole of an ordinary
binding-post, (Fig. 65), and using it just the same as a thick
wire.
=231. Directions.= (A) Stand the compass and D C near each
other (Fig. 66). Attach one end of an insulated copper wire, C
W, to the binding-post, C, which is on the carbon plate of the
cell. Do _not_ join the other end to the other binding-post,
Zn, of the zinc plate.
(B) With the left hand hold C W above and near the
compass-needle, and in the N and S line, so that it will extend
over the entire length of the needle.
(C) Take the free end of C W in the right hand and touch
binding-post, Zn, for an instant only, watching the needle.
Repeat.
_=232. Current Detectors.=_ We know that a magnet can act, by
induction, through the air upon a piece of iron or upon another magnet.
The deflection of the needle in this experiment shows that there must
be a magnetic field around a wire carrying a current. This fact is of
the greatest possible importance. The simple magnetic needle, when used
as above, becomes a detector of electricity.
[Illustration: Fig. 66.]
[Illustration: Fig. 67.]
=EXPERIMENT 102. To study the construction and use of a simple
"key."=
_Apparatus._ A key, K (No. 55) (§ 233); a dry cell, D C, (No.
51); a compass, O C (No. 18). _Arrange_ as shown in Fig. 67,
which is a top view or plan. Connect the pieces of apparatus
with wires and spring connectors (§ 226). Binding-post, C,
is joined to I (in) of the key; O (out) of key is joined to
binding-post, Zn, the wire, C W, passing directly over and near
O C, which is to be used as a detector. The current cannot pass
until the lever, L, is pressed. A metal plate, M P, is used to
connect two short wires (§ 228) in case C W is not long enough.
[Illustration: Fig. 68.]
=233. A key= is merely a piece of apparatus by which the
circuit can be conveniently and rapidly opened and closed at
the will of the operator; that is, by it the electricity can
be quickly turned on or off. Fig. 68 shows a simple form of
key. To the base, B, are fastened two metal pieces or straps,
the upper one, L, being the lever or key proper. The front end
of L is raised above O, so that the two do not touch each
other unless L is firmly pressed down. A screw, S, keeps L from
springing too far above O. For convenience we shall suppose
that the wire leading to the key joins it at I (in); the wire
_from_ the key is joined to O (out), by means of connectors (§
226).
The key may be put into any circuit by first cutting a wire and
then joining the ends to I and O. Spring connectors make the
best connections with this form of key. (For Home-Made Keys see
Apparatus Book.)
=234. Directions.= (A) The magnetic needle being directly under
the wire, press L down for an _instant only_ and note the
action of the needle.
(B) Press L again, hold it down for 3 seconds, not over that,
and watch the needle.
_=Discussion.=_ The key allows us to easily regulate the length of time
during which the current passes. This experiment shows, also, that the
magnetic field about the wire disappears as soon as the current ceases
to pass.
[Illustration: Fig. 69.]
=EXPERIMENT 103. To study the construction and use of a simple
"current reverser."=
_Apparatus._ A dry cell, D C (No. 51); a compass, O C (No. 18);
a current reverser, C R (No. 57) (See § 235); an insulated
copper wire, C W, 2 or 3 feet long, with spring connectors
joined to its ends (§ 226).
_Arrange_ as in Fig. 70. The wire, C W, leading from X should
be held by the left hand so that it will be just above (or
below) and parallel to the magnetic needle.
The current cannot pass through C W until one of the straps or
levers on C R is pressed. (See Apparatus Book for Home-Made
Reversers.)
=235. The Current Reverser.= (No. 57.) To the wooden base (Fig.
69) are fastened four metal straps, each turned up at the end
so that spring connectors (§ 227) can be slipped on to make
electric connections with other pieces of apparatus.
Suppose that at C and Z connections are made with the carbon
and zinc of the cell, by means of wires and spring connectors
(§ 226). The current comes from the cell to C. As the two
straps, 2 and 3, press firmly up against strap 4, and do not
touch 1, it is evident that no current can pass from 1 to 2 or
to 3 until they are pressed down upon 1. Two wires are joined
by spring connectors to 2 and 3 at their turned up ends, X and
Y, and these wires lead to any desired instrument.
[Illustration: Fig. 70.]
=236. Directions.= (A) Press down lever 2 (Fig. 70), for an
instant only, at the same time noting carefully in which
direction the N pole of the needle is deflected.
