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                         _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 soon as the liquids have soaked into P C the cell
    will be ready for use; but it is better to connect it with the
    galvanoscope at once and note the increase of current during
    the first few minutes while the liquids soak through P C. A
    crystal of copper sulphate should be put outside of P C to keep
    the solution full strength. This is a form of the well-known
    Daniell cell. Fig. 87 shows a form of home-made two-fluid cell
    as described in Apparatus Book. If you have the one furnished,
    use the rod instead of sheet zinc, and use connectors on the
    plates.

    =282. Care of Two-Fluid Cell.= This experimental cell should
    be taken apart when not in use. It should not be left in open
    circuit, even for half an hour. Even after the plates are
    removed, copper may be deposited upon P C in case there are
    any metallic impurities on it. Remove the plates and P C, and
    thoroughly wash them. The copper solution should be put into a
    bottle to prevent evaporation; the dilute acid may be thrown
    away to be replaced by fresh acid for the next experiment.

    =283. Copper Sulphate Solution= is made by adding the blue
    crystals to water until no more will dissolve--that is, the
    solution should be "saturated," extra crystals always being
    left in the stock bottle. An ounce of the crystals to half a
    tumbler of water will be about right, but a pint or so should
    be made at a time and be kept bottled to save time.

    =284. Directions.= (A) In case you have access to a pair of
    scales that weigh to within 0.1 gram, carefully weigh both
    copper and zinc before proceeding. They should be washed and
    dried with a cloth. See that there are no drops of mercury upon
    the zinc that may fall off during the experiment.

    (B) Replace the plates in the cell, and connect them with the
    15-turn coil of G V, placed N and S. Allow the circuit to
    remain closed for half an hour, and record the position of the
    needle every 5 minutes.

    (C) Again wash, dry with a cloth without rubbing, and weigh
    both the zinc and copper. Compare the new weights with those
    found in (A).


_=285. Chemical Action in the Two-Fluid Cell.=_ In this form of cell
the hydrogen is not allowed to collect upon the copper plate. The
action inside of P C is like that explained in § 270, hydrogen being
set free. As soon as this hydrogen (H_{2}) comes in contact with the
copper sulphate (CuSO_{4}), and it begins to do this in the walls of
P C, a new chemical reaction takes place. Hydrogen has a stronger
attraction for SO_{4} than Cu has, so it unites with the SO_{4},
forming H_{2}SO_{4} (sulphuric acid), and this at the same time throws
out the Cu bodily. This Cu is then free, instead of hydrogen, to be
deposited upon the copper plate. The current is constant, as there is
no polarization.


_=286. Various Galvanic Cells; Open and Closed Circuit Cells.=_ There
is no one form of cell that is best for all kinds of work. If momentary
currents only are wanted, such as for bells, telephones, etc., in which
the cell has plenty of time to rest, _open circuit_ cells are used.
These cells polarize, however, when the circuit has to be closed for
any length of time. This form of cell is always ready for use, and may
not need attention for months at a time. The most common forms of the
open circuit cells are the _Leclanché_ (§ 287) and _dry_ cell (§ 288).
Open circuit cells polarize quickly, because the depolarizer (§ 280) is
slow in destroying the hydrogen.

When a strong current is needed for a considerable time, such as for
telegraph lines, motors, etc., a _closed circuit_ cell is necessary.
The depolarization must be rapid and constant. The _bichromate_ (§ 289)
and the _Daniell_ cell (§ 290) are very common forms of closed circuit
cells. (See, also, Storage Cells.)


    =287. The Leclanché Cell= is an open circuit cell in which
    carbon and zinc are the plates. The carbon is surrounded
    with dioxide of manganese, a depolarizer; the two are either
    packed together in a porous cup, or the latter is compressed
    into blocks, which are fastened to the carbon. The porous cup
    stands in a jar containing a solution of sal ammoniac (ammonium
    chloride), which acts as the exciting fluid, and in which
    stands a zinc rod. The zinc is not acted upon when the circuit
    is open. The hydrogen given off by the decomposition of the
    ammonium chloride is destroyed by the oxygen contained in the
    manganese dioxide. The E. M. F. is nearly 1.5 volts.

    =288. Dry Cells= are for open circuit work. Sheet zinc forms
    at the same time the active plate and the outside cylinder
    or case. A carbon plate acts as the inactive or - plate. The
    exciting fluid is kept from spilling by its being absorbed by
    one of the various substances used for that purpose.

    =289. The Bichromate of Potash Cell= is a very common one
    for laboratory use. It gives a strong current, and although
    a single fluid cell, it does not readily polarize. Zinc and
    carbon plates are used. In the sulphuric acid, which is the
    exciting fluid, is dissolved bichromate of potash. This cell
    is used for running small motors, induction coils, etc. It
    has, with fresh solutions, an E. M. F. of about 2 volts. (See
    Apparatus Book, Chapter I., for Home-Made Batteries.)

    =290. The Daniell Cell=, of which the two-fluid cell used in Exp.
    112 is a form, is noted for its constant current. The E. M. F.
    is a little over 1 volt, and it should be kept working through
    a resistance when not in regular use; it should not be left in
    open circuit. The porous cup keeps the two fluids from mixing,
    but it does not stop the current.

    =291. The Gravity Cell= is a form of the above. As one of the
    fluids is heavier than the other, no porous cup is needed.
    Gravity, together with the action of the current, tends to
    keep the fluids separated. A copper plate is placed in the
    bottom of the jar, and upon this is put the copper sulphate
    solution. The zinc plate is supported by the top of the jar and
    rests in a solution of zinc sulphate, which is lighter than
    the blue solution below. An insulated wire extends from the
    copper through the liquids. This cell is used for telegraph and
    similar work. (See Apparatus Book for Home-Made Gravity Cell,
    its Regulation, etc.)




CHAPTER XVI.

THE ELECTRIC CIRCUIT.


    =EXPERIMENT 114. To see what is meant by "divided circuits" or
    "shunts."=

    _Apparatus._ The galvanoscope, G V (No. 58); astatic
    galvanoscope, A G (No. 59); two-fluid cell, 2-F C (see § 281);
    6 wires with connectors; small thin pieces of tin or other
    metal, M P, for rapidly making connections (§ 226). _Arrange_
    as in Fig. 88. The wires, 1 and 4, from 2-F C, lead to the
    metal plates M P-A and M P-B, for convenience. The wires, 2 and
    3, from G V, are also connected with these plates. The wires,
    5 and 6 (dotted lines), lead from A G, to be used as directed
    in part (B) of the experiment. See that G V is properly placed.
    See that A G is adjusted.

[Illustration: Fig. 88.]

    =292. Directions.= (A) Without A G in place, take the reading
    of G V. The current now passes from Cu through 1, M P-B, 2, G
    V, 3, M P-A, 4 to Zn.

    (B) Connect wires 5 and 6 to the plates, as shown by the dotted
    lines. Again take reading of G V, and compare it with the first
    reading. Does some of the current pass through A G?


_=293. Divided Circuits; Shunts.=_ The current divides at M P-B into
two parts; one part may be called a _shunt_ of the other. The circuit
is said to be _divided_; it has two branches. If the two ends of a wire
be fastened to another as in Fig. 101, the circuit is also divided.
When two or more conductors lead side by side from one point to
another, they are called _parallel_ circuits; that is, the conductors
are joined in parallel.

As strong currents would injure delicate galvanometers, a small part
only of the current may be allowed to pass through the galvanometer by
using a shunt. Fig. 89 shows such an arrangement, in which most of the
current passes through the shunt, S. There are many practical uses of
shunts.


[Illustration: Fig. 89.]

    =EXPERIMENT 115. To see what is meant by "short circuits."=

    _Apparatus._ About the same as in Exp. 114, Fig. 88. The
    astatic galvanoscope is not needed; in place of it provide
    a short piece of metal, such as a battery-plate, or even a
    jack-knife. _Arrange_ as in Fig. 88, but without A G.

    =294. Directions.= (A) With the current passing as described in
    Exp. 114 (A), take the reading of G V.

    (B) Lay the ends of the metal, or other thick conductor, upon M
    P-A and M P-B. Compare the new reading of G V with that in part
    (A).

    (C) Remove the conductor used to short circuit G V, take the
    reading in degrees, then touch M P-A to M P-B; watch G V.


_=295. Short Circuits=_ are very apt to occur unless care is taken. Do
not allow uninsulated wires to touch each other. As shown by the above
experiment, practically the whole of the current may be side-tracked
by a _shunt of low resistance_. A galvanic cell is short-circuited by
connecting the plates directly by a wire or other conductor.




CHAPTER XVII.

ELECTROMOTIVE FORCE.


_=296. Electromotive Force.=_ It has been stated that a galvanic cell
has the _power_ to charge one of its plates positively and the other
negatively; this power is called _electromotive force_, and, for short,
E. M. F. is written. The E. M. F. of a cell depends upon the kinds of
plates used and their condition, the chemicals used in the exciting
fluids, etc. The greater the E. M. F. of a cell the greater its power
to force the current through wires, etc. The E. M. F. of a cell does
not depend upon the size of its plates, as will be seen by later
experiments.


_=297. Unit of E. M. F.; The Volt.=_ A certain amount of E. M. F. has
been taken as the standard, and, in honor of Volta, it has been called
the volt. The E. M. F. of the two-fluid cell used in Exp. 113 is not
far from 1 volt. If a certain cell has the power to keep up twice the
difference of potential between its terminals that the Daniell cell
has, we say that it has an E. M. F. of about 2 volts.

_=Voltmeters=_ are instruments to measure E. M. F.


    =EXPERIMENT 116. To see if the E. M. F. of a cell depends upon
    the materials used in its construction.=

    _Apparatus._ Tumbler two-thirds full of dilute sulphuric acid
    (258); strips of zinc, Zn (No. 60); copper strips, Cu (No. 67);
    iron strip, I (No. 76); lead strip, L (No. 77); carbon rod (No.
    64); the galvanoscope, G V (No. 58); 2 wires with connectors
    (§ 226), so that the plates can be changed quickly; the wooden
    cross-piece, W C P (No. 70).

    _Arrange_ as in Fig. 90. The metal strips are all of the same
    size; they may be held with the hand firmly against W C P, in
    order to have them the same distance apart in each trial. They
    should be lowered to the bottom of the tumbler in each case, in
    order to have them acted upon by the same amount of acid. Place
    G V properly.

[Illustration: Fig. 90.]

    =298. Directions.= (A) With Zn and Cu connected to G V as shown
    (Fig. 90), take the reading in degrees, and note in which
    direction (east or west) the N end of the needle is deflected.
    Tabulate results, as shown in Fig. 91, filling in each column
    of your table made out on paper.

    (B) In like manner try the following combinations in the order
    given, in each case connecting the first-mentioned plate with
    the left-hand binding-post, L, of G V. For (B) use zinc-iron.

    (C) Use zinc-lead; (D) iron-copper; (E) iron-lead; (F)
    lead-copper; (G) copper-carbon.

[Illustration:

  +------+---------------+------------+-------------+------------+
  | PART |    PLATES.    |   LIQUID.  | DEFLECTION. | CURRENT IN |
  |      |               |            |             | CELL FROM  |
  +------+-------+-------+------------+-------------+------------+
  | (A)  | ZINC. |COPPER.| DIL. SULP. |  65° WEST.  |  CU TO ZN  |
  |      |       |       |    ACID.   |             |            |
  | (B)  |       |       |            |             |            |
  |      |       |       |            |             |            |
  | (C)  |       |       |            |             |            |

Fig. 91.]

    =299. Note.= Some of the combinations produce but slight
    currents. In case G V is not delicate enough to show clearly
    which way the current passes, use the astatic galvanoscope in
    its place for such combinations.


_=300. Discussion.=_ Exp. 116 clearly showed that different
combinations of metals in the acid have different powers of pushing
electricity through the galvanoscope. Although some of the pairs of
metals furnished so weak a current that it was necessary to use the
astatic galvanoscope to study the current, all produced _some_ current,
and from the results can be formed an electromotive series (§ 301). The
strength of acid, condition of plates, etc., affect the E. M. F. of a
cell.

    =301. Electromotive Series.= All metals are not acted upon to
    the same degree by dilute acid. From the results of Exp. 116
    it is seen, part (B), that iron is electronegative to zinc;
    that is, the current in the cell flows from zinc to iron.
    Part (D) showed that iron is electropositive to copper, as the
    current flowed from iron to copper in the cell. It is possible
    to arrange the metals in a series, one below the other, in such
    a way that any one will be electronegative to those above it
    and electropositive to those below it; that is, the list should
    have the most electropositive metal at the top, and the one
    least acted upon by the acid at the bottom. Make such a list
    from your results. The farther the metals used are apart in
    the _list_, the greater will be the E. M. F. of the cell. Good
    carbon is acted upon the least of all, so zinc and carbon are
    better than zinc and copper.


    =EXPERIMENT 117. To see whether the electromotive force of a
    cell depends upon its size.=

    _Apparatus._ Galvanoscope; two glass tumblers; dilute acid; two
    wooden cross-pieces; two copper and two zinc strips, the same
    size as those used for Exp. 112. (See § 275). These materials
    will form two simple cells like Fig. 85. Have about 3 in. of
    acid in one tumbler, and but 1 in. in the other. The plates of
    one cell will then be about 2-1/2 in. in acid, and those of the
    other cell only 1/2 in. in acid. This gives us the same effect
    as a large and a small cell.

    =302. Directions.= (A) Join the large and small cells with G V
    so that their currents will oppose each other. To do this, join
    the two zinc plates by means of a wire and connectors. With two
    other wires connect the two copper plates with the galvanoscope
    binding-posts, and watch for any indication of current. Does
    one cell oppose the other?


_=303. Discussion.=_ The E. M. F. of a cell, then, does not depend upon
the size of its plates. The small piece of zinc--that is, the one in
but little acid--had the same potential as the large piece; they must
have had, as they were joined. The large cell will give a stronger
current, under certain conditions, than the small one; but this
depends upon other things than E. M. F. (See experiments under Current
Strength.) A zinc-copper cell, like the one just used (Exp. 117), has
the same _voltage_ as one of the same kind would have, even though it
were made as large as a barrel.




CHAPTER XVIII.

ELECTRICAL RESISTANCE.


_=305. Resistance.=_ It is harder for a horse to draw a wagon through
deep sand than over a smooth pavement. We may say that the sand holds
the horse back--that is, it offers a resistance. The electric current
does not pass through all sorts of substances with the same ease, and
when it succeeds in pushing its way through a circuit of considerable
resistance, we cannot expect it to arrive at the end of its journey
without being weaker than when it started. Do we expect this of a man
or horse? We shall soon see that there is a definite relation between
resistance and the strength of the current at the end of its journey.


    =EXPERIMENT 118. To study the general effect of "resistance"
    upon a current.=

    _Apparatus._ Galvanoscope, G V (No. 58); resistance coil, R
    C (No. 79) (§ 310); two-fluid cell, 2-F C (§ 281); 4 wires
    with connectors (§ 226). _Arrange_ as in Fig. 92. The current
    passes as shown by the arrow, and the circuit may be opened and
    closed at the metal plate, M P, or by using a key in its place.
    Properly place G V.

    =306. Directions.= (A) Take the reading of G V in degrees, the
    current passing through the entire length of R C. (See § 310.)

    (B) Change the end of wire 4 from binding-post R to M, on R C,
    so that the current will pass through one-half only of R C.
    Note the reading of G V.

    (C) Remove R C entirely and connect wires 3 and 4 by means of a
    metal plate. Compare the readings of (A), (B) and (C). What do
    they show?

[Illustration: Fig. 92.]

_=307. External Resistance; Internal Resistance.=_ When we consider
a circuit like that shown in Fig. 92 we see that it is composed of
two parts, and that we have two kinds of resistances. The wires,
instruments, etc., make up what is called the _external resistance_
of the circuit; that is, the part that is external to the cell. The
liquids in the cell offer a resistance to the current; this is called
_internal resistance_. (See § 314.) The strength of the current depends
upon the relation between these two resistances, as will be seen by
future experiments, as well as upon the E. M. F. of the cell. As
liquids are not as good conductors as metals, the internal resistance
of cells may be quite high.


_=308. Unit of Resistance; The Ohm.=_ Whenever anything is to be
measured, a standard, or unit, is necessary. The unit of resistance is
called the ohm, in honor of Ohm, who made careful investigations upon
this subject. A column of mercury having a length of a little over 3
feet has been taken as a unit. (The column taken is 106.3 cm. with a
weight of 14.4521 grams; it has a cross-section of about 1 sq. mm.,
at a temperature of 0°C.) Mercury is a liquid, and has no "grain" to
affect the resistance. For the use of students, 9 ft. 9 in. of No. 30
copper wire, or 39 ft. 1 in. of No. 24 copper wire will make a fairly
good ohm. We might, of course, take any other length as _our_ standard;
the above, however, will give results that are approximately correct.
(See wire tables at the end of this book.)


_=309. Resistance Coils; Resistance Boxes.=_ Coils of wire, having
carefully-measured resistances, are called _resistance coils_. The wire
for any coil is doubled at the center before it is wound into coils or
upon spools (Fig. 93) to avoid the magnetic effect. The ends of the
coils are attached to binding-posts, or to brass blocks, in regular
instruments, so that one or more coils can be used at a time; that is,
so that they may be handled in a manner similar to that in which the
different coils on the galvanoscope are used.

If we have 4 coils of 1, 2, 2, and 5 ohms resistance, we shall be
able to use any number of ohms from 1 to 10 by making the proper
connections. (See Apparatus Book, chapter XVII, for Home-made
Resistance Coils.)

For protection and convenience, coils are usually placed in a box, the
whole being called a _resistance box_. The ends of the coils are joined
to brass blocks, placed near each other on the top of the box, and
between which may be pressed plugs when it is desired to short circuit
the coils. By removing a plug, the coil, whose ends are joined to the
blocks touching it, is brought into the circuit.


[Illustration: Fig. 93.]

[Illustration: Fig. 94.]

    =310. Simple Resistance Coil.= Fig. 94 shows a simple form of
    coil, R C (No. 79). The total resistance is 2 ohms, L (left)
    and R (right) being binding-posts to which the ends of the
    coil, C, are joined. M (middle) connects with the middle of the
    wire, at which point the wire is doubled. The coil is fastened
    to a stiff pasteboard base, B.

    =Connections.= When 2 ohms resistance are wanted, let the
    current enter at L and leave at R (or the reverse). When 1 ohm
    is wanted, let the current leave or enter at M, the other wire
    being joined to L or to R. Connections should be made with
    spring connectors. See § 229.


    =EXPERIMENT 119. To test the power of various substances to
    conduct galvanic electricity.=

    _Apparatus._ Galvanoscope, G V (No. 58); dry cell, D C, or
    two-fluid cell, 2-F C; pieces of different metals; wood,
    dry and damp; tumbler of pure water; rubber; ebonite; silk;
    glass, etc., etc. _Arrange_ as in Fig. 92, leaving out R C,
    and instead of having M P between wires 1 and 2, use their
    free ends to press firmly upon the ends of the substance to be
    tested; that is, the body under test should take the place of M
    P in the Fig. G V will show a deflection, of course, when the
    particular thing under test is a conductor.

    =311. Directions.= (A) Make tests with the above substances,
    and with any others at hand, and note which are conductors and
    which are not.


_=312. Conductors and Nonconductors.=_ It is evident, from the
experiments, that bodies which conduct static electricity do not
necessarily conduct galvanic electricity. The greater the E. M. F.
of a current, the greater its power to overcome resistance. Some
bodies, like dry wood, that readily conduct the high potential static
electricity, make fairly good insulators for the low potential galvanic
currents. For convenience, substances may be divided into _good
conductors_, _partial conductors_, and _insulators_, or nonconductors.


_=Good Conductors.=_ Metals, charcoal, graphite,
acids, etc.


_=Partial Conductors.=_ Dry wood, paper, cotton, etc.


_=Insulators.=_ Oils, porcelain, silk, resin, shellac,
ebonite, paraffine, glass, dry air.


[Illustration: Fig. 95.]

    =EXPERIMENT 120. To find the effect of sulphuric acid upon the
    conductivity of water.=

    _Apparatus._ Galvanoscope, G V; cell; 2-F C; connecting wires;
    saucer or tumbler, S; a little sulphuric acid.

    _Arrange_ as in Fig. 95.

    =313. Directions.= (A) Put a little pure water in S, and see
    if enough current can pass through it to deflect the needle of
    G V. The ends of the wires, 1 and 2, should be gradually moved
    toward each other, the needle being watched.

    (B) Put 4 or 5 drops of concentrated acid into the water; stir
    it, then repeat the test. What effect has the acid?


_=314. Internal Resistance.=_ As found in Exp. 120, pure water is
not a good conductor of galvanic electricity. The acid in the simple
cell, and in other single-fluid cells, acts upon the zinc and at the
same time makes it possible for the current to pass, as it reduces the
internal resistance.

As seen later, this resistance in cells is greatly diminished by
bringing the plates near each other, and by increasing the surface of
the plates that are in contact with the acid. The larger the plates
the less the internal resistance, other things remaining the same. The
internal resistance of a _battery_ can be changed by connecting the
cells differently. (See Chap. on Arrangement of Cells.)


[Illustration: Fig. 96.]

    =EXPERIMENT 121. To find what effect the length of a wire has
    upon its electrical resistance.=

    _Apparatus._ A No. 30 German-silver wire, G-S W, a little over
    two meters long, un-insulated (No. 81); the two-fluid cell, 2-F
    C (Exp. 113); galvanoscope, G V (No. 58); plate binding-posts,
    X, Y and Z (No. 83-84-85); copper washers (No. 87).

    _Arrange_ as in Fig. 96, so that the current will flow, at
    first, as shown by the arrow. The metal plates, M P 1 and M
    P 2, are used so that the connections may be changed without
    disturbing G V. The binding-posts may be fastened directly
    to the top of the table; but it will be more convenient to
    permanently fix them to a board, B, as shown, so that the
    same arrangement can be used for future experiments. The
    binding-posts, X and Y, should be about 1/8 in. apart, just far
    enough so that their edges do not touch each other.

    The binding-post, Z, should be fastened to B with its inside
    end 1 meter(100 centimeters, cm.) from the ends of X and Y.
    Marks should be made upon B, 10 centimeters apart, as indicated
    by the cross lines. This distance may be taken from the scale
    on the rule (No. 88).

    Fasten one end of the No. 30 wire, G-S W, to X. To do this
    twist its end around the screw, S, between X and the copper
    washer, then turn the screw in with a screw-driver until it
    firmly holds X to the board. Pass the wire around the screw in
    Z, and bring its free end to the other binding-post, Y, to be
    fastened (Fig. 96). Two meters of wire then form a path for the
    current from X to Y. Have the board wide enough so that another
    set of binding-posts can be put by the side of Y. It will be
    best to permanently leave the No. 30 wire upon the board, and
    to fasten the No. 28 wire (next experiment) to another set of
    binding-posts, placed in the same manner as those in Fig. 96.
    Make holes in the wood with an awl before forcing in the screws.

    =315. Note.= This experiment is usually done with a reverser in
    the circuit, first taking readings with the current passing in
    one direction, and then in the opposite direction. Considerable
    time will be saved by taking all the readings for one direction
    of the current at a time, simply using different lengths of
    German-silver wire, and allowing the current to flow constantly
    during each part. This obviates all danger of poor contacts
    in the reverser, etc.; it saves the trouble of handling the
    reverser, and much of the time needed for the needle to come to
    rest.

[Illustration:

  +--------------------+----+----+----+---+---+---+---+---+----+---+
  |LENGTH OF CIRC., CM.|200 |180 |160 |140|120|100| 80| 60|200 | O |
  +--------------------+----+----+----+---+---+---+---+---+----+---+
  |DEFLECTION; WEST    |26° |28° | 30°|   |   |   |   |   | 26°|67°|
  +--------------------+----+----+----+---+---+---+---+---+----+---+
  |DEFLECTION; EAST    |25° |27° | 30°|   |   |   |   |   | 25°|67°|
  +--------------------+----+----+----+---+---+---+---+---+----+---+
  |  AVERAGE           |25.5|27.5| 30 |   |   |   |   |   |25.5|67 |
  +--------------------+----+----+----+---+---+---+---+---+----+---+

Fig. 97.]

    =316. Directions.= (A) With the circuit arranged as in Fig.
    96, and with G V properly placed, take the reading of G V, the
    current passing through 200 cm. of No. 30 G-S W. Record your
    results in a diagram made like Fig. 97. The row of figures
    across the top shows the length of the circuit. The table is
    started with results from one experiment. Your results will
    probably be different from these.

    (B) Get the deflection with the current passing through 180
    cm. of wire. To do this press a piece of copper (O, Fig. 96)
    upon the wire at the mark 10 cm. from Z, another thin piece of
    metal, U, having been slipped under the wire. This will allow
    the current to pass across from one wire to the other. Record
    the deflection in the col. marked 180.

    (C) Record the deflections for the lengths, 160 cm., 140, 120,
    100, 80, and 60; then repeat (A) to be sure that the cell has
    been working uniformly. This deflection should agree with that
    in (A).

    (D) Change the direction of the current through G V; to do
    this, change wire, 1, from M P 2 to M P 1, and wire 5 to M P 2.
    This must be done without disturbing G V.

    (E) Repeat (A), (B), and (C), and record the deflections for
    the different lengths.

    (F) Get the average deflections.

    (G) Take, for future use, the deflection produced without G-S
    W being in the circuit. Swing the end of wire, 3, that is
    joined to Y, around to M P 2. The current will then pass simply
    through G V. Record deflection in col. marked O.

    =Note.= It is best to do the next experiment at once with the
    same cell, so that the results of the two experiments can be
    compared. In case this is impossible, get your cell to produce
    the same deflection when you use it again, as shown in col. O,
    Fig. 97. You can regulate the deflection of the needle of G V
    by varying the strength and quantity of the acid in P C.

    _=317. Discussion.=_ The resistance of a wire evidently depends
    (Exp. 121) upon its length. The _exact_ relation between
    resistance and length cannot be seen from these results,
    however, which are used in the next experiment. It will be
    shown later that in a wire, other things remaining the same,
    the resistance varies directly as its length.


    =EXPERIMENT 122. To find what effect the size (area of
    cross-section) of a wire has upon its electrical resistance.=

    _Apparatus._ Same as in last experiment, with one change,
    however. Replace the No. 30 G-silver wire with a No. 28
    G-silver wire (No. 82), or, what is better still, fasten it to
    another set of binding-posts on the board and leave the No. 30
    for future use. The two should be stretched side by side for
    constant use.

    =318. Directions.= (A) See that your cell is in the same
    condition as for Exp. 121; that is, it should produce the same
    deflection of the needle of G V as before, when the two, only,
    are in the circuit. (See Exp. 121, G.) The deflection may be
    changed by changing the strength and quantity of the dilute
    acid and copper solution.

    (B) Find the average deflection of the needle with the 2 meters
    of No. 28 G-s wire in the circuit, arranged as in Fig. 96.

    (C) Compare this average deflection with the results obtained
    in Exp. 121, in order to find what length of the No. 30
    wire has the same resistance as 2 meters of No. 28 wire. To
    find how many times greater one length is than another, we
    divide the larger length by the smaller; hence, to find the
    relation between the two lengths of wire that gave the same
    deflection,--lengths of equal resistance,--we divide the 200
    centimeters (the length of the No. 28) by the length of No. 30
    found as directed.

    (D) From the wire tables it will be found that the area of
    cross-section of No. 28 wire is about 1.59 times that of No.
    30 wire. How does this quotient, or ratio, compare with that
    found in part (C)? What is the relation between the area of
    cross-section of a wire and its resistance? (See § 319, also
    Exp. 136.)


_=319. Discussion.=_ If we find that a certain wire, X, which is 576
feet long, has the same resistance as a shorter one, Y, 360 feet long,
we see (576 divided by 360) that the ratio of their lengths is 1.6.
This means that the longer one, X, is 1.6 times as good a _conductor_
as Y; or, in other words, that the _resistance_ of Y is 1.6 times that
of X.

It is easier for water to flow through a large pipe than it is through
a small one. The same general principle is true of electricity. A large
wire offers less resistance to the current than a small one of the same
material. If one wire is twice the size of another of equal length, it
will be twice as good a conductor as the other; that is, it will have
one-half the resistance of the smaller, provided they are of the same
material. (See Laws.)


    =EXPERIMENT 123. To compare the resistance of a divided circuit
    with the resistance of one of its branches.=

    _Apparatus._ Same as in last experiment. Arrange as in Fig. 98.

    =320. Directions.= (A) Note the deflection of the needle when
    the current passes through 1 meter of G-s wire, as shown. This
    will be considered as one branch of the divided circuit.

    (B) Still allowing the current to pass as in part (A), press
    a piece of copper firmly across the binding-posts X and Y, to
    electrically connect them, and note the reading of the needle.
    In this case the current divides at Z through the two branches.
    What is learned from the results of (A) and (B)?

    (C) See if you can show the same results with apparatus
    arranged as in Fig. 99.

[Illustration: Fig. 98.]

[Illustration: Fig. 99.]


_=321. Discussion.=_ Two wires placed side by side as in (B), Exp. 123,
really form a conductor having twice the size (area of cross section)
of one of the branches. The more paths a current has in going from one
place to another, the less the resistance. (See Exp. 135.) The wires
are said to be in "parallel" or in "multiple arc."


[Illustration: Fig. 100.]

[Illustration: Fig. 101.]

    =EXPERIMENT 124. To study the effect of decreasing the
    resistance in one branch of a divided circuit.=

    _Apparatus._ Galvanoscope, G V (No. 58); resistance coil, R
    C (No. 79); two-fluid cell, 2-F C (§ 281), or a dry cell; 6
    connecting wires; metal plates, M P.

    _Arrange_ as in Fig. 100, so that the current divides into two
    branches at M P 1. The branches unite at M P 2.

    =322. Directions.= (A) Take the reading of G V with 2 ohms
    resistance in the lower branch; that is, with the whole of R C
    in circuit.

    (B) Take the reading of G V with one ohm in circuit; that is,
    with the end of wire, 5, connected to M instead of to R.

    (C) Cut out R C from the lower branch by replacing it with a
    metal plate, thus joining wires 3 and 5. Compare the results
    from (A), (B), and (C), and explain.


_=323. Current in Divided Circuits.=_ Let us consider a circuit like
that shown in Fig. 101. If the points, C and Z, were at the same
potential, no current would pass from C to Z. As the current does pass,
Z must be at a lower potential than C; there is a _fall of potential_
from C to Z. If the branch, A B, has the same resistance as R X, the
same amount of current will pass through each. Exp. 124 has shown that
when the branches have unequal resistances, most of the current passes
through the one of small resistance. If R X has a greater resistance
than A B, most of the current will pass through A B.




CHAPTER XIX.

MEASUREMENT OF RESISTANCE.


[Illustration: Fig. 102.]

    =EXPERIMENT 125. To study the construction and use of a simple
    "Wheatstone's Bridge."=

    _Apparatus._ Fig. 102. A Wheatstone's bridge, W B (No. 80), (§
    324); astatic galvanoscope, A G (No. 59); dry cell, D C (No.
    51); key, K (No. 55); 7 wires with spring connectors, two of
    which, R and X, are equal in length; metal plate, M P, for
    connecting wires.

    _Arrange_ as in Fig. 102. The carbon of D C is joined to K, and
    this to the point, C, of the bridge. The zinc of D C connects
    with the point Z on W B. The A G is placed between the branches
    for clearness. Wire 3 is joined to the left-hand binding-post
    of A G, and wire 4 joins M P with the right-hand one. When the
    end of wire 3 does not touch G-s W, it is evident that as soon
    as K is pressed, the current divides at C on its way to Z,
    where the branches unite again. K is used so that D C will not
    be polarized by steady use.


[Illustration: Fig. 103.]

    =324. The Simple Wheatstone's Bridge= (Fig. 103) consists of a
    wooden base, W, at the ends of which are fastened two aluminum
    conductors, 1 and 3. At one side of W is fastened another
    conductor, 2. In Fig. 104 are side views of the conductors.
    These are used merely for convenience in making connections,
    and take the place of the metal plates used in previous
    experiments. A German-silver wire, G-s W, is stretched between
    1 and 3, and under this is a scale, S, divided into 100 small
    parts, these being tenths of the larger divisions. The ends of
    G-s W are held between eyelets, as shown at E, Fig. 104.

[Illustration: Fig. 104.]

    =Reading the Scale.= The value of part A can be read
    directly from the scale, using the lower row of figures.
    The point marked P, for example, would be read 3.7 (three
    and seven-tenths large divisions); B would be 6.3, found by
    subtracting 3.7 from 10. The sum of A and B must always equal
    10. The 6.3 may also be read directly by using the upper row of
    figures for the whole numbers, counting the tenths to the left.
    Try to divide the smallest divisions into halves, at least;
    that is, if A = 3.75, B = 6.25. Take the readings carefully.

    =325. Directions.= (A) Touch the free end of wire, 3, to the
    point, C, which has a higher potential than M P. Press down K
    for an instant only. Some current should pass through A G, as
    a shunt. Should it pass from C to M P or the reverse? Note in
    which direction the right-hand end of the astatic needle is
    deflected.

    (B) Swing the end of 3 around and touch it to the point, Z,
    which has a lower potential than M P. Press K for an instant,
    watch the needle, and compare with the results in (A).

    (C) Move the free end of 3 along on G-s W, touching K at
    intervals, until a point is found at which the needle of A G
    is not deflected. How does the potential of this point compare
    with that of M P?


_=326. Discussion; Equipotential Points.=_ Since one end of the G-s W
has a higher, and its other end has a lower potential than M P, there
must be, somewhere on it, a point at which the potential is the same as
at M P. This place is quickly found by sliding the free end of wire, 3,
along, pressing K occasionally, until A G shows that no current tends
to pass through it in either direction, when the current passes from C
to Z through the two branches of the divided circuit. This point and M
P are called _equipotential points_.

If the resistance of the part, X, be increased, it should be evident
that the part of the bridge-wire, B, should be also increased to find a
point having the same potential as M P; that is, the end of 3 should be
moved towards C.

We have, in the bridge-wire, a simple means of varying the resistance
of its parts, A and B.

    =327. Use of Wheatstone's Bridge.= It will be found, upon
    trial, if we put a resistance of 2 ohms in place of R, Fig.
    102, and 2 ohms in place of X, that the free end of wire 3
    will have to be at the center of the bridge-wire in order
    to get a "balance"; that is, to find the place where A G is
    not affected. No matter what the resistance of R and X are,
    provided they are equal, this will be true. The value of both
    A and B, on the scale, will be 5 whole spaces, no tenths. From
    this we see that A: B:: R: X, which reads A _is to_ B _as_ R
    _is to_ X; this means that A × X = B × R. Supplying the values
    of the letters, we have 5 × 2 = 5 × 2. If we did not know the
    value of X, that is, if we were measuring the resistance of
    a coil of wire, using a 2-ohm coil as the standard, or R, we
    could find the value of X, knowing the other 3 parts of the
    proportion. 5 × X = 5 × 2, which means that 5 times the value
    of X is 10; hence the value of X is 10 ÷ 5 = 2 ohms.

    Suppose that we have R = 2 ohms, which is the standard
    resistance coil (No. 79), and are trying to find the resistance
    of a coil, X. We slide the end of wire, 3, along on the
    bridge-wire until the correct place is found. (See Exp. 125,
    126, for details.) Take the values of A and B (§ 324), supply
    them in the equation given, and work out the value of X.

    =328. EXAMPLE.= R = 2 ohms; A = 3.7; B = 6.3; to find the value
    of X in ohms.

    A: B:: R: X, which means that A × X = B × R, or 3.7 × X = 6.3 ×
    2. X must equal, then (6.3 × 2) ÷ 3.7 = 3.405 ohms.

    =Note.= In practice it is most convenient to make connections
    as shown in Fig. 105 when measuring resistances (Exp. 126). The
    arrangement given in Fig. 102 is simply for explanation. It
    will be seen that the smaller A is, compared with B, the larger
    the unknown resistance compared with your standard.


[Illustration: Fig. 105.]

    =EXPERIMENT 126. To measure the resistance of a wire by means
    of Wheatstone's Bridge; the "bridge method."=

    _Apparatus._ Same as in Exp. 125; the two-ohm resistance coil,
    R C (No. 79); a coil of wire, X, as, for example, the 15-turn
    coil on the galvanoscope, G V (No. 58).

    _Arrange_ as in Fig. 105. You will observe that the central
    conductor of the bridge (2, Fig. 104) takes the place of M P in
    previous explanations. We still have the same kind of a divided
    circuit as explained in Exp. 125, A G being connected with
    points of equal potential. It will be found convenient to have
    D C at the right, and A G facing you at the left, the key being
    in front. (See Exp. 107 in regard to adjusting A G.)

    Notice that you have a standard resistance (2 ohms) in place of
    R, Fig. 102, and an unknown resistance (galvanoscope coil) in
    place of X. (See § 330.)

    =329. Directions.= (A) Touch the free end of wire, 3, to
    the left-hand side of the bridge-wire, press the key for an
    instant, only, and note the direction taken by the right-hand
    end of the needle. Move the end of wire, 3, to the right-hand
    side of the bridge-wire, touch key, watching needle. Does the
    needle move more or less than before? In the same or opposite
    direction? If the deflections are opposite, the point that has
    the same potential as binding-post, 2, must be _between_ the
    two points touched.

    (B) Be sure that all connections are good. Find the point on
    G-s W, at which there is no deflection, as directed in Exp. 125
    (C). Note the readings on the scale, as explained in § 324.

    (C) Make the proper calculation, § 327, 328, and find the
    resistance of the coil of G V, the resistances of the wires
    joining R C and G V to the bridge being neglected.

    (D) Make proper allowances for the resistances of the wires
    just mentioned (see § 330), and compare them with the results
    found in part (C).