(B) After allowing lever 2 to spring up again, and after the
needle comes to rest, press down lever 3 for an instant,
watching the needle. Is the N pole of the needle deflected in
the same direction as it was in (A)?
_=237. Discussion.=_ The reverser gives us a quick and easy means of
reversing the current which is to pass through any desired instrument,
first in one direction and then in the opposite direction. Suppose
(Fig. 69) that the current enters C R at C, as it does when C is joined
to the carbon of the cell; the current can go no farther until one
lever is lowered. If lever 2 (Fig. 69) be now pressed down, as in part
(A), the current will pass along 2, which does not now touch 4, out
through X to a coil of wire or any instrument, and back to the reverser
by the wire joined to Y. It will then pass from 3 onto 4, to Z, and
back to the Cell; that is, the current enters C W at X. When lever
3 is pressed, the current still entering C R at C, the electricity
will pass onto 3 and out at Y, and back through X, 4 and Z to the
cell. The current, then, can be made to pass out of X or Y at will by
pressing the proper lever. This experiment also teaches something about
currents, but these will be discussed later.
[Illustration: Fig. 71.]
=EXPERIMENT 104. To study the simple current detector.=
_Apparatus._ The compass (No. 18); dry cell, D C (No. 51);
current reverser, C R (No. 57); copper wire, C W, a few feet
long, with spring connectors on its ends. (See Apparatus Book,
Chapter XIII, for Home-Made Detectors.)
=238. Directions.= (A) Join the ends of the wire to X and Y of
the reverser, C R, as in the last experiment. Coil up C W so
that you can hold the coil with your left hand, as shown in
Fig. 71, the magnetic needle being inside of it and parallel to
it.
(B) Press lever 2 of the reverser for an instant only. Is the
needle deflected more or less than it was when the wire simply
passed over or under it once?
(C) Reverse the current through C W by pressing lever 3, and
note the result.
(D) Get clearly in mind which way the N pole of the needle is
deflected when the current enters C W at X, also when it enters
at Y.
_=239. Discussion.=_ The current passed _over_ the needle in one
direction, and _under_ it in the opposite direction; that is, the part
of the wire above _helps_ that below. Each turn of the wire increases
the strength of the magnetic field about the coil, and helps to deflect
the needle. In this way, by increasing the number of turns, detectors
may be made that will show the presence of very weak currents. The
magnetic fields about wires and coils will be studied in a later
chapter.
[Illustration: Fig. 72.]
=EXPERIMENT 105. To study the construction and use of the
simple galvanoscope.=
_Apparatus._ The galvanoscope, G V, complete (No. 58),
described in § 240-246; dry cell, D C (No. 51); current
reverser, C R (No. 57) (§ 235); wires, with spring connectors,
to join the different pieces of apparatus (§226). (See
Apparatus Book, Chapter XIII, for Home-Made Galvanoscopes.)
=240. The Galvanoscope= (Fig. 72) is more than a mere detector
of electricity. With it we shall be able to study, more fully,
cells, currents, etc., etc. We must first understand its
construction.
=241. The Coil-support=, C S, is fastened to the cross-piece,
C P, on which are the 3 binding-posts or coil-ends, L, M and
R (left, middle, right). The legs, G L, are screwed to C P in
such a way that C P is held a little above the table: this
allows C S to be tipped to the front or rear to adjust it
vertically. On account of the peculiar arrangement of the legs,
the galvanoscope can be made to stand firmly, even upon uneven
surfaces. The screws holding G L should not be put in far
enough to tear the threads in the wood, C P.
=242. The Galvanoscope Coils=, G C, are two in number, both
being fastened to the coil-support, C S. The first coil has
five turns of wire, its ends being fastened to L and M; the
other coil has _ten_ turns, with ends at M and R. The current
can, at will, be made to pass through 5, 10 or 15 turns of wire
by making the proper connections.
Suppose that we have two wires direct from a cell, or from the
current reverser, with spring connectors on them so that we
can slip them onto L, M or R, which stand for left, middle or
right. When the wires are joined to L and M the current can
pass through but 5 turns; when joined to M and R it will go
through 10 turns; and when to L and R it will pass through the
entire 15 turns. When the current enters the galvanoscope at
L and passes out at M or R, it will pass through the turns of
wire from left to right, at the top; that is, it will pass in a
"clockwise" direction.
=243. The Compass-needle=, furnished with O C (No. 18), will
do also for this galvanoscope. It should be placed upon the
pin-point after fixing on the pointers (§ 246). The length of
the needle should be parallel to the plane of the coil when no
current passes; that is, the coil and coil-support should be
in the N and S line.
The needle can be centered in regard to right and left, and in
regard to up and down, by properly adjusting the position of
the pin-point support, P P S; this is held firmly to C S by two
spring-connectors. By removing S C, the support, P P S, may be
raised or lowered.
=244. To place the coil= in the N and S line, simply swing the
galvanoscope bodily around, at the same time looking down upon
the needle, until the length of the needle becomes parallel
to the coil-support. When once carefully adjusted N and S, a
line may be drawn upon the table as a guide for its position in
future experiments. The coil should stand in a vertical plane,
and this straight up and down position can be easily adjusted.
To place the coil in the E and W line, turn it until the
pointers are at the 90° (90 degree) marks,--the 0° (zero
degree) marks remaining, of course, as described above.
=245. The Degree-Card, G D C= (Fig. 72) has a dot at its
center, to show where to make a pin-hole for the pin that
supports the compass-needle. With this you can tell how many
degrees the needle is deflected when the current passes. This
card, G D C, should be pressed down over the pin-point. The
zero points of G D C should be N and S, also, when the coil
is in that position; that is, they should be in the plane of
the coil. The pointers on the needle (§ 246) will then be at
O, when the needle is at rest, no current passing through the
coils. (See Apparatus Book § 272 for Home-Made Degree-Card.) G
D C may be held permanently in position after it is adjusted,
by sticking a short pin through it into P P S. Do not let this
pin interfere, however, with the swinging of the needle.
[Illustration: Fig. 73.]
=246. Pointers= (Fig. 73) should be fastened to the needle,
in order to make the readings of degrees accurate. Fasten to
the compass-needle a piece of No. 30 insulated copper wire, as
shown. It may be cut to the proper length after it is wound
around the needle. See that the wire does not touch the pin
when needle is in place; balance needle by cutting a little
from the heavier end of wire with shears; bend the ends of wire
so that they are at opposite sides of the degree-card, both
pointing at O, for example. The needle must swing freely, be
nicely balanced, and the wire must not touch pin or degree-card.
[Illustration: Fig. 74.]
=247. Directions.= (A) Arrange as in Fig. 74, the coil being N
and S (§ 244). Join the ends of the wires, 2 and 3, with the
5-turn coil of G V as shown. Wire, 2, is connected to L (Fig.
72). Press lever 2 of C R (Fig. 69) for an instant, watching
the compass-needle and noting how many degrees it swings the
first time. Get thoroughly in mind the direction in which the N
pole of the needle is deflected when the current passes around
G C in a "clockwise" direction. There must be no magnets, iron,
or pieces of steel within 3 feet of A G.
(B) Press lever, 3, for an instant, watching the needle. The
current will now pass in an "anti-clockwise" direction. Is the
needle deflected about the same number of degrees as in (A)?
(C) Change the ends of the wires, 2 and 3, to the 10-turn coil
(§ 242) and repeat (A) and (B).
(D) Change 2 and 3 to L and R (Fig. 72), thus allowing the
current to pass around 15 turns; then repeat (A) and (B).
_=248. Discussion; True Readings.=_ Is not possible to get the magnetic
needle, M, exactly in the center of G C; the pointers will not exactly
be in the axis of M; the coils will not be exactly N and S: hence, if
you pass a certain current through the coil and the pointer reads 20
degrees, you will find, if you reverse the current, that the pointer
may read 24 degrees on the other side of the zero mark. To get the
_true reading_, average the two, in this case the average being 22
degrees.
The galvanoscope gives us an instrument with which we can study, more
fully, cells, currents, etc.
=249. Note of Caution.= It has already been stated that the
compass-needle should be in the center of the coil (§ 243),
and that the coil should be in the N and S line (§ 244). In
addition to the above, see that there are no magnets near G V,
when using it; tap G V occasionally to be sure that the needle
swings freely, hold the eye directly over the pointers when
reading degrees; the pointers should be at zero when no current
passes through G V; be sure that the electrical connections are
good.
There are several sources of error in taking readings, and in
all the quantitative experiments given. The author takes it for
granted that such errors will be looked out for by the teacher.
[Illustration: Fig. 75.]
=EXPERIMENT 106. To study the construction and use of a simple
astatic needle.=
_Apparatus._ Two unmagnetized sewing-needles (No. 1); horseshoe
magnet, H M (No. 16); piece of stiff paper doubled and cut as
in Fig. 75; a pin-point on which to support the paper. The pin
may be stuck through a cork, or that of O C (No. 18) may be
used.