    =330. Allowances for connections.= It should be remembered that
    the wires joining R C and G V to the bridge also have some
    resistance. Such connections, in regular instruments, are made
    by heavy copper straps or by thick, short wires, so that their
    resistances can be neglected. In case you use the ordinary No.
    24 copper wire, as directed, the resistances of the pieces
    can be measured by means of the bridge, or you can calculate
    their resistances from the wire tables. The resistances should
    be allowed for. It is evident that your standard resistance
    is 2 ohms _plus_ the resistance of the connecting wires, and
    that the resistance of the coil, X, is found by deducting the
    resistance of its connecting wires from that found from the
    proportion previously used.

    _Example._ We see from the table that the resistance of about
    39 ft. 1 in. of No. 24, B and S copper wire is 1 ohm. This
    equals 469 in. If 469 in. have a R (resistance) of 1 ohm, 1 in.
    will have a R of one-469th of an ohm; that is 1 divided by 469,
    which equals a little over .002 ohm. For every inch of No. 24
    wire used, then, for connections, we may allow .002 ohm. This
    will be near enough for our purposes.

    Suppose that each connection is 18 in. long, the regular wires
    with connectors being used. The R of the 36 in. joined to R
    C will then be 36 times .002 = .072 ohm. Our standard R must
    then be considered as 2.072 ohm. If we substitute this in the
    example, as stated in § 328, we have 3.7 × X = 6.3 × 2.072. X
    must equal (6.3 × 2.072)/3.7 = 3.528 ohm, which includes the
    unknown resistance and 36 in. of connections, the R of which
    is .002 ohm; 3.528 - .072 = 3.456, the resistance of X alone.
    Compare this with the answer to example, § 328. Make allowances
    according to length of connectors used.


_=Note.=_--Carefully keep all the results of these experiments in a
note book for future reference. Be sure that connections are good.


    =EXPERIMENTS 127-137. To measure the resistances of various
    wires, coils, etc., by the "bridge method."=

    _Apparatus._ The coils of wire, etc., as stated in the
    "Directions" of each experiment. The details of each piece of
    apparatus may be found by referring, from the numbers given,
    to the "Apparatus List," and to descriptions in the paragraphs
    mentioned. Also all the apparatus of Exp. 126.

    =Note.= Make proper allowances for connections (§ 330) in all
    experiments in measuring resistances.


    =EXPERIMENT 127.=

    =331. Directions.= (A) As explained in Exp. 126, measure the
    resistance of the 10-turn coil of G V, allowing for connections
    (§ 330). Read the bridge-scale carefully.

    (B) Use one-half of the 2-ohm coil as standard and repeat.


    =EXPERIMENT 128.=

    =332. Directions.= (A) Measure the resistance of the 5-turn
    coil of G V (see Exp. 126, etc.), using 2 ohms as standard.

    (B) Use 1 ohm as standard, repeat, and compare results.

    (C) Add the resistances of the 5 and 10-turn coils, and compare
    the sum with the resistance of the 15-turn coil, as found in
    Exp. 126, D. The difference should be but a few hundredths of
    an ohm.


    =EXPERIMENT 129.=

    =333. Directions.= (A) Measure the resistance of the coil
    of No. 24 copper wire (No. 89). This coil is used for later
    experiments. Spring connectors are fastened to the ends of this
    coil, allowing it to be directly connected to the conductor on
    the bridge, so no allowance should be made for its connecting
    wires. (See Exp. 126 for details.) Mark the resistance upon the
    coil for future use. (See Note.)

    =Note.= The student will be surprised, perhaps, to find that
    different results are obtained for the resistance of a given
    wire in case he uses different standard resistances in the
    various tests; that is, he will probably get a different result
    in Exp. 127 (A) from the result of Exp. 127 (B). The difference
    here, however, may not be large. The best results are obtained
    by making the standard resistance as nearly equal as possible
    to the resistance to be measured, so that a balance can be
    found when the end of wire 3 (Fig. 105) is near the center of
    the bridge-wire. If R, Fig. 105, is much larger or smaller than
    X, the point desired on G-s W will be near one of its ends, and
    large errors thereby produced. The approximate resistance of
    X can be found by trial, then more or less resistance can be
    used for R to suit. The student should make several coils as
    explained in Apparatus Book, Chapter XVII. The resistance of
    the different coils furnished should be measured and marked.
    These can be used to vary the value of R.


    =EXPERIMENT 130.=

    =334. Directions.= (A) Measure the resistance of the coil of
    No. 25 copper wire (No. 90). (See Exp. 126 for details and the
    Note, Exp. 129.)


    =EXPERIMENT 131.=

    =335. Directions.= (A) Measure the combined resistance of the
    two coils used in Exps. 129 and 130, when they are joined in
    "series"; that is, when one end of one coil is joined to one
    end of the other by means of a metal plate, the free ends being
    connected to the bridge (Exp. 126). The current has to travel
    through the entire length of both coils.

    (B) Compare this result with the sum of their separate
    resistances found in Exps. 129 and 130. (See Exp. 129, Note.)


    =EXPERIMENT 132.=

    =336. Directions.= (A) Measure the resistance of the two coils
    (Exp. 131) when they are joined "in parallel." (See § 293.)
    They may be joined in parallel by connecting them both to the
    bridge at the same time, one end of each being slipped onto 2
    (Fig. 103), the other end of each being joined to 3. In this
    way the current has two paths, side-by-side, to get from 2 to
    3. (See Exp. 129, Note.)

    (B) Compare this resistance with that of Exp. 131.


    =EXPERIMENT 133.=

    =337. Directions.= (A) Measure the resistance of 1 meter of No.
    28 German-silver wire. Use the wire as arranged on a board,
    Exp. 122 (Figs. 96 and 98), making the connections with the
    bridge from binding-posts, X and Z. (See Exp. 129, Note.) The
    wires connecting the bridge with the ends of the G-s wire will
    each have to be about 2 ft. long. In making deductions (§ 330)
    figure according to the length used.

    (B) Divide the total resistance by 100 to get the resistance of
    1 cm. of the wire, and carefully mark off the board into cm.
    This will give 100 parts between X and Z.


    =EXPERIMENT 134.=

    =338. Directions.= (A) Using the No. 28 G-s wire on the board,
    as arranged for Exp. 122, measure the resistance of the 2
    meters in series, the connections being made with the bridge
    from X and Y, Fig. 98.

    (B) Compare the result with that of Exp. 133. What is the
    relation between the length of a wire and its resistance? See
    Summary of Laws. (See Exp. 129, Note.)


    =EXPERIMENT 135.=

    =339. Directions.= (A) Measure the resistance of the above two
    meters of No. 28 G-s wire when joined in parallel. (§ 293.)
    The binding-posts, X and Y, can be joined by a short wire with
    connectors on its ends, or by clamping a thin strip across by
    means of spring connectors. Use the 2-ohm coil as the standard,
    and make proper allowances. (§ 330.)

    (B) From the results of Exps. 132 and 135 what can be said
    about the resistances of parallel circuits as compared with the
    resistances of the separate branches?


    =EXPERIMENT 136.=

    =340. Directions.= (A) Arrange the 2 meters of No. 30 G-s wire
    on the table or board, again (Exp. 121, Fig. 96).

    (B) Measure the resistance of one meter. Find the value of X
    approximately, and use a resistance for R that will suit. (See
    Exp. 129, Note.)

    (C) Divide the result by 100 to get the resistance of 1 cm. of
    the wire.

    (D) Compare the resistance of one meter of No. 28 G-s wire,
    found in Exp. 133, with the resistance of 1 meter of No. 30 G-s
    wire. What is the relation, then, between the size (area of
    cross-section) of a wire and its resistance? (See the results
    of Exp. 122, and § 319, also Summary of Laws.)


    =EXPERIMENT 137.=

    =341. Directions.= (A) Measure the resistance of 2 meters of
    No. 30 copper wire, arranged on a board as in Fig. 96. (See
    Exp. 129, Note.) Get the resistance of 1 meter.

    (B) Compare the conductivities of copper and German silver
    by studying the results of Exps. 136 and 137. Which has the
    greater resistance? To find out how many times greater one
    resistance is than the other, divide the larger by the smaller.


    =EXPERIMENT 138. To study the effect of heat upon the
    resistance of metals.=

    _Apparatus._ Same as for Exp. 126; the coil of No. 24 wire (No.
    89); a lamp or other source of heat. Arrange as in Fig. 105.

    =342. Directions.= (A) Measure the resistance of the coil as
    before, Exp. 129. The result should nearly agree with that of
    Exp. 129, provided connections, etc., are the same.

    (B) Remove the coil from the bridge, hold it about a foot above
    a lamp or stove, to warm it thoroughly, but do not heat it
    enough to injure the covering. It will take a minute or so to
    warm it so that the heat will get to the inside also.

    (C) Replace the coil, measure its resistance, and compare the
    result with its resistance when cold. Does heat increase or
    decrease the resistance of a copper wire?


_=343. Effect of Heat upon Resistance.=_ Although there was but the
fraction of an ohm difference in the resistances of the hot and cold
coil, it was evident that changes of temperature affect the conducting
power of copper. This is true of all metals; but German silver and
other alloys are much less affected than pure metals, so they are used
in making standard resistance coils. The resistance of liquids that
can be decomposed by the electric current decreases as the temperature
rises. Carbon acts like the liquids, while the resistance of metals
_increases_ as their temperature rises.


    =EXPERIMENT 139. To measure the resistance of a wire by the
    method of "substitution."=

    _Apparatus._ The coil of No. 24 wire (No. 89), the resistance
    of which has been measured, but which will be considered an
    unknown resistance, X; G V, 2-F C, M P, connecting wires, etc.,
    previously used; rheostat (§ 344). Arrange as in Fig. 106
    first, then as in Fig. 107.


_=344. Simple Rheostat.=_ The No. 28 and No. 30 G-s wires stretched
upon the board (Fig. 96), make a convenient form of rheostat. The
resistance per cm. being known from the results of Exp. 133 and 136,
the resistance for any number of cm. is easily found. The 10-cm.
divisions should be divided into centimeters. These spaces may be
marked off from the rule (No. 88).


[Illustration: Fig. 106.]

    =345. Directions.= (A) Be sure that 2-F C gives a constant
    current, shown by the uniform deflection at G V, when arranged
    as in Fig. 106. Do not use a cell that quickly polarizes. The
    coil, X, forms a part of the circuit; it is joined to wires,
    1 and 2, by means of metal plates, so that it may be quickly
    removed without disturbing either G V or 2-F C. Carefully read
    the deflection at G V.

    (B) Remove X from the circuit, and join the free end of
    wire, 2, to binding-post, X, and the free end of wire, 1, to
    a small piece of sheet copper, which can be firmly pressed
    upon the G-s wire to make a contact. Move this along on the
    G-s wire until the deflection produced equals that of part
    (A), remembering that the longer the G-s wire in the circuit
    the less the deflection. Make two or three trials, as one or
    two cm. difference in length make but a little difference in
    the deflection. Note the number of cm. of G-s wire used, the
    resistance of which must equal that of the coil, X.

    (C) Find the resistance of X by multiplying the length just
    found by the resistance of each cm., and compare the result
    with the value found by using the bridge method directly.


[Illustration: Fig. 107.]

    =EXPERIMENT 140. To measure the E. M. F. of a cell by
    comparison with the two-fluid cell.=

    _Apparatus._ Rheostat (§ 344); the two-fluid cell, 2-F C (Exp.
    113), the E. M. F. of which may be taken as 1 volt; dry cell, D
    C; galvanoscope, G V. Arrange first as in Fig. 107.

    =346. Directions.= (A) Be sure that 2-F C gives a constant
    current. Take the reading of G V without the rheostat in the
    circuit; that is, with wires, 2 and 1, joined directly. The
    deflection should be 50 or 60 degrees at least, and be constant.

    (B) Attach a small piece of copper to the end of 1, and firmly
    rub it along upon the G-s wire, thus introducing resistance
    into the circuit, until the deflection is, say, 60° (50 or 55
    degrees will do). Note the length of G-s wire used and call it
    (B).

    (C) Gradually add more resistance by moving the end of 1 along
    until the deflection is 50°, 10 degrees less than before. (If
    the original was 50° make the new 40°). Call the number of cm.
    of wire used (C).

    (D) Replace 2-F C with the dry cell D C. Add resistance, as
    before, until G V indicates a deflection of 60°, being careful
    not to keep the circuit closed long enough to partially
    polarize D C. Make 2 or 3 trials, allowing D C to rest a few
    minutes between each. Call the number of cm. of G-s wire used
    (D).

    (E) Again add more resistance, as in (C), until the deflection
    is reduced to 50°. Call the length used (E).

    =347. Calculation.= It is known that resistances that are able
    to reduce the strength of the currents equally are proportional
    to the electromotive forces; that is, the electromotive forces
    of the two cells are to each other as the two resistances
    necessary to produce equal changes in the deflections, which,
    of course, indicate equal changes in the strength of the
    currents. Since the resistances used in the two cases are
    directly proportional to the lengths used, we have:

    Length (C-B): Length (E-D):: E. M. F. of 2-F C: E. M. F. of D C.

    Substitute the values found and find the E. M. F. of D C.


    =EXPERIMENT 141. To measure the internal resistance of a cell
    by the "method of opposition."=

    _Apparatus._ All the apparatus of Exp. 126. Two simple cells
    (§ 275), the plates of which should be of the same size, the
    same distance apart, and immersed in acid to the same extent in
    both. The acid in both should be of the same strength.

    =348. Directions.= (A) Connect the two cells in opposition, so
    that no current will be generated by them, and so that the two
    can be treated as a dead resistance. Do this by joining the
    two zinc plates by a wire with connectors, and use wires to
    connect the copper plates to the bridge like any other unknown
    resistance.

    (B) Measure the resistance of the two by the regular bridge
    method, allowing for wires used for connections. One-half of
    the resistance found will give the internal resistance of one
    cell. (See Note.)

    =Note.=--The standard resistance will have to be arranged
    to suit each particular case to make the calculations even
    approximately correct. (See Exp. 129, Note.) The standard
    resistance may be increased by adding the various coils and
    rheostat wires, their values being known.


_=349. Summary of Laws of Resistance.=_ 1. _The resistance of a wire
is directly proportional to its length, provided its cross-section,
material, etc., are uniform._

    =EXAMPLE.= If 39.1 ft. of No. 24 copper wire has a resistance
    of 1 ohm, 78.2 ft. will have a resistance of 2 ohms, because
    78.2 is twice 39.1; 70.38 ft. will have a resistance of 1.8
    ohms, as (70.38 ÷ 39.1 = 1.8) it is 1.8 times 39.1.

2. _The resistance of a wire is inversely proportional to its area
of cross-section._ The areas of cross-section of round wires vary as
the squares of their diameters; so _the resistance of a wire is also
inversely proportional to the square of its diameter, other things
being equal_.

    =EXAMPLE.= A No. 30 wire has a diameter of about .01 inch,
    while the diameter of a No. 24 wire is about .02 in.; that is,
    the No. 24 has _twice_ the diam. that the No. 30 has. The area
    of cross-section of the No. 24, however, is four times that of
    the No. 30, so its resistance is but 1/4 that of the No. 30,
    the lengths, etc., being the same. (See Wire Tables.)

3. _The resistance of a wire depends upon its material, as well as upon
its length, size, etc._

4. _The resistance of a wire depends upon its temperature._ (See
Elementary Electrical Examples.)




CHAPTER XX.

CURRENT STRENGTH.


_=350. Strength of Current.=_ The water in a certain tank may be under
great pressure, but if it is obliged to pass through long tubes before
it can turn a water-wheel, for example, it is evident that the work
done will depend not only upon the pressure in the tank, but upon the
resistance to be overcome before the water gets to the wheel. The work
that the water can do depends upon its _rate of flow_, and may be used
to measure the _strength_ of the current.

The strength of a current of electricity is measured also by the _work_
that it can do, and it depends upon its _rate of flow_ at the point
measured. The strength may be determined from its magnetic, heating, or
chemical effects.


_=351. Unit of Current Strength; The Ampere.=_ A current having the
strength of 1 ampere, when passed through a solution of silver nitrate
under proper conditions, will deposit 0.001118 gramme of silver in _one
second_; if passed through a solution of copper sulphate, copper plates
being used for the electrodes, in the solution, 0.0003277 gramme of
copper will be deposited in _one second_. (See Chemical Effects of the
Current.) The thousandth of an ampere is called the milliampere. The
strength of a current is proportional to the amount of chemical work
that it can do per second. (See § 357.)


_=352. Measurement of Current Strength.=_ The _galvanoscope_ previously
described simply shows the presence of a current, or whether one
current is larger or smaller than another. When the degree-card is
used to get the relative deflections, the instrument may be called a
_galvanometer_.


_=The Tangent Galvanometer=_ is made on the same general idea as our
galvanoscope, the diameter of the coil being twenty times, or more,
the length of the needle. In these the strengths of the two currents
compared are proportional to the tangents of the angles of deflection
produced. (See Elementary Electrical Examples.) There are several
varieties of galvanometers, each designed for its special work. They
are often calibrated or standardized so that the amperes of current
passing through them can be read off directly from the scale.


_=353. The Ammeter=_ is really a galvanometer from which may be read
directly the strength of a current. The coil has a low resistance so
that it will not greatly reduce the strength of the current to be
tested.


_=The Voltameter=_ measures the strength of a current by chemical means.


_=354. Unit of Quantity; The Coulomb.=_ A current having a strength of
1 ampere will do more chemical work by flowing one hour than it can do
in 1 second. In speaking of the _quantity_ of electricity we introduce
the element of _time_. The unit of quantity is called the _coulomb_,
just as a cubic foot of water may be taken as a unit of quantity for
water. A coulomb is the quantity of electricity given, in one second,
by a current having a strength of 1 ampere. Coulombs are found by
multiplying amperes by seconds; thus, a current of 5 amperes will give
20 coulombs in 4 seconds.


_=355. Electrical Horse-power; The Watt.=_ The electric current has
power to do work, and we speak of the horse-power of an electric motor
in the same way as for a steam-engine. A current with the strength of 1
ampere and an E. M. F. of 1 volt has a unit of power called the watt.
746 watts make an electrical horse-power.

  Watts = amperes × volts.

  Watts ÷ 746 = the number of horse-power.

(See Transformers, also Elementary Electrical Examples.)


_=356. Ohm's Law.=_ It was first shown by Ohm that the strength of
a current is equal to its E. M. F. divided by the resistance in the
circuit; that is,

  Strength of current (amperes) = E. M. F. (volts). / resistance (ohms).

If we let C stand for the strength in amperes, E for the E. M. F. in
volts, and R for the resistance in ohms, we have the short formula,
easily remembered,

  C = E/R


_=357. An Ampere=_ would be produced by a current of 1 volt pushing
its way through a resistance of 1 ohm. Knowing any two of the three,
C, E, or, R, the other may be found. The resistance, R, it must be
remembered, is the total resistance in the circuit, and is composed of
the total internal and external resistances.

(See Elementary Electrical Examples.)