=250. Directions.= (A) Draw each needle across the N pole of H
M five times from point to head (Exp. 9). This should make them
of nearly equal strength, both points being N poles.
(B) Stick the needles through the paper as shown, the N poles
being at the same end of the paper. Balance the paper upon the
pin-point. Has this combination a strong or weak pointing-power?
(C) Turn one of the needles end for end. Again test the
pointing-power.
_=251. Discussion; Astatic Needles.=_ By arranging the needles so
that their poles oppose each other, the pointing-power becomes almost
nothing. This sort of a needle is needed in some experiments in
electricity. Their magnetic fields are still retained. The combination
is called an _astatic needle_; it is used to detect very feeble
currents. The more nearly equal the magnets are in strength, the
better. They are usually arranged with one above the other (Fig. 76).
[Illustration: Fig. 76.]
[Illustration: Fig. 77.]
=EXPERIMENT 107. To study the construction and use of a simple
astatic galvanoscope.=
_Apparatus._ An astatic galvanoscope, A G (No. 59) (§ 252-254);
dry cell, D C (No. 51); current reverser, C R (No. 57) (§ 235);
wires for connections (§ 226).
_Arrange_ as shown in Fig. 80, which is a top view. The wires
from C R are connected to the binding-posts of A G at the back,
the spring connectors being slipped into them (§ 229).
=252. Construction of the Astatic Galvanoscope.= When not to be
used for a long time, or for shipping, the legs, A (Fig. 77)
may be removed, and the whole packed inside of the box, B.
The _Coil_, C, has a resistance of about 5 ohms, and is
fastened to the coil-support, C S. The ends of the coil are
permanently fastened to the binding-posts, L and R (left and
right). The ends are so arranged that when the current enters
at L it will pass around the coil in a clockwise direction.
=253. The Astatic Needle= (Exp. 106) is supported by a small
thread, T, which is tied to the thread-wire, T W. This T W
springs into an eyelet, E, which, in turn, rests in a hole
made in the end of B. E should turn easily in the hole, but it
should not wabble.
Fig. 78 shows a sectional view of the coil and needle. The
wire, W, should be bent, as shown, so that the magnets can be
as near the center-line of C as possible. Fig. 79 shows a front
view of the needle. As a matter of convenience it will be best
to arrange the poles of the needles, as shown, to agree with
the descriptions of the experiments.
To keep the needle from being affected by air currents, the
glass plate (No. 38) may be placed in front of the box, B.
Stand it upon the legs, A, and tie a string around it, and B,
to hold it in place.
[Illustration: Fig. 78.]
[Illustration: Fig. 79.]
=254. Adjusting the Needle.= As T is tied to T W, the needle
may be swung completely around by turning T W. This should be
done until the length of the needle is parallel to the turns
of C. The up and down position of the needle should be fixed
as nearly as possible when fastening T to T W, the exact place
being finally fixed by raising or lowering T W through E. The
spring in T W should hold it firmly in E after adjustment. The
wire, W, joining the needle-magnets should not touch the coil.
It may be made to just swing free from C by tilting the box
forward or backward a little. The construction of the legs,
etc., makes it possible to tilt the box, and to make it stand
firmly upon an irregular surface.
[Illustration: Fig. 80.]
=255. Directions.= (A) See that there are no magnets within
3 feet of A G. Test the astatic needle, after you have it
properly suspended, to convince yourself that it does not try
to swing around in a N and S line. In case the needle-magnets
have been in contact with other magnets, or are not equally
magnetized, remagnetize them as directed in Exp. 106. They must
remain in any position given them by turning T W. Finally,
bring them parallel to the turns of the coil. (See § 254.)
Arrange as in Fig. 80.
(B) Press lever 2 of C R (§ 235) for an instant only. This
allows the current to enter A G at L. Repeat several times
until you thoroughly fix in your mind the direction in which
the right-hand end of the needle is deflected. Does the needle
jump suddenly when the current passes?
(C) Press lever 3 for an instant only. Study the result.