_=358. Internal Resistance and Current Strength.=_ It is evident that
the internal resistance of a cell varies with the position and size of
the plates. We shall now study the effects of these changes upon the
strength of the current.


[Illustration: Fig. 108.]

    =EXPERIMENT 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.=

    _Apparatus._ Galvanoscope, G V; materials for simple cell (Exp.
    110); connecting wires. Arrange as in figure 108, omitting the
    wooden cross-piece.

    =359. Directions.= (A) Connect the wires with the 5-turn coil
    of G V, which has but little resistance. Have the tumbler
    nearly full of dilute acid to get the effect of large plates;
    that is, the current has a large liquid conductor to pass
    through in the cell, and the _internal_ resistance will be
    small. G V should be properly placed N and S.

    (B) Place the copper and zinc plates as far apart as possible
    in the acid, and press them against the bottom of the tumbler.
    Note the reading of G V. It is not necessary to take readings
    with reversed current.

    (C) Still pressing them against the bottom of the glass, to
    keep the same amount of surface under acid, slowly bring them
    near each other and watch the needle.

    (D) Hold the plates about an inch apart, and against the
    bottom, and note the reading of G V. Slowly raise the plates,
    keeping them the same distance apart until they are out of the
    acid. Watch the action of the needle.

    Make a note of your readings in degrees and write your
    conclusions. Does a change in internal resistance affect the
    strength of the current?


    =EXPERIMENT 143. Having a cell with SMALL PLATES to find how
    the strength of the current is affected by changes in the
    position of the plates, the external resistance being small.=

    _Apparatus._ Same as in Exp. 142, the acid, however, being but
    1 in. deep in the tumbler; that is, we have the effect of a
    cell with small plates, each being about 1 in. by 1/2 in.

    =360. Directions.= (A) Repeat (B) and (C) of Exp. 142,
    recording the reading of G V in each case.

    (B) Compare the results with those of Exp. 142, remembering
    that the _internal_ resistance is larger than before. Is the
    current as strong with small plates as with large plates when
    the external resistance is small? When the external resistance
    is small (the 5-turn coil, for example), should the cell have a
    high or low internal resistance to produce the greatest effect
    upon the needle?


[Illustration: Fig. 109.]

    =EXPERIMENT 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.=

    _Apparatus._ Same as for Exp. 142, 143, also the rheostat
    containing the two meters of G-s wire (Exp. 121).

    =361. Directions.= (A) Arrange as in Fig. 109, the external
    resistance being 2 meters of No. 30 G-s wire in series with G
    V. The 2-F C in the Fig. is replaced, however, by the simple
    cell as in Exp. 143.

    (B) Find the effect upon the strength of the current of moving
    the plates about when but 1 in. of acid is in the tumbler.

    (C) Nearly fill the tumbler with acid and repeat (B), taking
    readings with plates near each other and as far apart as
    possible. Lift them nearly out of the acid and take the reading.

    (D) Still increase the external resistance of the circuit
    by adding coils of wire or the meter of No. 28 G-s wire and
    repeat. Is the strength of the current greatly affected by
    _slight_ changes in the internal resistance when the external
    resistance is large?


_=362. Discussion.=_ We shall study, by means of figures, how changes
in internal resistance affect the strength of the current.

Let R stand for the total external resistance of a circuit, and r for
the total internal resistance of the cell or cells; ohm's law, then,
will be expressed by

  C = E / (R + r)

    =EXAMPLE.= Let us take a circuit (A) when the external
    resistance, R, is small, and (B) when R is large compared with
    r, E being taken as 1 volt in both cases.

    (A) Let R = 1, and r = 2; substituting these values in the
    formula above, we have:

    C = 1 / (1 + 2) = 1 / 3 = .33+ ampere.

    Now let the internal resistance, r, be slightly increased from
    2 to 3 ohms; the value of C then becomes 1/4 ampere, as R + r
    = 4. The change in C, then, is the difference between 1/3 and
    1/4; and this expressed in decimals becomes .33 - .25 = .08
    ampere.

    (B) Let R = 200 ohms, and r = 2 ohms as in (A). Substituting
    these values we have,

    C = 1 / (200 + 2) = 1 / 202 = .00495 ampere.

    Increasing r from 2 to 3, as before, etc., we find that C = 1
    divided by 203 = .00492 ampere.

    The above shows clearly (A) that the value of C is changed
    considerably by changes in r when R is _small_, and (B) that
    changes in r produce very slight changes in C when R is
    _large_. Review your results of Exps. 142-144. (See Elementary
    Electrical Examples.)


_=363. Arrangement of Cells and Current Strength.=_ We have seen that
internal resistance affects current strength. In joining cells, then,
attention must be given to the internal resistance as well as to the E.
M. F. of the combination.


_=364. Cells in Series.=_ It has been shown by careful experiments that
the E. M. F. of two cells joined in series (Fig. 110) is equal to the
sum of the E. M. F. of each. Ten cells, joined in series, have ten
times the E. M. F. of one cell, provided they have the same E. M. F. As
the Zn of one is joined to the Cu of the other, the current is obliged
to pass through one solution after the other; that is, the internal
resistance of the two in series is equal to the sum of their internal
resistances. Ten cells, joined in series, have ten times the internal
resistance of one cell, provided they have equal internal resistances.


[Illustration: Fig. 110.]

[Illustration: Fig. 111.]

_=365. Cells Abreast.=_ When the positive plates are joined together
and the negative plates are also joined together (Fig. 111), the cells
are said to be _abreast_, _in parallel_, or in _multiple arc_. It has
been shown that two cells of equal strength, joined abreast, have the
same E. M. F. as one cell. The two Cu plates, being joined, must have
the same potential; all the Zn plates have the same potential, so the
difference of potential at the terminals of the combination is the same
as that at the terminals of a single cell.

In two cells abreast (Fig. 111) the current has two liquid paths, side
by side, to get from Cu to Zn; this makes the internal resistance
one-half that of one cell, provided their internal resistances are
equal. Ten cells, of equal internal resistance, when joined abreast,
have one-tenth the internal resistance of one cell.


    =EXPERIMENT 145. To find the best way to join two similar cells
    when the external resistance is small.=

    _Apparatus._ Two simple cells using dilute sulphuric acid, with
    copper and zinc elements, as in Exp. 112; galvanoscope, G V;
    connecting wires, etc. Have the zincs well amalgamated. Remove
    them from the acid as soon as readings are taken.

    =366. Directions.= (A) Partly fill the tumblers with the acid.
    Join the cells in series (Fig. 110), then connect wire 1 (Fig.
    110) with the left-hand binding-post of G V, and wire 2 with
    the middle one, thus putting the 5-turn coil into the circuit.
    Take the reading of G V.

    (B) Join the cells in multiple arc (Fig. 111), connecting them
    as in (A) with G V. Write down the reading, and compare it with
    that found in (A).

    (C) Take the reading with but 1 cell joined to G V.


    =EXPERIMENT 146. To find the best way to join two similar cells
    when the external resistance is large.=

    _Apparatus._ Same as for Exp. 145, also the rheostat containing
    2 metres of No. 28 or 30 G-s wire. Arrange the G-s wire in
    series with the 15-turn coil of G V, as shown in Fig. 109, two
    simple cells being used, however, instead of 2-F C as shown.

    =367. Directions.= (A) Take the reading of G V when the two
    cells are in series (Exp. 145), the external resistance being
    the 15-turn coil and G-s wire.

    (B) Join the cells in parallel and take the reading, using the
    same external resistance as in (A).

    (C) Increase the external resistance by adding coils of wire or
    2 metres of No. 28 G-s wire and repeat (A) and (B). What does
    the experiment show?

    (D) Take the reading with 1 cell and large external resistance.


_=368. Best Arrangement of Cells.=_ It will be seen by experiments that
with a given number of cells the strongest current is produced when
they are arranged so that the internal resistance of the battery nearly
equals the external resistance of the circuit.

When the external resistance is small, the internal resistance may be
kept down by joining the cells in parallel; and, although the E. M. F.
is also kept small, the value of C will be larger than it would be with
a larger internal resistance and a larger E. M. F.

When the external resistance is large, the internal resistance can
be made large by joining the cells in series. The advantage comes,
however, from having a large value of E. A large resistance can not
hold back a current of large E. M. F. By joining the cells in series
the value of E is made large, and the value of C becomes large even
though there is an increased internal resistance. (See Elementary
Electrical Examples.)




CHAPTER XXI.

CHEMICAL EFFECTS OF THE ELECTRIC CURRENT.


_=369. Chemical Action and Electricity.=_ We have learned that the
electric current is produced, in the cell, by chemical action. There
is a definite relation between the chemical action and the current
produced. We are now to study the changing of electrical energy back,
again, to chemical energy.


[Illustration: Fig. 112.]

_=370. Electrolysis=_ is the name given to the process of decomposing
chemical compounds by passing the electric current through them. The
compound decomposed is the _electrolyte_. Fig. 112 shows a tumbler
of liquid (electrolyte) through which the current is to pass in the
direction of the arrow. Two carbon plates, A and C, are in the liquid,
and are joined to the source of electricity. The current enters at A
(_anode_) and leaves at C (_cathode_).


[Illustration: Fig. 113.]

    =EXPERIMENT 147. To study the electrolysis of water.=

    _Apparatus._ The two simple cells (§ 275) joined in series (§
    364), although two Daniell or two dry cells will be better. A
    tumbler of water containing a few drops of sulphuric acid to
    make the water a conductor. Two pieces of sheet copper will
    serve as the electrodes. The galvanoscope may also be put into
    the circuit as in Fig. 113.

    =371. Directions.= (A) Allow the current to pass, and note (1)
    whether gas is set free at both electrodes, A and C, and (2)
    at which the quantity of gas is the greater. If very little
    gas is produced use more cells.

    (B) Remove A and C from the liquid, to remove the gas, then
    watch the action of the needle of G V as the water is again
    decomposed.


_=372. Composition of Water.=_ The two gases liberated in Exp. 147 were
hydrogen (H) and oxygen (O). The chemical formula for water is H_{2}O,
which means that it is composed of two parts, by volume, of H and one
part of O. With proper apparatus these gases may be collected, tested,
and the amounts measured.


_=373. Electromotive Force of Polarization.=_

We know that H and O have a strong chemical attraction, or affinity,
for each other. In order, then, for the current to decompose water,
this attraction between the gases must be overcome; and as soon as the
current ceases, these gases try to rush together again to form water.
This sets up an electromotive force of almost 1.5 volts; in fact, a
current is produced if the H and O be allowed to form water again (See
Storage Cells). To decompose water the current must have an E. M. F. of
over 1.5 volts to overcome this E. M. F. of polarization. It was seen
in the study of simple cells that the current became rapidly weaker
as hydrogen was deposited upon the copper plate, on account of this
opposing electromotive force.

In decomposing other compounds, the anode is made of the metal which
is to be deposited at the cathode. If copper is to be deposited from a
solution of copper sulphate the anode should be a copper plate; this
keeps the solution at same strength, and avoids the opposing E. M. F.
of polarization; that is, a very weak current will do the work (See
Exp. 149), because the electrodes are of the same metal.


    =EXPERIMENT 148. To coat iron with copper.=

    _Apparatus._ Iron nail, solution of copper sulphate (§ 283).

    =374. Directions.= (A) Clean the nail with sandpaper, then hold
    it in the copper solution for a few seconds. Machinists often
    cover iron or steel tools with a thin coating of copper in this
    way.


[Illustration: Fig. 114.]

    =EXPERIMENT 149. To study the electrolysis of a solution of
    copper sulphate.=

    _Apparatus._ Galvanoscope, G V; two-fluid cell, 2-F C; a
    tumbler, T, containing about an inch of copper sulphate
    solution (§ 283); a wooden cross-piece to which is fastened a
    copper strip; carbon rod, C; wire 2 is held to C by a rubber
    band. _Arrange_ as in Fig. 114, so that Cu will be the _anode_
    (§ 370), the current passing as shown by arrow. A dry cell may
    be used for short experiments instead of the 2-F C.

    =375. Directions.= (A) The carbon being clean, allow the
    current to pass, C and Cu being kept about 1/2 in. apart. Watch
    the surface of C, and note the beautiful color of the deposited
    copper. Save the coated rod for the next experiment. Has the Cu
    plate been acted upon?


_=376. Electroplating=_ is the name given to the process of coating
substances with metal with the aid of the electric current. The copper
sulphate, CuSO_{4}, is broken up into Cu and SO_{4} by the current. The
Cu goes to the cathode, and the SO_{4} attacks the anode, gradually
dissolving it if it be copper; that is, the _metal_ part of CuSO_{4} is
carried in the direction of the current.

Most metals are coated with copper before they are silver or gold
plated. A solution of silver is used for silver plating, silver being
used as the anode.


    =EXPERIMENT 150. To study the chemistry of electroplating.=

    _Apparatus._ Same as in last experiment, but use two carbon
    rods for the electrodes. Arrange as in Fig. 114, with the Cu
    replaced by another carbon. Two simple cells (§ 275) are also
    needed.

    =377. Directions.= (A) Allow the current to pass as before. Is
    copper still deposited? Does anything occur now at the surface
    of the anode? Is the copper deposited as rapidly as before?

    (B) Try the effect of the two simple cells joined in series,
    Instead of the two-fluid cell.

    (C) After a fair coating of copper has been deposited upon the
    carbon cathode, reverse the direction of the current through
    the copper solution; that is, use the coated rod for the anode.
    Allow the current to pass until a change takes place in the
    anode.


_=378. Discussion.=_ Ions are the names given to the parts into which
an electrolyte is decomposed by the electric current. In the case of
CuSO_{4}, the ions are Cu and SO_{4}, which is called an acid radical.
This SO_{4} can not dissolve carbon or platinum, so these are used
when water is to be electrolyzed. Where copper is used as the anode
for copper plating, the SO_{4} attacks it, forming CuSO_{4} again,
and this keeps the solution strong. If carbon were used instead, the
SO_{4} would take H_{2} from the water around the anode and H_{2}SO_{4}
(sulphuric acid) would be formed, the oxygen of the water being set
free at the anode. The amount of Cu dissolved from the copper anode
equals nearly the amount deposited upon the cathode. Exp. 150 shows
that the metal is carried in the direction of the current. As hydrogen
is produced at the cathode it is chemically considered a metal.


_=379. Electrotyping=_ consists in making a copy in metal, of a
woodcut, page of type, etc. A mould or impression of the type is first
made in wax, or other suitable material (the pages of this book, for
example, as set up by the printer). These moulds are, of course, the
reverse of the type. They are coated with graphite to make them
conduct electricity, and hung as the cathode, in a bath of copper
sulphate. After a thin coat of copper has been deposited by an electric
current, the wax is removed and the thin copper backed with soft metal.
The metal surface next to the wax will be just like the type, only made
of copper. These plates or _electrotypes_ can be printed from, the
original type being used to set up another page. (See "Things a Boy
Should Know About Electricity.")


_=380. Voltameters=_ are cells used to measure the strength of an
electric current. In the _Water Voltameter_ the hydrogen and oxygen
produced are measured. The H acts like a metal and goes to the cathode,
two parts of H being formed to one of O.

_Copper Voltameter._ This cell measures the amount of copper deposited
in a given time by a current. The copper cathode is weighed before and
after the current flows. The weight of Cu deposited is then divided by
the number of seconds during which the current passed, and this result,
in turn, by .000328, which will give the average strength of the
current in amperes. (See § 351.) Other forms of voltameters are also
used.

In all voltameters the quantity of metal deposited is proportional to
the time that the current flows, and to its strength.


[Illustration: Fig. 115.]

    =EXPERIMENT 151. To study the construction and action of a
    simple "storage" cell.=

    _Apparatus._ Two lead plates, L P, (Nos. 77, 78) fastened to a
    wooden cross-piece (§ 275). The spring-connectors should not be
    forced upon the thick lead. Fasten one end of the wire under
    the screw-head. A tumbler two-thirds full of dilute sulphuric
    acid (§ 258); the astatic galvanoscope, A G; wires to form
    connections; the two simple cells joined in series. _Arrange_
    as in Fig. 115. One L P is joined to binding-post, L, of A
    G by the wire marked 1; wire 2 connects the other L P to the
    copper Cu. Wire 3 joins the zinc to any thin metal plate, M P,
    which is used for convenience, so that the spring connectors
    can be quickly slipped on or off. Wire 4 joins M P with
    binding-post R of A G.

    =381. Directions.= (A) Get clearly in mind the direction in
    which the right-hand end of the astatic needle is deflected
    when the current passes, remembering that it passes into A G
    at L and leaves at R. Allow the current to flow for 10 or 15
    minutes through the circuit, at the same time watching the
    needle to see whether the strength of the current remains
    constant.

    (B) Remove the connector from Cu, swing it over into the
    position of the dotted line (Fig. 115), slip the connector
    upon M P and watch the needle. This cuts the cells out of the
    circuit; but, if you desire, also remove wire 3 from M P.
    Does the storage cell, S C, produce any current? Does it pass
    through A G in the same direction as that which came directly
    from the two cells?

    (C) Try the dry cell in place of the two simple cells. Try 2
    other cells in series if you have them.


_=382. Secondary or Storage Cells=_ must be charged by a current
before they can give out a current. _Electricity_ is not really
stored. Chemical changes are produced in the storage cell by the
charging current, as in the voltameter or electroplating bath; and
it is, then, potential chemical energy that is stored. When the new
compounds are allowed to go back to their original condition by joining
the electrodes of the charged cell a current is produced. In other
words, an electric current produces chemical changes in the cell by
electrolysis, and these new compounds have an E. M. F. of polarization
because they are constantly willing and anxious to get back to their
old state. The plates are lead and are usually coated with compounds of
lead. Hydrogen and oxygen are given out at the electrodes. The current
from a dynamo is used to charge secondary batteries. (See "Things a Boy
Should Know About Electricity.")




CHAPTER XXII.

ELECTROMAGNETISM.


_=383. Electromagnetism=_ is the name given to magnetism that is
developed by electricity. You have already seen that if a magnetic
needle be placed in the magnetic field of a _magnet_, its N pole will
point in the direction in which the lines of force pass on their way
from the N to the S pole of the magnet. You have also seen that in the
galvanoscope, etc., a coil of wire acts like a magnet when a current
passes through it. Can we not, then, use the needle to study the lines
of force about wires and coils?


[Illustration: Fig. 116.]

[Illustration: Fig. 117.]

    =EXPERIMENT 152. To study the lines of magnetic force about a
    straight wire carrying a current.=

    _Apparatus._ The compass, O C; key, K; dry cell, D C. Arrange
    as in Fig. 116.

    =384. Directions.= (A) Arrange the wire so that the current
    will flow through it from N to S over the compass-needle as
    soon as the circuit is closed (Fig. 117, A). Press K for an
    instant only, and note the direction in which the N pole is
    deflected. Repeat two or three times until you get clearly in
    mind the direction taken by the needle. Sketch the result in
    your note-book, and compare with Fig. 118, A. The arrow shows
    the direction of the current.

    (B) Let the current pass for an instant from N to S and _under_
    the needle, as shown in Fig. 117, B. Sketch result.

    (C) Let the current pass for an instant from S to N _above_ the
    needle (Fig. 117, C). Sketch result.

    (D) Let it pass from S to N _under_ the needle (Fig. 117, D).
    Sketch result.

    (E) Let it pass through the wire from east to west (Fig. 117,
    F) above the needle, then under it, and note result. Compare
    the results with those indicated in Fig. 118.


[Illustration: Fig. 118.]

[Illustration: Fig. 119.]

_=385. Lines of Force About a Wire.=_ When a current passes through a
wire, the needle, over or under it, tends to take a position at right
angles to the wire. This shows that the lines of force pass _around_
the wire and not in the direction of its length. The needle does not
swing entirely perpendicular to the wire; that is, to the E and W line,
because the earth is at the same time pulling its N pole towards the N.
If the needle had no pointing power, and at the same time retained its
magnetic field, it would point exactly at right angles to the wire as
soon as the current passed.

If you look along the wire, Fig. 119, from the point, C, towards the
positions, A and B, you will see (A) that _under_ the wire the lines
of force pass to the left, and that _above_ the wire (B) they pass
towards the right. This is because the N pole points in the directions
mentioned. (See Fig. 118.) Looking along the wire from Z towards
position, D and C, you will see just the opposite to the above, as the
current comes _towards_ you.

_Rule._--When the current goes from you, the lines of force pass around
the wire in a clockwise direction, and when the current comes toward
you they pass around it in an anti-clockwise direction.


_=386. Ampere's Rule=_ may be used to remember what has been learned in
Exp. 152.

_If you imagine yourself swimming in the wire with the current, always
facing the needle, the N-seeking pole of the needle will always be
deflected towards your left hand._

When the needle is above the wire you must imagine that you swim upon
your back, in order to _face_ the needle.

_Another Rule._--Hold the right hand with the thumb extended and with
the fingers pointing in the direction of the current, the palm being
towards the needle and on the opposite side of the wire from the
needle. The N-seeking pole will then be deflected in the direction in
which the thumb points.


_=387.=_ If a wire carrying a strong current be dipped in iron filings,
the magnetic field about the wire acts by induction upon the particles
of filings, making magnets of them. These cling to each other simply
because they are little magnets.


_=388. Lines of Force about Parallel Wires.=_ When a current passes
in the same direction in two parallel wires the lines of force pass
around the wires in the same direction in both, and the magnetic fields
attract each other. When the currents flow in opposite directions the
magnetic fields repel each other.


    =EXPERIMENT 153. To study the lines of force about a coil of
    wire like that upon the galvanoscope.=

    _Apparatus._ Galvanoscope, G V; dry cell; key; compass. Arrange
    as in Fig. 116, using G V instead of the compass shown. The
    coil of G V should be placed in the E and W line. The current
    can pass only when the key is pressed. Connect the wires with G
    V, so that the current will pass through the 15-turn coil from
    W to E on top of the coil; that is, so that the current will
    have a "clockwise" motion. Fig. 120 represents a front view of
    the coil.

[Illustration: Fig. 120.]

[Illustration: Fig. 121.]

    =389. Directions.= (A) Hold the compass in the various places
    marked with a dot (Fig. 120) and note the directions taken by
    its N pole. Make a circle similar to the one shown to represent
    the coil, and sketch upon it the way in which the lines of
    force pass around it according to your observations.

    (B) Make a diagram like Fig. 121, which represents a
    cross-section of the coil through the center. Imagine that you
    have removed the top half of the coil and that you are looking
    down upon the ends of the wire of the lower half. Draw curved
    arrows about the coil at W and E to show which way the lines of
    force are passing. Compare your results with those in Fig. 119,
    remembering that at E, Fig. 121, the current is going away from
    you.

    (C) Move O C back and forth on the center-line that runs
    N and S through the coil, and note the positions of the
    compass-needle. Does the coil seem to have poles?

    (D) Reverse the current through the coil and repeat your
    observations.


    =EXPERIMENT 154. To study the magnetic field about a small coil
    of wire.=

    _Apparatus._ A coil of wire (No. 89), described in § 390;
    current reverser, C R (No. 57); dry cell; connecting wires, etc.

    =390. Coils= of wire for some of the following experiments
    should be wound upon wooden spools that have been turned down
    thin, so that the wire will be as near the central hole as
    possible. They should be wound with a winder. (See Apparatus
    Book, Chapter X.)

    For convenience we shall call the starting end of the coil,
    that is, the end that comes from the wire that is near the
    center, the _inside end_, I E. The end of the last layer of the
    coil we shall call the _outside end_, O E. These letters should
    be noted in the diagrams. See Apparatus List for details of the
    special coils used in these experiments.

[Illustration: Fig. 122.]

    =391. Directions.= (A) Arrange as in Fig. 122, so that the axis
    of the coil will lie in the E and W line. Place O C about 2 in.
    from the E end of the coil. Press one lever of C R so that the
    current will pass around the coil for an instant in a clockwise
    direction; that is, so that it will enter the coil at O E. Note
    the action of the needle. If the needle is not affected move it
    nearer the coil and press the lever again. Get clearly in mind
    the connections, the direction in which the N end of the needle
    is deflected, etc. Is the E end of the coil a N or a S pole?

    (B) Reverse the current through the coil. What effect has it
    upon the polarity of the E end of the coil?

    (C) Place O C at the west end of the coil and repeat (A) and
    (B).

    (D) Place O C in various positions about the coil and note the
    action of the needle when the current passes. Does this coil
    act like a magnet, having poles, magnetic field, etc.?


[Illustration: Fig. 123.]

_=392. Polarity of Coils.=_ It is evident from Exps. 153 and 154 that
a coiled conductor has poles, magnetic field, etc., when a current
passes, and that it strongly resembles a magnet, even though no
iron enters into its construction. We may say that the coil becomes
magnetized by the electric current. Fig. 123 shows a right handed coil
or helix of wire, the current passing as shown by the small arrows.
The left-hand end is a S pole because the current passes around it in
a clockwise direction. When you face the right-hand end of the coil the
current is seen to pass around it in an anti-clockwise direction; this
produces a N pole. As the N pole of the magnetic needle is attracted
toward the S pole of the coil, it is clear that the lines of force pass
through the inside of the coil as shown by the large arrows. They then
curve through the air and return to the S pole as with magnets.


    =EXPERIMENT 155. To test the attracting and "sucking" power of
    a magnetized coil or helix.=

    _Apparatus._ The coil, battery, etc., used in Exp. 154, Fig.
    122; a sewing-needle.

[Illustration: Fig. 124.]

    =393. Directions.= (A) Arrange the coil, etc., as described in
    Exp. 154. The coil need not lie in the E and W line, however,
    and a key may be used instead of the current reverser.

    (B) Magnetize the needle so that its point will be a N pole.

    (C) Tie a thread about the center of the magnetized needle,
    hold the thread in the hand so that the S pole of the needle
    will swing freely at the hole at the right-hand end of the coil
    (Fig. 124). If the current passes as directed, the right-hand
    end of the coil will be a N pole. What happens to the needle
    when the key is pressed for an instant.

    (D) Change the needle to the left end of the coil and repeat.

    (E) Try a nail, pen, iron, etc., instead of the needle.


    =EXPERIMENT 156. To find whether a piece of steel can be
    permanently magnetized by an electric current.=

    _Apparatus._ Same as for last experiment; an unmagnetized
    sewing-needle; the compass.

    =394. Directions.= (A) Be sure that the needle is not
    magnetized. It should attract both ends of the compass-needle.
    How can any magnetism in the needle be removed?

    (B) Place the needle inside of the coil with its _point_ to the
    east; that is, with its point at the N pole of the coil, and
    its head at the S pole. Close the circuit for an instant. Test
    the needle again for poles. Is the point a N or a S pole?

    (C) Turn the needle end for end in the coil, and see whether
    its polarity can be reversed.

    (D) Experiment with iron wire, nails, steel pens, spring steel,
    etc.


[Illustration: Fig. 125.]

    =EXPERIMENT 157. To study the effect of a piece of iron placed
    inside of a magnetized coil of wire.=

    _Apparatus._ Same as in Exp. 154; a short rod or iron _core_, I
    C, of soft iron (No. 92) that will fit inside of the coil. This
    combination is called an electromagnet.

    =395. Directions.= (A) Arrange first as for Exp. 154, Fig. 122,
    with the coil in the E and W line, no core being used, and
    place O C about 6 in. from the right-hand end of the coil.

    (B) Press the lever for an instant to see whether the field of
    the coil is strong enough to move the compass-needle at that
    distance. Move O C a little nearer or farther from the coil
    until the needle _just_ moves, when the circuit is closed.

    (C) Place I C inside of the coil (Fig. 125), and repeat (B)
    to see whether the magnetic field of the coil is stronger or
    weaker than before.

    (D) Study the location of the poles. Can they be reversed?




CHAPTER XXIII.

ELECTROMAGNETS.


_=396. Electromagnets=_ are important to the student of electricity.
They form the principal part of nearly every electrical instrument.
You have seen that a wire has a magnetic field about it the instant a
current passes through it. A coil, or helix of wire, has a stronger
field than a straight wire carrying the same current, because each
turn, or convolution, adds its field to the fields of the other
turns. By having a _core_ of soft iron instead of air, wood, or
other non-magnetic material, the strength of the magnet is greatly
increased. The central core may be permanently fixed in the coil, or
it may be removable. (See Apparatus Book, Chapter IX, for Home-made
Electromagnets.)


_=397. Cores of Electromagnets.=_ A strong magnet has more lines of
force passing from its N pole through the air to its S pole than a weak
magnet. By increasing the number of lines of force we increase the
strength of a magnet. It has been seen, in experiments with permanent
magnets, that lines of force do not pass as readily through air as
through soft iron, and that lines of force will go out of their way to
pass through iron. It was learned in Exp. 154 that inside of a helix
(Fig. 123) the lines of force pass from the S to the N pole; they then
spread out through the air and pass back on all sides of the coil to
its S pole, as in the case of permanent magnets. The air around and
inside of a helix offers a great resistance to the lines of force, and
tends to weaken the magnetic field. When part of the circuit consists
of an iron core, which is a splendid conductor of lines of force, the
magnetic field is greatly increased in strength.


    =EXPERIMENTS 158-163. To study straight electromagnets.=

    _Apparatus._ A good dry cell or other source of a fairly strong
    current; coil with soft iron core; key; wires with connectors,
    etc.; small nails; iron-filings; compass; large wire nail; tin
    box (No. 94) to act as a base for the electromagnets.


    =EXPERIMENT 158. Lifting power.=

    =398. Directions.= (A) Join the cell, key, and coil, as
    explained in Exp. 154, so that the current will pass only when
    the key is pressed. Place the core inside of the coil (Fig.
    125). Two good cells in series can be used to advantage.

    (B) Hold the coil in a vertical position near small nails, iron
    filings, tin boxes, etc.; then press the key and raise coil;
    carry the clinging iron to another place, break the circuit
    at the key, and explain the result. Why do nails cling more
    strongly to the core than filings after the circuit is broken?


    =EXPERIMENT 159. Residual magnetism of core.=

    =399. Directions.= (A) After the current has passed through
    the coil with the core in place, remove the core and test it
    for magnetism with the compass. Will the small end of the core
    attract both poles of the compass-needle, or is it slightly
    magnetized?

    (B) If there is any residual magnetism, strike the core with a
    hammer and test again.

    (C) Use a soft steel wire nail for the core, and repeat (A)
    and (B). Why does soft iron make a better core than steel for
    electromagnets? Which should be the more easily magnetized?


    =EXPERIMENT 160. Magnetic tick.=

    =400. Directions.= (A) Join the electromagnet with the cell
    and key as before (Exp. 154). Hold one end of the core firmly
    against the top of a tin box which should stand upon the table
    and which should act as a sounding-board. The flat boxes used
    in the experiments on static electricity are good for this, or
    use the tin box, No. 94, for a base. Rapidly open and close
    the circuit by means of the key and listen for any clicks made
    by the core.

    (B) Listen for this sound in telegraph sounders, electric
    bells, etc., if you have them. The armature should be held, of
    course, so that slight sounds can be heard.

    _=401. Discussion.=_ A bar of iron becomes slightly longer when
    it is magnetized, the particles of iron being made to point
    in the same direction. As soon as the current ceases to flow
    through the coil the particles of the soft core nearly all
    resume their mixed positions. The click heard is supposed to be
    due to the changes in the molecules of iron. The core becomes
    gradually warmer when it is rapidly magnetized and demagnetized
    by a strong current.


[Illustration: Fig. 126.]

    =EXPERIMENT 161. Magnetic figures.=

    =402. Directions.= (A) Arrange as in Fig. 126. The key should
    be used in case a dry cell acts as the source of the current.
    Two good cells joined in series can be used to advantage. Lay
    the coil flat upon the table and place on it a piece of stiff,
    smooth paper, or a sheet of glass.

    (B) Sprinkle a few iron filings upon the glass, which may be
    held in place by books. Gently tap the glass with a pencil
    while you close the circuit at the key. Do the filings arrange
    themselves as in the case of permanent magnets? Make a sketch
    of the field, remembering that you have both N and S poles, and
    compare it with previous results.


[Illustration: Fig. 127.]

    =EXPERIMENT 162. Magnetic figures.=

    =403. Directions.= (A) Arrange as in Fig. 126, but stand the
    coil on end, using the base as directed in § 407, to hold it
    firmly in position. Join the ends, O E and I E, to the key as
    before. Fig. 127 shows a top view of the coil and base.

    (B) With books, etc., fix a piece of stiff, smooth paper, or
    glass just over the top of the core, and proceed as in Exp. 161
    to study the field. See § 417 for making permanent pictures of
    magnetic fields.


    =EXPERIMENT 163. Magnetic field.=

    =404. Directions.= (A) Use same arrangement as for Exp. 162,
    except filings and glass, which are replaced by the compass.

    (B) Hold the compass about 2 in. from the top pole of the
    electromagnet, close the circuit for a second or two and note
    action of needle. Is the top N or S, when the current enters
    the coil at O E? Compare result with § 392.

    (C) Move the compass quickly about the pole, the circuit
    being closed, and note action of needle. Compare result with
    directions taken by particles of iron filings in Exp. 163.

    (D) Reverse the direction of the current through the coil and
    test the nature of the pole at the top.


[Illustration: Fig. 128.]

[Illustration: Fig. 129.]

_=405. Horseshoe Electromagnets.=_ Fig. 128 shows a simple form of
electromagnet with two coils which have a bent piece of iron as a core
for both. The coils have to be wound on by hand in this form. As this
is troublesome, the coils are generally wound on two separate cores
which are joined by a _yoke_ (§ 406), which takes the place of the
curved part in Fig. 128. The separate coils can be quickly made with
a "winder" and joined to suit. (See Apparatus Book, Chapter IX, for
Home-made Electromagnets.) Fig. 129 shows a top view of a home-made
experimental horseshoe electromagnet. The coils are joined by an iron
strap, called the _yoke_, which is screwed to a wooden base. A strip of
iron placed above the magnets to be attracted by them, when the current
passes, is called the _armature_. (See Telegraph Sounders.)


_=406. Use of Yoke.=_ It has been explained (§ 82) why horseshoe
magnets are, in general, better than straight ones. The same is true
of electromagnets; there are two poles to attract, and two to induce.
The lines of force pass through the yoke on their way from one core to
the other, and this reduces the resistance to them. The strength of
the horseshoe magnet would be greatly reduced if the lines of force
were obliged to pass through two air spaces instead of one; in fact, if
there were no yoke we should have simply two straight magnets. The yoke
should be made of soft iron.


[Illustration: Fig. 130.]

[Illustration: Fig. 131.]

    =407. Experimental Magnets= are quickly joined to a tin base
    (No. 94), which has 3 holes punched in, through which screws
    can be put to hold the cores in place. Fig. 127 shows plan of
    tin. Fig. 130 shows how removable cores are fastened to the
    base, the coils being on the spools, and Fig. 131 shows how
    home-made coils on bolts can be used. The coils on bolts should
    be wound as directed in Apparatus Book, Chapter X. The tin base
    also serves as the yoke.

    _Removable Cores._ Fig. 130. These are of soft iron (No. 92,
    93). In one end of each is a hole for the screws, S. Part of
    the tin has been cut away in the Fig. The copper washer, C W,
    should be used. (See § 408.) Connectors are fastened to the
    ends of the coils (§ 226-230).

    _Bolt Cores._ Fig. 131. After winding on the coils, as directed
    in Apparatus Book, remove the nut and put on an extra washer,
    E W, so that the ends of the coils will not be pressed against
    the tin, but come out between the two washers. Push the
    screw-end of the bolt through holes (about 2 in. apart) punched
    in the tin, then put on the nut, as shown. Do not force the nut
    on too far,--just far enough to hold the cores in place. The
    ends of the wires are not shown in Figs. 130, 131. Connectors
    are fastened to them (§ 408).

[Illustration: Fig. 132.]

    =408. Method of Joining Coils.= To produce the best results
    the poles of the horseshoe electromagnet should be unlike. As
    the coils are wound alike, their ends must be joined in such
    a manner that the current will pass around them in opposite
    directions; that is, if the current enters one coil at the
    outside end, O E, it must enter the other coil at the inside
    end, I E. Fig. 132 shows a plan of the connections, spring
    connectors being fastened to the coil-ends, to allow rapid
    and easy changes in the arrangement. L, M, and R are pieces
    of metal fastened to a strip of wood (No. 95), used to make
    connections from cells or other apparatus. They are turned up
    at each end as in Fig. 104, 3. Care should be taken not to get
    short circuits by allowing two wires to touch the tin base.

    By changing the ends of the coils upon L, M, and R (left,
    middle, and right), and by changing the direction in which the
    current enters the "combination connecting plates" (No. 95),
    it is evident that the nature of the poles can be regulated to
    suit.


    =EXPERIMENTS 164-173. To study horseshoe electromagnets.=

    _Apparatus._ Coils of wire with cores and yoke like those
    explained in this chapter. Coils fastened to tin base or yoke
    with wires leading from them to the combination connecting
    plates (No. 95, Fig. 132), are very handy. Cells; iron filings;
    compass; iron strip (No. 76).


    =EXPERIMENT 164. To test the poles.=

    =409. Directions.= (A) Arrange as in Fig. 126, but use the
    experimental magnets and combination connections (Fig. 132) in
    place of the single coil shown in Fig. 126. Join O of the key
    with L, and Zn of the cell with R of Fig. 132. When the key is
    pressed the current will enter the magnets from L and leave at
    R.

    (B) With the compass test the polarity of the cores as in Exp.
    163, B, C. Make a sketch of the arrangement, and note which
    pole is N and which S.

    (C) See which way the current must pass around each coil, by
    the way it is wound, and compare the results of (B) with Exp.
    154, Fig. 123.


    =EXPERIMENT 165. To test the poles.=

    =410. Directions.= (A) Arrange as in Exp. 164, but reverse the
    direction of the current through the coils. Do this by joining
    O of the key (Fig. 126) with R of Fig. 132, and Zn of the cell
    with L.

    (B) Repeat (B) and (C) of Exp. 164 and study results.