_=256. Astatic Galvanoscopes.=_ It is evident that in the ordinary
current detector (Exp. 104), the pointing power of the needle has to be
overcome by the magnetic field about the coil, before the needle can be
forced from its N and S line. Very weak currents will not visibly move
the needle in ordinary detectors. To make a sensitive instrument we
must have strong fields about both the needle and coil, and we must, at
the same time, decrease the pointing power of the needle. Both of these
things are accomplished by using an _astatic needle_ in connection
with a coil containing considerable wire. The uses of the astatic
galvanoscope will be studied more fully in later experiments.
CHAPTER XV.
GALVANIC CELLS AND BATTERIES.
=EXPERIMENT 108. To study the effect of dilute sulphuric acid
upon carbon and various metals.=
_Apparatus._ A piece each of copper and iron wire 4 in., (10
cm.) long; two narrow strips of sheet zinc (No. 60), one being
amalgamated (§ 257); a carbon rod (No. 64); a tumbler (No. 65),
partly filled with dilute sulphuric acid, (§ 258); mercury (No.
52).
=257. To amalgamate= one of the above zinc strips is to coat
it with mercury. Remove all jewelry from the hands before
proceeding. Wash the zinc with water, and with a cloth remove
all dirt from its surface. Amalgamated zinc is very brittle,
so lay it flat upon a piece of board or upon a plate, after
dipping it for an instant in the dilute acid. Place a small
drop or two of mercury upon the strip, and rub the mercury
about upon both sides of the zinc with a cloth made wet with
the dilute acid.
Mercury clings strongly to zinc or tin, so you may use a narrow
piece of either as a spoon to carry a small drop from the
supply to the zinc. Tap it upon the zinc to dislodge the drops.
Do not get on too much mercury, just enough to coat it, or, at
least, that part of it that will be under the acid. Be careful
not to break the thin zinc when amalgamating it, as it gets
very brittle. It should look bright. (See Apparatus Book § 32,
33.)
_Note._ Should any mercury get upon copper plates it may be
removed by heating them in a flame.
=258. Dilute sulphuric acid=, for these experiments, should be
made by mixing 1 part, by volume, of concentrated acid, with
20 parts of water. Do not let the acid get upon clothes or
carpets. Do not add water to acid. Mix by _slowly_ adding the
acid to the water in a glass or earthen dish, stirring at the
same time. Mix over a sink or out of doors. (For fuller details
see App. Book; § 21, 22, 23, 24, 25.) To save time, make at
least a quart of the mixture, bottle, and label it for use.
=259. Directions.= (A) Bend over one end of each of the wires
and metal strips, and hang them upon the edge of the tumbler
(Fig. 81), so that their lower ends shall be in the acid. Do
not let them touch each other. Stand the carbon rod in the acid.
If there is no visible action upon any of the above substances,
add a few drops of concentrated acid to the tumbler.
Note carefully what takes place in the tumbler, and state which
of the substances are dissolved, which simply made brighter,
and which not acted upon at all.
_=260. Discussion.=_ The bubbles of gas that arise from the zinc when
it dissolves are hydrogen, and they indicate that a vigorous chemical
action is going on in the tumbler, and that the zinc is being eaten
away.
[Illustration: Fig. 81.]
[Illustration: Fig. 82.]
=EXPERIMENT 109. To study the effect of dilute sulphuric acid
upon various combinations of metals.=
_Apparatus._ The same as in the last experiment. A small piece
of amalgamated zinc, however, will be better than the whole
strip.
=261. Directions.= (A) Twist one end of the clean copper wire
around the small piece of amalgamated zinc (Fig. 82). Hold
one end of the wire in the hand and dip the combination into
the acid. What takes place? Watch the surface of the copper,
remembering that each, alone, was not acted upon by the acid
(Exp. 108).
(B) Use the clean iron wire in place of the copper wire, and
repeat (A). Watch the surface of the iron.
(C) With a string or thread tie a small piece of well
amalgamated zinc to the carbon rod (Fig. 82), then dip the
combination into the acid. Watch the surface of the carbon.
_=262. Discussion.=_ While amalgamated zinc is not rapidly dissolved
by dilute sulphuric acid, a vigorous action of some kind takes place
when it is in contact with another metal or with carbon in the acid.
The bubbles of hydrogen that are liberated do not seem to come from the
zinc; they appear to grow, in the fluid, directly at the surface of
the copper, iron, or other metal used with the zinc. This shows that
something besides the mere dissolving of a metal takes place.
Can we arrange our apparatus so that we can get some useful results
from this action?