[Illustration: Fig. 133.]

    =EXPERIMENT 166. To test the poles.=

    =411. Directions.= (A) Arrange all connections as in Exp. 164,
    then reverse the positions of O E and I E of coil A; that is,
    join O E to M, and I E to L, Fig. 132. This will make unlike
    ends come together at M; in other words, when the current
    enters at L and leaves at R it will pass around both coils in
    the same direction.

    (B) Study the nature of the poles, as in Exps. 164, 165, and
    note results.

    _Note._--Fig. 133 shows simply the two cores of a horseshoe
    electromagnet with arrows to indicate in which direction the
    current is passing in each coil to produce N and S poles.


    =EXPERIMENT 167. To study the inductive action of one core upon
    the other.=

    =412. Directions.= (A) Arrange as for Exp. 164, but join the
    wire from Zn of the cell to M (Fig. 132). In this way coil B
    will be cut out of the circuit. Place the coils in the E and W
    line.

    (B) Find about how far the residual magnetism of the core of
    B can act upon the compass-needle, holding the compass on the
    side away from coil A, no current passing.

    (C) Press the key for an instant, and note whether the
    magnetism of coil B has been made stronger or weaker. Explain
    the action of core A on core B.


    =EXPERIMENT 168. Magnetic figures.=

    =413. Directions.= (A) Arrange as in Exp. 164. With books,
    etc., fix a piece of smooth, stiff paper, or a sheet of glass,
    just above the poles of the electromagnets.

    (B) Sprinkle iron filings upon the glass, and gently tap it
    while the circuit is closed at the key for a few seconds. Make
    a sketch of the magnetic figure produced. Do the lines of force
    from the opposite poles attract or repel each other? See § 417
    for making permanent figures. (See "Things a Boy Should Know
    About Electricity" for drawings of magnetic figures.)

    _Note._--If possible, use two or three good cells in series for
    making magnetic figures, as a fairly strong field is best.


    =EXPERIMENT 169. Magnetic figures.=

    =414. Directions.= (A) Arrange apparatus as for Exp. 165, and
    make the magnetic figure for this combination, as directed in
    Exp. 168. Sketch and study the results.


    =EXPERIMENT 170. Magnetic figures.=

    =415. Directions.= (A) Arrange the apparatus and connections as
    in Exp. 166, and make the magnetic figure of this combination
    as directed in Exp. 168. In this case the poles are alike.
    Sketch and study the results.

    =EXPERIMENT 171. Magnetic figures.=

    =416. Directions.= (A) Arrange apparatus and connections as in
    Exp. 167, and make the magnetic figure of the combination as
    directed in Exp. 168. Compare the figure produced with that of
    Exp. 168. In this case the current passes through but one coil.

    =417. Permanent Magnetic Figures= can be made in several ways
    for future study and comparison.

    (A) _Paraffine paper figures._ Make paraffine paper as directed
    in Apparatus Book, page 135. For this purpose smooth, stiff,
    _white_ paper is best, so that the filings will show plainly,
    and but a thin coating of paraffine should be given. Place the
    magnets upon the table, lay over them a piece of unparaffined
    paper, and fix the paraffine paper directly over this. This
    is necessary, as the coated paper sticks when heated. For
    electromagnets it will be necessary to support the edges of
    the paper with books, etc. Sprinkle on the filings and tap
    the paper to make them arrange themselves while the circuit
    is closed. After the lines of force show plainly, the current
    need not be used again, provided the paper be kept perfectly
    still. Pass the flame of a Bunsen burner over the paper to melt
    the coating. This will, no doubt, make the two pieces of paper
    stick together, and permanently fix the particles of filings in
    place. Do not heat the paper too much--just enough to melt the
    paraffine. If you have no gas, hold a fire-shovel, containing
    hot coals, over the paper. As soon as the paraffine cools, the
    figures will stand considerable handling.

    _Blue print figures_ are very pretty, and last indefinitely.
    Get some blue-print paper at a photographer's, who will give
    you directions about "developing" it with water. Keep this in
    the dark, and take out but one sheet at a time for experiments.
    To make the figures, take your apparatus near a window where
    bright sunlight comes in. Pull down the curtain so that you
    have but a dim light when you make the magnetic figure, as
    directed before. After the lines of force show plainly, raise
    the curtain, and let the bright sunlight shine on it for 5 or 6
    minutes, or until the surface of the paper has a rich, bronze
    color. The paper cannot be acted upon by the light under the
    particles of filings. Quickly shake the filings from the paper,
    and wash it in 3 changes of water to "develop" it, then pin the
    paper up to dry.


    =EXPERIMENT 172. Lifting power.=

    =418. Directions.= (A) Arrange the apparatus as in Exp. 164.
    Hold an iron strip (No. 76), a screw-driver, or other iron
    bar directly over and near the poles of the experimental
    electromagnet. Close the circuit at the key, then lift the
    magnets by the "armature," as the iron strip may be called, the
    circuit being kept closed for a few seconds. If your cell is
    good there should be no trouble in lifting the magnets by the
    armature. Open the circuit, and see whether the magnets drop.

    (B) Hold the magnets upside down directly over nails, tin
    boxes, iron filings, or other pieces of iron. Close the
    circuit, move the attracted iron to another place on the table,
    and open the circuit. Can this principle be used for practical
    purposes?

    _Note._--Some experiments illustrating practical uses of
    electromagnets will be given in a future chapter.


    =EXPERIMENT 173. Residual magnetism when magnetic circuit is
    closed.=

    =419. Directions.= (A) Arrange as in Exp. 164. You have already
    seen that each core retains some magnetism after the circuit
    is closed. Place the iron strip firmly across the poles, close
    the circuit for an instant, open the circuit, then see whether
    the armature still clings to the cores with some strength. The
    armature should fit well upon the cores for this experiment.

    (B) Again press the armature upon the cores, no current being
    used; then lift it as in (A). Compare the attraction with that
    found in (A).


_=420. Closed Magnetic Circuits.=_ It was seen in the study of the
permanent horseshoe magnet, that the armature clung strongly to the
magnet. The armature closed the magnetic circuit, the lines of force
having almost no resistance. In the case of electromagnets the magnetic
circuit becomes closed when the armature touches both poles at the same
time. The armature clings strongly to the poles even after the current
ceases to flow. As soon as the magnetic circuit is broken, however, but
little residual magnetism remains. The armatures of electromagnets are
usually arranged so that they can not quite touch the cores, to avoid
this sticking.




CHAPTER XXIV.

THERMOELECTRICITY.


[Illustration: Fig. 134.]

    =EXPERIMENT 174. To find whether electricity can be produced by
    heat.=

    _Apparatus._ The home-made thermopile described in §421;
    astatic galvanoscope; connecting wires; candle or alcohol lamp.

    =421. Home-made Thermopile.= (Fig. 134.) For this you need 3
    hairpins, copper wire, a piece of wood about 3 in. long and 1
    in. square on the ends, 2 pieces of tin, and some small nails.

    Straighten the hairpins and scrape the coating off with
    sandpaper or a file. Scrape the insulation from 4 pieces of
    copper wire, each about 8 in. long. Twist the ends of the
    copper wire about the ends of the hairpins (Fig. 134), and then
    fasten the hairpins to the block. They may be held firmly by
    small nails which should be driven partly into the block and
    bent over. The hairpins at the right-hand side of the Fig. are
    shown to be near but not touching each other. This allows all
    to be heated at the same time.

    The tin binding-posts may be nailed or screwed to the block,
    and if the bare copper wires 1 and 4 be placed under X and
    Y before they are screwed down they will be electrically
    connected. The ends of 1 and 4 may be held under the
    screw-heads. The block may be supported upon other blocks
    to raise it to the proper height, which will depend upon the
    length of the candle.

[Illustration: Fig. 135.]

    A thermopile in the form of a circle with several pairs of
    metals, can easily be made by fastening the hairpins to a piece
    of cardboard (Fig. 135) with a hole at the center. This may be
    supported by blocks, the heat being applied under the center.

    =422. Directions.= (A) Arrange the apparatus as in Fig. 134.
    See that the astatic needle is properly adjusted, no magnets
    being near it.

    (B) Heat the joints as shown, and watch the needle. Can a
    current be produced by heat?

    (C) Remove the connector on wire 6 from Y to M, thus cutting
    one pair out of the circuit. Heat the joints again and compare
    the strength of the current with that produced in (B).

    (D) See whether much current is produced by one pair. From
    results obtained do you see any relation between the strength
    of the current and the number of pairs?


_=423. Thermoelectricity=_ is produced by heating the junction between
two metals. Different pairs of metals produce different results.
Antimony and bismuth are often used. If the end of a strip of bismuth
be soldered to the end of a similar strip of antimony, and the free
ends be connected to a galvanometer of low resistance, the presence
of a current will be shown when the point of contact becomes hotter
than the rest of the circuit. The current will flow from the bismuth
to antimony across the joint. By cooling the junction below the
temperature of the rest of the circuit a current will be produced in
the opposite direction.

Thermoelectric currents have a low potential. The energy of the current
is kept up by the heat absorbed.


_=424. Peltier Effect.=_ The action noted in § 423 can be reversed;
that is, if a current from a battery be sent through the metals, the
parts at the junction become slightly warmer or cooler than before,
depending upon the direction of the current. This is known as the
_Peltier Effect_, the heat not being due to the resistance to the
current.


_=425. Thermopiles.=_ As the E. M. F. of the current produced by a
single pair of metals is small, several pairs are usually joined in
series in such a way that the different currents help each other and
flow in the same direction. Such combinations, usually made of antimony
and bismuth, are called thermoelectric piles, or simply thermopiles.
They are useful in detecting very small differences in temperature. The
heat of a match, or the cold of a piece of ice, will produce a current
even at some distance, the thermopile being connected with a sensitive
short-coil astatic galvanometer. (See "Things a Boy Should Know About
Electricity.")




CHAPTER XXV.

INDUCED CURRENTS.


_=426. Electromagnetic Induction.=_ You have seen, by experiments,
that a magnet has the power to induce another piece of iron or steel
to become a magnet. You have also seen, in the study of static
electricity, that an electrified body has the power to act through
space upon another conductor. A body may be polarized and charged with
static electricity by induction.

Several questions now come up. Can a _current_ of electricity in a
conductor induce a _current_ in another conductor not in any way
connected with the first? Can current electricity produce effects
through space? Is there an electromagnetic induction?

It has been seen that a current-carrying wire has a magnetic field,
and that magnetic fields can act through space. It is evident, then,
that a conductor will be surrounded and cut by lines of force when it
is placed in a magnetic field, or near a wire or coil through which a
current passes. Let us study this by experiments.


    =EXPERIMENTS 175-182. To study induced currents.=

    _Apparatus._ The two coils of wire (Nos. 89, 90); two short,
    soft iron cores (Nos. 92, 93); long iron core (No. 96); bar
    magnet (No. 97); astatic galvanoscope (No. 59); dry cell (No.
    51); key (No. 55); horseshoe magnet; connecting wires with
    spring connectors (No. 54) on the ends (§ 226-230); coil of
    wire (No. 98) wound on an iron core; compass.


[Illustration: Fig. 136.]

    =EXPERIMENT 175. To find whether a current can be generated
    with a bar magnet and a hollowed coil of wire.=

    =427. Directions.= (A) Arrange as in Fig. 136. The coil (No.
    90) of fine wire is joined to A G (No. 59) as shown. Small
    pieces of tin or copper, 1 and 2, are used to make connections
    between the coil ends and wires, 3 and 4, which are attached
    to the galvanoscope. It is best to use the wires, 3 and 4, so
    that the coil will be 2 feet at least, from A G; otherwise the
    needle of A G might be affected by the magnet, M (No. 97).

    (B) Get clearly in mind in which direction the right-hand end
    of the needle is deflected when a current enters A G at L, the
    left-hand binding-post. If you have forgotten the results of
    previous experiments, use the cell for an instant, touching the
    wire from the carbon to L and that from the zinc to R. If any
    currents come from the coil, later, you should be able to tell
    in which direction they flow, the coil and A G forming a closed
    circuit.

    (C) Hold the magnet, M, as shown, and quickly push it into
    the coil until it has the place of a core, at the same time
    watching the needle. If a current is produced, in which
    direction does it flow from the coil? Does the needle remain
    deflected? Is the current constant or temporary?

    (D) After the magnet, M, has been placed in the coil, as in
    (C), and the needle has come to rest, quickly pull M from the
    coil, watching the needle. If a current is produced, does it
    pass from the coil in the same direction as before, in (C)?

    (E) Turn M end for end, repeat (C) and (D), and study the
    results. Are lines of force made to cut the turns of the coil?

    (F) Repeat (C) and (D), moving M slowly.


_=428. Discussion.=_ An induced current, produced as in the above
experiment, is a momentary one. No current passes when the magnet and
coil are still; at least one of them has to be in motion. When the
magnet is inserted, the induced current is said to be an _inverse_ one,
as it passes in a direction opposite to that which would be necessary
to give the magnet its poles, it being considered a core magnetized
by the current. A _direct_ current is produced when the magnet is
withdrawn from the coil. Rapid movements produce stronger currents than
slow ones. (See § 439.)


_=429. Induced Currents and Work.=_ It takes force to move a magnet
through the center of a coil, and it is this work that is the source of
the induced current. When the coil is pushed on to the magnet, or when
it is moved through a magnetic field, force is also required. We have,
in this simple experiment, the key to the action of the dynamo and
other important electrical machines. These will be discussed later.


    =EXPERIMENT 176. To find whether a current can be generated
    with a bar magnet and a coil of wire having an iron core.=

    =430. Directions.= (A) Arrange as in Exp. 175, Fig. 136, and,
    in addition, place an iron core (No. 92) inside of the coil
    (No. 90).

    (B) Hold the bar magnet (No. 97) as in Fig. 136, and quickly
    lower it until it touches the core, at the same time watching
    the needle. Study results, direction of current, etc., as
    before.

    (C) Suddenly withdraw M from the core. Is the current produced
    in the same direction as that from (B)?

    (D) Turn M end for end and repeat (B) and (C).

    (E) Repeat (C) and (D), moving magnet slowly.

    How does the strength of the current compare with that of Exp.
    175? Are lines of force made to cut the turns of the coil?


[Illustration: Fig. 137.]

    =EXPERIMENT 177. To find whether a current can be generated
    with a horseshoe magnet and a coil of wire having an iron core.=

    =431. Directions.= (A) Arrange the apparatus as in Exp. 176,
    but use the horseshoe magnet, H M, instead of the bar magnet.
    Fig. 137 shows the coil (No. 90) with one pole of H M held over
    the core.

    (B) Study the effect of quickly lowering and raising first one
    pole and then the other over the core, as with the bar magnet.
    Get clearly in mind the direction in which the induced current
    flows in each case.


_=432. Induced Currents and Lines of Force.=_ In the experiments just
given, it should be remembered that the permanent magnets are sending
out thousands of lines of force from their N poles, and receiving them
again at their S poles. As the magnet is pushed into the coil (Exp.
175), the lines of force not only cut through the turns of the coil,
but the number of lines of force that cut the coil at any instant
varies rapidly as the magnet is moved.

Motion is necessary, with this arrangement, to make a change in the
number of cutting lines of force. The current passes only while the
magnet moves; and the direction of the current at any moment depends
upon whether the number of lines of force is increasing or decreasing
at that moment. (See § 438, 439.)


[Illustration: Fig. 138.]

    =EXPERIMENT 178. To find whether a current can be generated
    with an electromagnet and a hollow coil of wire.=

    =433. Directions.= (A) The hollow coil (No. 90) should be
    joined to the astatic galvanoscope, as shown in Fig. 136.
    Instead of the bar magnet in Fig. 136, an electromagnet is to
    be used, and this should be joined in series with a cell and
    key, as shown in Fig. 138. The current from the cell will pass
    only when K is pressed.

    (B) Note from the winding which way the current must pass
    around the coil when the circuit is closed at K, and determine
    whether the lower end of the long iron core, L I C (No. 96)
    should be N or S. With the compass test the poles of the core
    to be sure you are right.

    (C) Quickly lower the end of L I C into the hollow coil (H,
    Fig. 136), the circuit being kept closed long enough to allow
    the needle to partially come to rest again. Withdraw L I C
    before you open the circuit. Explain action of needle.

    (D) Reverse the direction of the current through the
    electromagnet, by changing the connections, and repeat (C).
    Does any induced current pass through A G when the core is held
    still in the coil H, even though a current passes through coil
    E?


[Illustration: Fig. 139.]

    =EXPERIMENT 179. To find whether a current can be generated
    with an electromagnet and a coil of wire having an iron core.=

    =434. Directions.= (A) Fig. 139 shows simply the arrangement of
    coils. Coil H (No. 90) with core, is joined to the galvanoscope
    as in Fig. 136. Coil E, with short core, should be joined to
    key and cell as shown in Fig. 138.

    (B) Keeping in mind the polarity of the lower end of core E,
    quickly lower it to the core of H, the circuit being kept
    closed for a few seconds. Does the needle remain deflected
    after the motion ceases?

    (C) Quickly raise E, the circuit being still closed, then
    open the circuit. Compare the directions taken by the induced
    currents in (B) and (C).


_=435. Discussion of Exps. 178, 179.=_ This motion in straight lines
is not suitable for producing currents strong enough for commercial
purposes. In order to produce currents of considerable strength, the
coils of wire have to be pushed past magnets with great speed. Special
machines (see Dynamos) are constructed in which the coils are wound
so that they can be given a rapid _rotary motion_ as they fly past
strong electromagnets. In this way the coil can keep on passing the
same magnets, in the same direction, as long as force is applied to the
shaft that carries them.


[Illustration: Fig. 140.]

    =EXPERIMENT 180. To study the effect of starting or stopping a
    current near a coil of wire or other closed circuit.=

    =436. Directions.= (A) Arrange as in Fig. 140. Place the two
    coils, H and E, on the same core, L I C. Connect E with the
    key and cell as before (Fig. 138). Connect H with the astatic
    galvanoscope, A G, as in Fig. 136. Keep the coils 2 or 3 feet
    from A G, so that the needle will not be affected by them.

    (B) Close the circuit at the key, watching the needle, then
    as soon as the needle regains its former position, open the
    circuit again. Compare the direction of the induced current in
    H with that of the current in E, (1) when the main circuit is
    closed, and (2) when it is opened. Is any current induced in H
    by a steady current in E? (See Transformers.)


[Illustration: Fig. 141.]

    =EXPERIMENT 181. To study the effect of starting or stopping a
    current in a coil placed inside of another coil.=

    =437. Directions.= (A) Arrange as in Fig. 141. Join coil H with
    the astatic galvanoscope, A G. Place the small coil P (No. 98)
    with core, inside of H, and connect the ends of P with the key
    and cell, as shown.

    (B) Close the circuit at K; watch the needle, and as soon as it
    regains its position, open the circuit again.

    Compare the direction of the induced current in H with that of
    the inducing current in P, (1) when the inducing circuit is
    closed, and (2) when it is broken. (See Induction Coils.)


_=438. Discussion of Exps. 180, 181.=_ When a current suddenly begins
to flow through a coil, the effect upon a neighboring coil is the same
as that produced by suddenly bringing a magnet near it; and when the
current stops, the opposite effect is produced.

We may consider that when the inducing circuit is closed, the lines of
force shoot out through the turns of the outside coil. Upon opening the
circuit the lines of force cease to exist; that is, we may imagine them
drawn in again.


[Illustration: Fig. 142.]

_=439. Direction of Induced Current.=_ Fig. 142 shows the magnet on its
way into the coil; the number of lines of force is increasing in the
coil, and the induced current passes in an anti-clockwise direction
when looking down into the coil along the lines of force. This produces
an _indirect_ current. If a current from a cell were passed through the
coil in the direction of this indirect current, the lower end of a bar
of iron would become a S pole. (See § 428.)


_=440. Laws of Induction.=_ (1) An increase in the number of lines of
force that pass through a closed circuit produces an indirect induced
current; while a decrease produces a direct one. (See § 428.)

(2) The E. M. F. of the induced current is equal to the rate of
increase or decrease in the number of lines of force that pass through
the circuit.

(3) A constant current produces no induced current, provided there is
no motion.

(4) Closing a circuit produces an indirect current.

(5) Opening a circuit produces a direct current.

(6) _Lenz's Law._ Induced currents have a direction that tends to stop
the motion that produces them.


_=441. Primary and Secondary Currents.=_ In the preceding experiments
in induction, it must be kept in mind that the current from the cell
did not pass through the galvanoscope. There were two entirely separate
circuits, in no way connected. The _primary_ current comes from the
cell, while the _secondary_ current is an induced one.


[Illustration: Fig. 143.]

    =EXPERIMENT 182. To see what is meant by alternating currents.=

    =442. Directions.= (A) Arrange as in Fig. 143. Connect coil H
    with A G, as before. Place one pole of H M against the end of
    the core I C, hold H with one hand, and with the other quickly
    push the other pole of H M onto the core. This should produce a
    momentary current through A G, first in one direction, and then
    in the other. Let the needle come to rest.

    (B) Move H M back and forth upon the end of I C, changing
    its polarity rapidly. A minute's practice will enable you to
    slide the core from one pole of H M to the other and back
    again rapidly--3 complete vibrations per second being about
    right. The needle should be parallel to the coil of A G, and
    if properly done, the needle will be made to vibrate back and
    forth slightly at each change in the polarity of I C.


_=443. Direct and Alternating Currents.=_ A current that flows steadily
in one direction is said to be a _direct_ current. A cell gives a
direct current when the circuit is closed. When the current passes in
one direction for an instant, and then reverses immediately and flows
in the opposite direction, it is said to _alternate_. The induced
current which flowed through the galvanoscope in Exp. 182 was an
alternating one. Currents of this class have great practical uses.


_=444. Self-Induction; Extra Currents.=_ It has been shown that
a magnetized coil can act through space and induce a current in
a neighboring coil. The lines of force which reach out from an
electromagnet will generate a current in any conductor which happens
to be in the field, or which is moved across the lines. It is evident,
then, since the lines of force from each turn of a coil cut all the
other turns of the same coil, that each turn acts as a conductor placed
in the field of every other turn. The instant a current begins to flow
through a coil, there is an inverse current of self-induction started
in the coil, which opposes the current in the cell. When the circuit is
broken, this _extra current_, as it is also called, is a direct one and
adds its strength to that of the current from the cell; as this takes
place at the instant the circuit is broken, a bright spark is seen at
the key, and this shows that the E. M. F. of this extra current is
high. Practical uses are made of it.




CHAPTER XXVI.

THE PRODUCTION OF MOTION BY CURRENTS.


_=445. Currents and Motion.=_ We have seen, in the experiments on
induced currents, that a current of electricity can be generated by
properly moving magnets near coils of wire. (See Dynamo-electric
Machines.) Can we reverse this process? Can motion be produced by the
electric current?


    =EXPERIMENTS 183-190. To study the production of motion by
    means of the electric current.=

    _Apparatus._ The support, including base, rod, and support
    wire, S W (Fig. 144.) Coils of wire (No. 89, 90); iron cores
    for coils; cell; key; connecting wires; compass; current
    reverser; bar magnet; horseshoe magnet.


[Illustration: Fig. 144.]

    =EXPERIMENT 183. Motion produced with a hollow coil and a piece
    of iron.=

    =446. Directions.= (A) Arrange as in Fig. 144. Coil H (No. 90)
    is to be used as a pendulum, and can be supported by fastening
    a string to it, the upper end of which should be tied to S W.

    Connect the ends of H with K and D C. There will be a slight
    magnetic field about H as soon as the circuit is closed.

    (B) Hold I C near the end of the coil. Close the circuit for an
    instant. Is there any motion produced in H? While the motion
    will be slight, there should be enough to be noticed if the
    cell is strong.

    (C) Swing the suspended coil back and forth like a pendulum for
    a minute, until you get in mind the rapidity of its vibrations.
    Stop it, then repeat (B), closing and opening the circuit at
    regular intervals, so that the little impulses given by the
    attraction for I C will gradually cause H to vibrate. The
    wires leading from H should not drag upon the table.


    =EXPERIMENT 184. Motion with hollow coil and bar magnet.=

    =447. Directions.= (A) Substitute the bar magnet M (No. 97)
    for the iron of Exp. 183 (Fig. 144). Get clearly in mind the
    polarity of the coil from the way the current flows through it,
    then test it with the compass to find whether you are right.

    (B) Hold the N pole of M near the left-hand end of the coil,
    close the circuit for an instant and study results.

    (C) Reverse the magnet and repeat (B). Compare the results with
    those of Exp. 183. Try to make the coil vibrate.


    =EXPERIMENT 185. Motion with electromagnet and piece of iron.=

    =448. Directions.= (A) Arrange as described in Exp. 183, Fig.
    144. Place a short core inside of the coil and repeat. (See §
    446 for directions.) Why is the motion produced much larger
    than that given by a hollow coil?

    (B) The coil can gradually be made to swing through quite a
    little space by closing and opening the circuit regularly (§
    446, C). Could any use be made of such a motion, if it were on
    a large scale? Could it be made to run a machine?


[Illustration: Fig. 145.]

    =EXPERIMENT 186. Motion with electromagnet and bar magnet.=

    =449. Directions.= (A) Arrange as in Fig. 145, the coil being
    suspended and connected as in Exp. 183 (Fig. 144).

    (B) Study the effect of closing the circuit when the N pole of
    M is held near the core of H. Reverse M, and repeat.


[Illustration: Fig. 146.]

    =EXPERIMENT 187. Motion with electromagnet and horseshoe
    magnet.=

    =450. Directions.= (A) Arrange as in Fig. 146. The ends of H
    (No. 89) are joined to X and Y of the current reverser C R (No.
    57). It is evident, then, that the direction of the current
    through H can be easily and rapidly reversed by C R. (See Exp.
    103.) Either pole of the horseshoe magnet H M will attract I C
    when it is not magnetized.

    (B) Place the end of I C near the N pole of H M so that it will
    be attracted to it. You have learned that like poles repel each
    other, so press the lever of C R that will produce a N pole at
    the left-hand end of I C. The core I C should be repelled by
    the N pole of H M and be instantly attracted by its S pole.

    (C) Rapidly reverse the current and make I C jump back
    and forth from one pole to the other. The results of this
    experiment should be remembered, as they will aid in
    understanding motors. A core 1/4 in. in diameter can be placed
    in between the poles and be made to vibrate rapidly as the
    current is reversed.


[Illustration: Fig. 147.]

    =EXPERIMENT 188. Motion with two electromagnets.=

    =451. Directions.= (A) Arrange as in Fig. 147. Join the two
    coils, H and E, in parallel. Connect their two outside ends
    O E to a metal plate A, and their inside ends I E to B. Join
    wires 1 and 6 to K, D C, A and B, as shown. When the circuit
    is closed at K, the current will pass along wire 1 and divide
    at A, entering E and H at the same time by wires 2 and 4 and
    returning through 3 and 5 to B, and thence to D C.

    (B) Close the circuit for an instant with wires arranged as in
    Fig. 147. Do the electromagnets attract or repel each other?
    Study out the direction in which the current passes around the
    coils, and see whether they _should_ attract or repel.

    (C) Change wire 4 to B, and wire 5 to A. The polarity of H,
    only, will be changed when this circuit is closed. Press the
    key for an instant and study the results.


_=452. Discussion of Exps. 183-188.=_ From the results it is evident
that motion can be produced with the aid of the electric current in
many different ways. It can be produced at the ends of wires which
simply reach across the room, or which reach miles from the source
of the current. To get practical results for commercial purposes we
require a proper source of current, proper conductors, and proper
apparatus to convert the motions into useful work. The motions given to
the parts of the apparatus in the previous experiments are not suitable
for commercial purposes, as they are in straight lines. A rotary motion
is needed to do good work; and when this is applied to a shaft, belts
can be used to run all sorts of machinery. (See Electric Motors.)


[Illustration: Fig. 148.]

    =EXPERIMENT 189. Rotary motion with a hollow coil of wire and a
    permanent magnet.=

    =453. Directions.= (A) Arrange as in Fig. 148. A key can be
    used instead of the reverser. The coil of the galvanoscope, G
    V, has a magnetic field about it when the circuit is closed.
    The needle has a permanent field.

    (B) Close the circuit for an instant, let the needle swing back
    past the zero mark, close the circuit again, etc., until the
    added impulses give the needle a complete turn.

    (C) Keep the needle turning on its axis by opening and closing
    the circuit at the proper time. With a little practice you can
    make it turn rapidly.

    (D) Reverse the motion of the needle. (See § 455.)


[Illustration: Fig. 149.]

    =EXPERIMENT 190. Rotary motion with an electromagnet and a
    permanent magnet.=

    =454. Directions.= (A) Arrange as in Fig. 149. Place the
    compass a short distance from the end of the core of the coil
    H (No. 89). Close the circuit, and as soon as the needle gets
    part way around open it again, closing it at the proper time
    to give the needle a new impulse. The speed can be regulated,
    somewhat, by changing its distance from the core. A key may be
    used in place of a reverser.

    (B) Reverse the direction of rotation.


_=455. Discussion of Exps. 189-190.=_ We have, in these experiments,
the key to the action of electric motors. By properly opening and
closing the circuit, the rotary motion can be kept up as long as
current is supplied. If a small pulley were attached to the top of
the compass-needle in Exp. 190, a tiny belt could be attached, and we
should have a machine that could do, perhaps, a fly-power of work. (See
Electric Motors.)




CHAPTER XXVII.

APPLICATIONS OF ELECTRICITY.


_=456. Things Electricity Can Do.=_ Among the almost countless things
that electricity can do are the following: It signals without wires. It
drills rock, coal, and teeth. It cures diseases and kills criminals. It
protects, heats, and ventilates houses. It photographs the bones of the
human body. It rings church bells and plays church organs. It lights
streets, cars, boats, mines, houses, etc. It pumps water, cooks food,
and fans you while eating. It runs all sorts of machinery, elevators,
cars, boats, and wagons. It sends messages with the telegraph,
telephone, and search-light. It cuts cloth, irons clothes, washes
dishes, blackens boots, welds metals, prints books, etc., etc.

As this book deals almost exclusively with experiments, to be performed
with simple, home-made apparatus, space cannot be given for a
discussion of the many instruments and machines which make electricity
a practical every-day thing. (See "Things A Boy Should Know About
Electricity.") The principles upon which a few important instruments
depend, however, will be given.


[Illustration: Fig. 150.]

    =EXPERIMENT 191. To study the action of a simple "telegraph
    sounder."=

    =457. Directions.= (A) Arrange as in Fig. 150. The
    electromagnet is supported upon its base, as directed in § 407.
    Coil H, K, and D C are joined in series. The iron strip, I, can
    be held by the left hand, while K is worked with the right.

    (B) Press the key, closing the circuit for different lengths of
    time, and note that the _armature_, I, responds exactly to the
    motions at K.


_=458. Discussion.=_ The downward click makes a distinct sound, and in
regular instruments the armature is allowed to make an upward click,
also. The time between the two clicks can be short or long to represent
_dots_ or _dashes_, which, together with _spaces_, represent letters.
(For telegraph alphabet, and complete directions for making and
connecting a home-made telegraph line, see Apparatus Book.)


[Illustration: Fig. 151.]

_=459. Telegraph Line; Connections.=_ Fig. 151 shows complete
connections for a home-made telegraph line. The capital letters are
used for the right side, R, and small letters for the left side, L.

Gravity cells, B and b, are used. The _sounders_ S and s, and the
_keys_, K and k, are shown by a top view, or plan. The broad black
lines of S and s represent the armatures, which are directly over the
electromagnets. The keys have switches, E and e.

The two stations, R and L, may be near each other or in different
houses. The _return wire_, R W, passes from the copper of b to the zinc
of B. This is important, as the cells must help each other; that is,
they are in series. The _line wire_, L W, passes from one station to
the other, and the return may be through a wire, R W, or through the
earth; but for short lines a return wire is best.


_=460. Operation of Line.=_ Suppose R (right) and L (left) have a line.
Fig. 151 shows that R's switch, E, is open, while e is closed. The
entire circuit, then, is broken at but one point. As soon as R presses
his key, the circuit is closed, and the current from both cells rushes
around from B through K, S, L W, s, k, b, R W and back to B. This makes
the armatures of S and s come down with a click at the same time. (See
Exp. 191.) As soon as the key is raised, the armatures raise, making
the up-click. (See § 458.) As soon as R has finished, he closes his
switch, E. L then opens e and answers R. Both E and e are closed when
the line is not in use, so that either can open his switch at any time
and call up the other. Closed circuit cells are used for such lines. On
large lines the current from a dynamo is used.


[Illustration: Fig. 152.]

    =EXPERIMENT 192. To study the action and use of the "relay" on
    telegraph lines.=

    =461. Directions.= (A) Arrange as in Fig. 152. Place K and D C
    at one end of the table to represent the sending station. At
    the other end of the table place E, which is the electromagnet
    of the relay, and H, the electromagnet of the sounder. Connect
    the ends of E with K and D C, L W being the line wire, and R W
    the return. In practice, the return is through the earth. The
    relay armature, R A, should vibrate towards E every time K is
    pressed. C is a piece of copper against which R A presses each
    time it is attracted by E, and this closes what is called the
    local circuit. Connect the poles of another battery, L B, with
    C and H, and the other end of coil H with R A. The sounder
    armature, S A, should be arranged as in Exp. 191. Small springs
    are shown on the two armatures, and these keep them away from
    the cores when the circuits are open.

    (B) Fasten the parts to a board, and study the connections and
    action of this home-made outfit.

    =462. The Relay= replaces the sounder in the line wire circuit,
    and its coils are usually wound with many turns of fine
    wire, so that a feeble current will move its nicely adjusted
    armature. Owing to the large resistance of long telegraph
    lines, the current is weak when it reaches a distant station,
    and not strong enough to work an ordinary sounder. The current
    passes back from the relay to the sending station through the
    earth. The relay armature acts as an automatic key to open and
    close the local circuit, which includes also a battery and
    sounder. The line current does not enter the sounder. (See
    "Things A Boy Should Know About Electricity.")


[Illustration: Fig. 153.]

    =EXPERIMENT 193. To study the action of a two-pole telegraph
    instrument.=

    =463. Directions.= (A) Arrange as in Fig. 153. Connect the two
    coils to the connecting plates, as described in § 408. Join a
    strip of copper Cu with wire 2 leading from D C, and join the
    zinc of D C to M. The ends of wires 1 and 3 should be near Cu
    but they must not touch it. If Cu be slightly curved so that
    its ends are raised above the table, the ends of wires 1 and
    3 may be put directly under the ends of Cu; each half of Cu
    can then be used as a key. Two armatures, A and B, should be
    held as shown. D C can be placed at one side, of course, its
    terminals being joined to M and Cu.

    (B) Press first one end and then the other of Cu, so that the
    current will pass through H or E at will.

    (C) Paste pieces of paper to the armatures, the left one being
    marked with a dot, and the other with a dash. The one who
    sends the message can make dots or dashes at the instrument by
    pressing the proper key. This form of instrument can be easily
    made by boys, and the messages are more easily read by the eye
    than by the ear, as in regular sounders.


[Illustration: Fig. 154.]

    =EXPERIMENT 194. To study the action of a simple "single needle
    telegraph instrument."=

    =464. Directions.= (A) Arrange as in Fig. 154. Stick a pin on
    each side of the N pole of the galvanoscope-needle through the
    degree-card, so that the needle can make but part of a turn
    when the circuit is closed.

    (B) Touch one lever of the reverser C R, then the other, to
    see whether connections are right. The needle should be forced
    against one pin and then against the other. If motions to
    the left represent _dots_, and those to the right _dashes_,
    combinations of dots and dashes can be used for letters as in
    the "sounder" (Exp. 191).

    (C) Arrange the apparatus shown in Fig. 122 so that messages
    can be sent.


[Illustration: Fig. 155.]

    =EXPERIMENT 195. To study the action of a simple automatic
    "contact breaker," or "current interrupter."=

    =465. Directions.= (A) Arrange as in Fig. 155. Slip a spring
    connector attached to wire 1 upon the iron strip I, a short
    distance from its end. Hold the left-hand end of I firmly in
    one hand, and with the other hold the connector on wire 2 just
    above that on 1. The right-hand end of I should be just above
    the core of H.

    (B) Allow the current to pass through the circuit by touching
    the two connectors together gently. Does the armature make one
    click, as in the telegraph sounder, or does it vibrate rapidly?

    (C) Try the connectors in various positions on I.


_=466. Automatic Current Interrupters=_ are used on bells, buzzers,
induction coils, etc. The principle upon which they work is shown in
the above experiment (Fig. 155). The current, as it comes from the
carbon of D C, is obliged to stop when it reaches I, unless the two
connectors touch. As soon as the current passes, I is pulled down
and away from the upper connector, and this breaks the circuit. I,
being held firmly in the hand, immediately springs back to its former
position, closing the circuit. The rapidity of the vibrations depends
somewhat upon the position of the connectors upon I. In regular
instruments, a platinum point is used where the circuit is broken; this
stands the constant sparking at that point.


[Illustration: Fig. 156.]

    =EXPERIMENT 196. To study the action of a simple "electric
    bell," or a "buzzer."=

    =467. Directions.= (A) Fig. 156 shows the circuit explained in
    Exp. 195, with a key or push-button put in, so that the circuit
    can be closed at a distance from the vibrating armature.

    (B) Have a friend work the key while you hold I and wires 1 and
    2 as directed in Exp. 195. The circuit must not be broken at
    two places, of course, so begin by holding the two connectors
    together. The armature should vibrate rapidly each time K is
    pressed.


_=468. Electric Bells and Buzzers=_ are very nearly alike in
construction; in fact, you will have a buzzer by removing the bell from
an ordinary electric bell. Buzzers are used in places where the loud
sound of a bell would be objectionable.