=EXPERIMENT 110. To study the construction of a simple Voltaic
or Galvanic cell.=
_Apparatus._ A narrow strip of zinc (No. 60), amalgamated as
directed in § 257. (An amalgamated zinc rod (No. 74) may be
used in place of the strip); a narrow strip of sheet copper
(No. 67); the tumbler of dilute acid of Exp. 108; a flexible
copper wire about 2 feet long, with spring connectors (No. 54)
attached to its ends. (See Electrical Connections, § 226.)
[Illustration: Fig. 83.]
=263. Directions.= (A) Holding the amalgamated zinc strip in
one hand and the copper strip in the other (Fig. 83), dip them
into the acid, but do not let them touch each other. Note any
chemical action.
(B) Touch the copper and zinc together, _below_ the surface of
the acid. Watch the copper.
(C) Separate the lower ends of the strips, then touch them
together _above_ the acid. Anything still happen to the copper?
(D) Slip one spring connector with the attached wire upon the
zinc strip, then stand the strips in the tumbler, so that they
can not touch each other. Now touch the copper strip with the
free end of the wire, at the same time watching the copper.
(E) Raise the wire from Cu, touch it to Cu again, and repeat
several times until you are sure that something takes place
every time the wire touches Cu.
_=264. The Electric Current.=_ Something must happen in or through
the wire, and it can only happen when the two metals are joined in
some way. This peculiar, invisible action in the wire is called the
_electric current_, and such an arrangement is called a _Galvanic cell_.
_=265. Source of the Electrification.=_ When two different metals are
placed in acid they are electrified unequally by chemical action.
Each has a higher potential than the acid, but their potentials are
not the same. This electrification tends to pass from the place of
higher to the place of lower potential, and the conducting wire allows
this transfer to take place. As the difference of potential is kept
up by the continued chemical action, the current is continuous, and
not simply instantaneous, as in discharges of frictional electricity.
As heat is produced by the burning of coal, so electrification is
produced by the chemical burning of zinc. Chemical energy is the source
of electrification in the Galvanic cell, just as muscular energy was
the source of the electrification in the experiments with frictional
electricity.
_=266. The Electric Circuit; Open and Closed Circuits.=_ The simple
Galvanic cell just used, together with the wire which joined the metal
strips, is called an _electric circuit_. If the current should pass
through a telegraph instrument, for example, on its way from one strip
to the other, the telegraph instrument would also be in the circuit.
When the wire is cut or removed from one metal strip, the circuit is
said to be _open_--that is, we have an _open circuit_. When the current
passes, the circuit is _closed_. We also say _make_ and _break_ the
circuit, and that the circuit has been _broken_.
_=267. Plates or Elements.=_ The copper and zinc strips are called the
_plates or elements_ of the cell. The zinc, Zn, Fig. 84, is dissolved
by the acid, and is called the positive plate (+ plate). The copper,
Cu, is the negative plate (- plate). The copper is also called the
_cathode_, and the zinc the _anode_.
_=268. Direction of Current.=_ It has been agreed, for convenience,
that _in_ the cell the current passes from the zinc through the liquid
to the copper, where the hydrogen bubbles are deposited. It then passes
through the wire, or other conductor furnished, back to the zinc,
through the liquid to Cu again, and so on around and around thousands
of times per second. The current really starts at the surface of the
zinc, where the chemical action is. When carbon and zinc are used, the
action and direction of the current are the same, carbon being the -
plate.
[Illustration: Fig. 84.]
_=269. Poles or Electrodes.=_ If the wire were cut, the electricity
coming from the + plate would be stopped at the end of the wire marked
+, Fig. 84, after passing through the acid and up Cu. This end of the
wire is called the + _pole or electrode_ (positive). The end of the
wire joined to Zn is called the - _pole_ or _electrode_; that is, the +
electrode is the end of the wire attached to the - plate. The tops of
Cu and Zn may be considered electrodes. The top of Cu is the + _pole_
of the cell, while Cu is the - _plate_.
=270. Chemical Action in the Simple Galvanic Cell.= The
chemical formula of sulphuric acid is H_{2}SO_{4} (read H,
2, S, O, 4). This means that it is a compound of hydrogen
(H), sulphur (S), and oxygen (O). The H_{2}SO_{4} stands
for _molecule_ of acid, and the small figures show that the
molecule is made up of 2 _atoms_ of hydrogen (H_{2}), one
of sulphur (S), and 4 of oxygen (O_{4}). The atoms are held
together by _chemical attraction_ or _affinity_.
There is a stronger chemical affinity between zinc (Zn) and
SO_{4} than between H_{2} and SO_{4}; so, as soon as the Zn
gets a chance, as it does in the cell, it drives out the
H_{2}, and it takes its place in the molecule. This chemical
_reaction_ may be shown by the following chemical _equation_:
Zn + H_{2}SO_4 = ZnSO_4 + H_2.
Zinc and sulphuric acid produce zinc sulphate and hydrogen.
The zinc sulphate produced weakens the effect of the acid; in
fact, the acid has to be renewed occasionally, as it is changed
to the sulphate which remains in solution.
=271. Action in Cell Using Impure Zinc.= The above action
takes place in the cell when impure zinc is used, even when no
current passes, heat being produced by the reaction instead of
useful electricity. (See Local Currents.)
=Action in Cell Using Pure Zinc.= When pure zinc (or impure
zinc that has been properly amalgamated) is used in the cell,
it dissolves, or is eaten away, only when the current passes.
It should be noted that the bubbles of hydrogen do not even
then appear at the zinc; they are not seen throughout the body
of the liquid. There seems to be an unseen transfer of hydrogen
from the zinc, through the liquid, to the copper (or other -
plate used), and it appears there in the shape of bubbles. The
larger the quantity of pure zinc dissolved, the stronger the
current, and the larger the amount of hydrogen produced.
As the zinc dissolves it parts with its latent energy, and this
energy forces the electric current through the circuit. While
the hydrogen of the decomposed acid makes its way towards the -
plate, the SO_4 part of it travels towards the Zn plate, where
the ZNSO_4 is formed. (See § 270.)
=EXPERIMENT 111. To see what is meant by "local currents" in
the cell.=
_Apparatus._ Tumbler of dilute sulphuric acid. (§ 258); strip
of unamalgamated zinc; crystal of copper sulphate (blue
vitriol) (No. 86); a galvanized iron nail (No. 69), this being
iron covered with zinc.
=272. Directions.= (A) Hold the nail in the acid for a few
seconds, and note result.
(B) Rub the copper sulphate upon the zinc simply in one spot,
then place the zinc in the acid, noting the result at the spot.
_=273. Local action; Local Currents.=_ Ordinary commercial zinc
contains such impurities as carbon, iron, lead, etc., in small
quantities. It was seen, Exp. 109, that when different metals were in
contact with the zinc, the zinc was rapidly dissolved by the acid. The
impurities in the zinc act like the copper plate in the simple cell,
thus producing _local currents_ in the zinc, which rapidly destroy it
without doing any good. These currents pass from zinc to impurities,
and back to the zinc, without going out into the main wire. This local
action takes place even when the main circuit is open.
_=274. Reasons for Amalgamating Zinc Plates.=_ Pure zinc is not
affected by dilute sulphuric acid, but it is too expensive to use in
cells; so amalgamated zinc is used instead, because it is cheaper, and
acts the same as pure zinc. The impurities are removed from the surface
of the zinc, as the zinc alone is dissolved by the mercury. There is,
then, a liquid layer of pure zinc with mercury upon the surface of
the amalgamated plate. This is not acted upon by the acid when the
circuit is open. A stronger and more regular current is produced with
amalgamated zinc than with the impure unamalgamated zinc.
=EXPERIMENT 112. To study the "single-fluid" Galvanic cell.=
_Apparatus._ The galvanoscope G V (No. 58), (See § 240, etc.);
the simple cell arranged as described in § 275, the zinc being
amalgamated.
=275. The Simple Cell= should be arranged so that the plates
will be held firmly in position. The zinc, Zn (No. 60),
and copper, Cu (No. 67), should be fastened to the wooden
cross-piece, W C P (No. 70), as shown in Fig. 85. Care should
be taken not to use longer screws than those provided for (No.
72). If the screws touch each other they will short circuit the
cell. Partly fill the tumbler (No. 65) with dilute sulphuric
acid (§ 258), join wires with connectors to the plates. The
free ends of the wires are then ready to join to apparatus. The
ends of wires _may_ be fastened under the screw-heads instead
of using connectors on the plates. Do not put the plates into
the acid until you read the "directions."
=276. Directions.= (A) Arrange as in Fig. 86. Place the coil
of G V, N and S (§ 244). _Before_ putting the plates in
the acid join them to the 15-turn coil of G V (§ 242). The
compass-needle should point to zero. See that the needle swings
freely.
(B) Place the plates in the acid, and _quickly_ bring the
needle to rest with the aid of the hand, so that you can take
the reading at once before the hydrogen bubbles entirely
cover the copper plate. Watch the action of the needle for a
few minutes. Make a note of the reading, in degrees, at the
beginning of the experiment and at the end of five minutes.
(See Note.)
[Illustration: Fig. 85.]
[Illustration: Fig. 86.]
=Note.= If no change takes place in the position of the needle,
the change beginning inside of 10 seconds after the plates are
let down into the acid, withdraw the plates, then clean and
thoroughly dry the copper to remove all traces of hydrogen.
This may be done by heating the copper over a gas flame. Let
the copper remain in the air 15 minutes, then try again. In
taking the first reading you must work quickly. Catch the
needle during its _first_ swing. If you allow it to swing back
and forth until it comes to rest, your first reading will not
be what it should be.
(C) After the needle has remained in one position, without
change, for 2 or 3 minutes, take hold of the wooden
cross-piece, move the plates back and forth in the acid to
dislodge the hydrogen bubbles, and note carefully the action of
the needle. Does the current seem stronger when the plates are
moved? Can you get the needle back to the first reading?
(D) Remove the plates from the acid, dry and clean the copper,
let them stand in the air for 15 minutes, then take another
quick reading and compare it with the first.
_=278. Polarization of Cells.=_ The acid gets a little weaker, of
course, as it is decomposed by the zinc (§ 270), but the chief cause of
the weakening of the current is hydrogen, which forms a filmy coating
upon the copper plate. This coating even seems to soak into the copper,
and it takes some time for it to be thoroughly removed. The zinc plate
is kept comparatively free from hydrogen by amalgamation.
_=279. Effects of Polarization.=_ The hydrogen bubbles weaken the
current in at least two ways. In the _first_ place, hydrogen is not a
conductor of electricity; so it holds the current back, as any other
resistance would.
_Secondly_, acid acts upon hydrogen as it would upon another metal.
When the copper plate becomes well covered with hydrogen, the acid
cannot touch it; so we really have a _hydrogen plate_ in the cell.
Hydrogen acts very much like zinc in the acid; we say that it is more
electro-positive than copper. The result is, then, that a new current
starts up, and as this is towards the zinc, in the acid, it partially
destroys or neutralizes the main zinc-to-copper current. Practical use
is made of the principles of polarization (see Secondary Batteries).
_=280. Remedies for Polarization; Depolarizers.=_ Any scheme by which
the hydrogen may be destroyed and kept from the inactive, or negative
plate, will prevent polarization. _Mechanical_ means have been employed
to brush away the hydrogen by keeping up a constant circulation of
the liquid. _Chemical action_ is another means by which the hydrogen
may be side-tracked before it gets to the - plate in single-fluid
cells. Substances like nitric acid and bichromate of potash, called
_depolarizers_, contain large quantities of oxygen, and, during
the chemical changes that take place, this oxygen unites with the
hydrogen. These substances are used in zinc-carbon cells. (See § 286,
etc. for various forms of cells.)
There is another form of cell, the _two-fluid_ type, in which
_electro-chemical_ means are employed, and in which a metal is
deposited upon the - plate instead of hydrogen. The - plate is usually
copper, copper being deposited upon it.
=EXPERIMENT 113. To study the "two-fluid" Galvanic cell.=
_Apparatus._ The glass tumbler, G T, (No. 65); porous cup, P C,
(No. 73); the strip of zinc (No. 60), well amalgamated (§ 257),
or the amalgamated zinc rod (No. 74); piece of sheet copper
(No. 75), bent so that it will surround P C; copper wires, C
W, with connectors; a saturated solution of copper sulphate,
commonly called blue vitriol or blue stone (See § 283);
dilute sulphuric acid (See § 258). With the above, set up the
two-fluid cell (See § 281). The galvanoscope, G V, complete,
is also needed, and if quantitative work is to be done, a pair
of scales weighing to 0.1 gram is necessary. (See App. Book,
Chapter I, for Home-Made Two-Fluid Cells.)
[Illustration: Fig. 87.]
=281. Setting Up the Two-Fluid Cell.= Fig. 87. Stand the
amalgamated zinc rod, Zn, in P C, then place P C in the
tumbler, G T; put in the copper plate as shown. Pour diluted
acid (§ 258) into P C until it stands about 2-1/2 in. deep;
then at once pour the copper solution (§ 283) into G T, on the
outside of P C, until it stands at the same height as the acid
_in_ P C. As s
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