By placing a bell near the end of the vibrating armature (Fig. 156), so
that the bell would be struck by it at each vibration, we should have
an electric bell. By making the wires 1 and 3 long, the bell or buzzer
can be worked at a distance. (See Apparatus Book, Chapter XV, for
Home-made Bells and Buzzers.)


[Illustration: Fig. 157.]

    =EXPERIMENT 197. To study the action of a simple telegraph
    "recorder."=

    =469. Directions.= (A) Cut from a tin box, or can, a piece of
    tin about 4 in. long and 1-1/2 in. wide. Bend this double to
    make two thicknesses. This will serve as an armature I (Fig.
    157). Nail to one end of I a small spool, S, and into this put
    a short length of lead-pencil, P, which may be held firmly in S
    by wrapping a little paper around it. Connect the ends of coil
    H to a key and cell as in Fig. 156.

    (B) Hold or fasten I in place, and have a friend make dots and
    dashes at the key, while you draw a piece of paper past the end
    of P. A little adjusting will be necessary to get the pencil to
    write only while the circuit is closed. In regular machines all
    the parts are automatic.


[Illustration: Fig. 158.]

    =EXPERIMENT 198. To study the action of a simple "annunciator."=

    =470. Directions.= (A) Arrange as in Fig. 158. Fasten the
    two electromagnets, H and E, to a board or a piece of stiff
    cardboard. They may be held in place by passing strings over
    them and through the board, tying on the other side. The ends
    of coils H and E should be joined to pieces of tin, A, B, C, by
    means of connectors. K and K are keys or push-buttons, which
    in real instruments are in different rooms. Two steel pens may
    be swung on pins a short distance from the ends of the cores,
    so that their lower ends will be attracted to the cores the
    instant the current passes through them. The residual magnetism
    should hold them against the cores until removed. Hairpins,
    nails, or needles can be used instead of pens.

    (B) Press first one K and then the other to see whether your
    connections are correct.


_=471. Annunciators.=_ There are many forms of annunciators in use to
indicate, in a hotel for example, a certain room when a bell rings at
the office. If a bell be included in the circuit between D C and A
in Fig. 158, it will ring each time a key is pushed. This will call
attention to the fact that some one has rung, and the annunciator will
show the location of the special call. Large instruments are made with
hundreds of electromagnets, each one answering to a special room. The
instrument should be set, of course, after each call. A nail or screw
wound with insulated wire can be used for the electromagnets of a
home-made annunciator.


    =EXPERIMENT 199. To study the shocking effects of the "extra
    current."=

    =472. Directions.= (A) Use the two electromagnets joined to
    the connecting plates (Fig. 132), to generate a self-induced
    or extra current. Connect R of Fig. 132 with the zinc of a dry
    cell, and between L and the carbon of the cell place a key; in
    other words, join the electromagnets, cell, and key in series.
    Two good cells in series can be used to advantage.

    (B) Wet the ends of two fingers of the left hand, press one
    upon L and the other on R, thus making a shunt with your hand.
    With the right hand work the key rapidly. If the current is
    strong enough you should feel a slight shock in the fingers
    each time the circuit is broken. The extra current (§ 444)
    causes the shock as it shoots through the fingers.

    (C) If you have electric bells or telegraph sounders use them
    for this experiment.


_=473. Induction Coils=_ are instruments for producing induced currents
of high E. M. F. The apparatus shown in Fig. 141 forms a simple
induction coil. The _primary_ coil is made of coarser wire and has less
turns of wire than the _secondary_ coil. The current in the primary
circuit is usually interrupted by an _automatic interrupter_ (Exp.
195), thus producing an alternating current in the secondary coil,
the voltage of which depends upon the relative number of turns in the
two coils. Induction coils are used in telephone work, for medical
purposes, for X-ray work, etc., etc.

(For Home-made Induction Coils see Apparatus Book, Chapter XI.)

[Illustration: Fig. 159.]

    =474. Action of Induction Coils.= Fig. 159 shows a top view of
    one of the home-made induction coils described, in full, in the
    Apparatus Book. Wires 5 and 6 are the ends of the primary coil,
    while wires 7 and 8 are the terminals of the secondary coil.
    The battery wires should be joined to binding-posts W and X,
    and the handles to Y and Z. Fig. 160 shows the details of the
    automatic interrupter which is placed in the primary circuit.

[Illustration: Fig. 160.]

    If the current enters at W, it will pass through the primary
    coil and out at X, after going through 5, R, F, S I, B, E and
    C. The instant the current passes, the bolt becomes magnetized;
    this attracts A, which pulls B away from the end of S I, thus
    automatically opening the circuit. B at once springs back to
    its former position against S I, as A is no longer attracted;
    the circuit being closed, the operation is rapidly repeated.
    (For commercial forms and uses of induction coils see "Things A
    Boy Should Know About Electricity.")


_=475. Transformers=_, like induction coils, are instruments for
changing the E. M. F. and strength of currents. There is very little
loss of energy in well-made transformers. They consist of two coils of
wire on the same core; in fact, an induction coil may be considered
a transformer. If the secondary coil has 100 times as many turns of
wire as the primary, a current with an E. M. F. of 100 volts can
be taken from the secondary coil, when the E. M. F. of the current
passing through the primary is 1 volt; but the _strength_ (amperes) of
the secondary current will be but one-hundredth that of the primary
current. By using the coil of fine wire as the primary, the E. M. F. of
the current that comes from the other coil will be but one-hundredth
that in the fine coil. It will have 100 times its strength, however.
Continuous currents from cells or dynamos must be interrupted, as
in induction coils, to be transformed from one E. M. F. to another.
Transformers are now largely used in lighting and power circuits, etc.
(See "Things A Boy Should Know About Electricity.")


_=476. The Dynamo.=_ We saw in the Exps. of Chapter XXV. that currents
of electricity can be generated in a coil of wire (closed circuit)
by rapidly moving it through the field of a magnet. As shown by the
experiments, this can be accomplished in many ways. The dynamo is a
machine for doing this on a large scale, the coils being given a rotary
motion in a very strong magnetic field; and as the number of lines of
force that cut the coil is constantly changing, there is a current in
the coil as long as power is applied, and this current is led from the
machine by proper devices.

_The dynamo is a machine for converting mechanical energy into an
electric current, through electromagnetic induction._

If a loop of wire (Fig. 161) be so arranged on bearings at its ends
that it can be made to revolve, a current will flow through it in
one direction during one-half of the revolution, and in the opposite
direction during the other half, it being insulated from all external
conductors. Such a current inside of the machine would be of no value;
it must be led out to external conductors. Some sort of sliding contact
is necessary to connect a revolving conductor with a stationary one.

[Illustration: Fig. 161.]

[Illustration: Fig. 162.]

Fig. 162 shows the ends of a coil joined to two rings, X, Y, which
are insulated from each other, and which rotate with the coil. Two
stationary pieces of carbon, A, B, called _brushes_, press against
the rings, and to these are joined wires which complete the circuit,
and which lead out where the current can do work. The arrows show the
direction of the current during one-half of a revolution. The rings
form a _collector_, and this arrangement gives an alternating current.

[Illustration: Fig. 163.]

In Fig. 163 the ends of the coil are joined to the two halves of a
cylinder. These halves, X and Y, are insulated from each other and
from the axis. The current flows from X onto the brush A, through some
external circuit where it does work, and thence back through brush
B onto Y. By the time that Y gets around to A the direction of the
current in the loop has reversed, so that it passes towards Y; but it
still enters the outside circuit through A because Y is then in contact
with A. This device is called a _commutator_, and it allows a constant
or direct current to leave the machine.

In regular machines there are many loops of wire and several segments
to the commutator. The rotating coils are wound upon an iron core,
so that the lines of force, in passing from one pole to the other,
will meet with as little resistance as possible. The coils, core, and
commutator, taken together, are called the _armature_. The magnets
which furnish the field are called the _field-magnets_. These are
electromagnets, the current from the dynamo, or a part of it, being
used to excite them. There are many forms of dynamos, and many ways
of winding the armature and field-magnets, but space will not permit
a discussion of them here. (See "Things a Boy Should Know About
Electricity.")


_=477. The Electric Motor.=_ Experiments have shown that motion can be
produced by the electric current in many ways. The galvanoscope may be
considered a tiny motor.

_An electric motor is a machine for transforming electric energy into
mechanical power._

While the electric motor is similar in construction to the dynamo, it
is opposite to it in action. Motors receive current and produce motion.
The motion is a rotary one, the power being applied to other machines
by means of belts or gears.


[Illustration: Fig. 164.]

    =EXPERIMENT 200. To study the action of the telephone.=

    =478. Directions.= (A) Join the ends of coil H (Fig. 164) to
    the astatic galvanoscope. Move magnet M back and forth in front
    of the soft iron core, while H is held in position. Watch the
    needle. Imagine that vibrations in the air caused by the voice
    are strong enough to give M a slight motion to and fro, and you
    can see how a current would be sent through the galvanoscope by
    speaking against M.


_=479. The Telephone=_ is an instrument for reproducing sounds at a
distance, and electricity is the agent by which this is generally
accomplished. The part spoken to is called the _transmitter_, and the
part which gives the sound out again is called the _receiver_. Sound
itself does not pass over the line. Although the same apparatus may be
used for both transmitter and receiver, they are generally different in
construction.


[Illustration: Fig. 165.]

_=480. The Bell or Magneto-transmitter=_ generates its own current,
and is, strictly speaking, a dynamo that is run by the voice. You have
seen, by experiments, that a current can be generated in a coil of wire
by moving a magnet back and forth in front of its soft iron core. In
the telephone this process is reversed, soft iron in the shape of a
thin disc (D, Fig. 165) being made to vibrate by the voice immediately
in front of a coil having a permanent magnet, M, for a core.

The soft iron diaphragm is fixed near, but it does not touch the
magnet. The coil consists of many turns of fine insulated wire. The
current generated is an alternating one and exceedingly feeble; in
fact, it can not be detected by a galvanoscope.


_=481. The Receiver=_ has the same construction as the bell
transmitter, and receives the currents from the line. As the diaphragm
is always attracted by the magnet, it is under a constant strain.
This strain is increased when a current passes through the coil in a
direction that adds strength to the magnet, and decreased when the
current weakens the magnet.

When the current through the coil is always in the same direction,
but varies in strength, the diaphragm will vibrate on account of the
varying pull upon it.

[Illustration: Fig. 166.]

When the current through the coil is an alternating one, the same
result is obtained, as the magnet gets weaker and stronger many times
per minute. Fig. 166 shows two bell instruments joined, either being
used as the transmitter and the other as the receiver.


_=482. The Carbon Transmitter=_ does not in itself generate a current
like the magneto-transmitter; it merely produces changes in the
strength of a current that flows through it, and that comes from some
outside source.

In Fig. 167, X and Y are two carbon buttons, X being attached to the
diaphragm, D. Button Y presses gently against X. When D is caused to
vibrate by the voice, X is made to press more or less against Y, and
this allows more or less current to pass through the circuit, in which
also is the receiver, R. This direct undulating current changes the
pull upon the diaphragm of R, causing it to vibrate and reproduce the
original sounds spoken into the transmitter.

[Illustration: Fig. 167.]


_=483. Induction Coils in Telephone Work.=_ As the resistance of
telephone lines is large, a current with a fairly high E. M. F. is
desired. While the current from one or two cells is sufficient to
work the transmitter, it is not strong enough to force its way over a
long line. To get around this difficulty an induction coil is used to
transform the battery current, that flows through the transmitter and
primary coil, into a current with a high E. M. F. that can go into the
main line and force its way to a distant receiver.

The battery current in the primary coil is undulating, but always in
the same direction, the magnetic field around the core getting weaker
and stronger. This causes an alternating current in the secondary coil
and main line.

[Illustration: Fig. 168.]

Fig. 168 shows the two coils, P, S, of the induction coil. The primary,
P, is joined in series with a cell and transmitter. The secondary
coil, S, is joined to the receiver. One end of S can be grounded, the
current completing the circuit through the earth and into the receiver
through another wire entering the earth. There are many forms of
transmitters. (See "Things a Boy Should Know About Electricity.")


_=484. Electric Lighting and Heating.=_ Whenever resistance is offered
to the electric current, heat is produced. By proper appliances, the
heat of resistance can be applied just where it is needed, and many
commercial processes depend upon electricity for their success. Dynamos
are used to generate currents for lighting and heating purposes. There
are two great systems of lighting, the one by _arc_ lamps and the
other by _incandescent_ lamps. (See "Things a Boy Should Know About
Electricity.")


_=485. Arc Lamps=_ produce a light when a current passes from one
carbon rod to the other across an air-space. As the current starts
through the lamp, the ends of the carbons touch, and the imperfect
contact causes resistance enough to heat the ends red-hot. They are
then automatically separated, and the current passes from one to the
other, causing the "arc." The resistance of the air-space is reduced by
the intensely heated vapor and flying particles of carbon.


_=486. The Incandescent Lamp=_ consists of a glass bulb, in which is a
vacuum, and the light is caused by the passage of a current through a
thin fibre of vegetable carbon, enclosed in the vacuum. The fibre would
burn instantly if allowed to come in contact with the air. The fibres
have a high resistance, and are easily heated to incandescence.




CHAPTER XXVIII.

WIRE TABLES.


_Copper Wire Tables_ are very convenient, and a necessity when working
electrical examples. The tables here given are taken from a dealer's
catalogue, and will be found sufficiently accurate for ordinary work.

_Explanation of Tables._ In the _first_ column are given the sizes of
wires by numbers. The B & S or American gauge is used. In the table
below is given a comparison between the B & S and the Birmingham gauges.

The _second column_ gives the diameters of wires. The diameter of No.
36 wire is 5 thousandths of an inch; the diameter of No. 24 wire is a
little over 20 thousandths or 2 hundredths of an inch.

The _third column_ contains what is called circular mils, a mil being
a thousandth of an inch. The figures in this column are obtained by
squaring those in the second; thus, for No. 36 wire, 5 × 5 = 25.
This column is useful when working examples where the squares of the
diameters are wanted. The rest of the table explains itself.

The table at the bottom gives a comparison between the fractional and
decimal parts of an inch. Space can not be given here for a series
of examples showing the many uses of this table. (See "Elementary
Electrical Examples.")


COPPER WIRE TABLES.

(Based on the
B. A. Unit.)

    =====+=======+=========+=======+===================================+
    Gauge| DIAM- |Sectional|Capac- |                OHMS               |
         | ETER. |  AREA   | ity.  |                                   |
    -----+-------+---------+-------+-----------+----------+------------+
    B.&S.|   In  |  In     |In Amp-|    Per    |    Per   |    Per     |
     No. |1000ths|Circular | eres. |   1,000   |   Mile.  |   Pound.   |
         |       |  Mils.  |       |    feet.  |          |            |
    -----+-------+---------+-------+-----------+----------+------------+
     0000|.460   |211600.  | 312.  |     .04906|    .25903|     .000077|
      000|.40964 |167805.  | 262.  |     .06186|    .32664|     .00012 |
       00|.3648  |133079.  | 220.  |     .07801|    .41187|     .00019 |
        0|.32486 |105534.  | 185.  |     .09831|    .51909|     .00031 |
        1|.2893  | 83694.  | 156.  |     .12404|    .65490|     .00049 |
        2|.25763 | 66373.  | 131.  |     .15640|    .8258 |     .00078 |
        3|.22942 | 52634.  | 110.  |     .19723|   1.0414 |     .00125 |
        4|.20431 | 41743.  |  92.3 |     .24869|   1.313  |     .00198 |
        5|.18194 | 33102.  |  77.6 |     .31361|   1.655  |     .00314 |
        6|.16202 | 26251.  |  65.2 |     .39546|   2.088  |     .00499 |
        7|.14428 | 20817.  |  54.8 |     .49871|   2.633  |     .00792 |
        8|.12849 | 16510.  |  46.1 |     .6529 |   3.3    |     .0125  |
        9|.11443 | 13094.  |  38.7 |     .7892 |   4.1    |     .0197  |
       10|.10189 | 10382.  |  32.5 |     .8441 |   4.4    |     .0270  |
       11|.090742|  8234.  |  27.3 |    1.254  |   6.4    |     .0501  |
       12|.080808|  6530.  |  23.  |    1.580  |   8.3    |     .079   |
       13|.071961|  5178.  |  19.3 |    1.995  |  10.4    |     .127   |
       14|.064084|  4107.  |  16.2 |    2.504  |  13.2    |     .200   |
       15|.057068|  3257.  |  13.6 |    3.172  |  16.7    |     .320   |
       16|.05082 |  2583.  |  11.5 |    4.001  |  23.     |     .512   |
       17|.045257|  2048.  |   9.6 |    5.04   |  26.     |     .811   |
       18|.040303|  1624.  |   8.1 |    6.36   |  33.     |    1.29    |
       19|.03589 |  1288.  |  .... |    8.25   |  43.     |    2.11    |
       20|.031961|  1021.  |  .... |   10.12   |  53.     |    3.27    |
       21|.028462|   810.  |  .... |   12.76   |  68.     |    5.20    |
       22|.025347|   642.  |  .... |   16.25   |  85.     |    8.35    |
       23|.022571|   509.  |  .... |   20.30   | 108.     |   13.3     |
       24|.0201  |   404.  |  .... |   25.60   | 135.     |   20.9     |
       25|.0179  |   320.  |  .... |   32.2    | 170.     |   33.2     |
       26|.01594 |   254.  |  .... |   40.7    | 214.     |   52.9     |
       27|.014195|   201.  |  .... |   51.3    | 270.     |   84.2     |
       28|.012641|   159.8 |  .... |   64.8    | 343.     |  134.      |
       29|.011257|   126.7 |  .... |   81.6    | 482.     |  213.      |
       30|.010025|   100.5 |  .... |  103.     | 538.     |  338.      |
       31|.008928|    79.7 |  .... |  130.     | 685.     |  539.      |
       32|.00795 |    63.  |  .... |  164.     | 865.     |  856.      |
       33|.00708 |    50.1 |  .... |  206.     |1033.     | 1357.      |
       34|.006304|    39.74|  .... |  260.     |1389.     | 2166.      |
       35|.005614|    31.5 |  .... |  328.     |1820.     | 3521.      |
       36|.005   |    25.  |  .... |  414.     |2200.     | 5469.      |
       37|.004453|    19.8 |  .... |  523.     |2765.     | 8742.      |
       38|.003965|    15.72|  .... |  660.     |3486.     |13772.      |
       39|.003531|    12.47|  .... |  832.     |4395.     |21896.      |
       40|.003144|     9.88|  .... | 1049      |5542.     |34823.      |
    -----+-------+---------+-------+-----------+----------+------------+


    =====+=======================+========================
    Gauge|         FEET.         |         POUNDS.
         |                       |
    -----+-----------+-----------+-----------+------------
    B.&S.|   Per     |   Per     |    Per    |    Per
     No. |  Pound.   |   Ohm.    |1,000 feet.|    Ohm.
    -----+-----------+-----------+-----------+------------
     0000|    1.56122|20497.7    |  640.51   |12987.
      000|    1.9687 |16255.27   |  507.95   | 8333.
       00|    2.4824 |12891.37   |  402.83   | 5263.
        0|    3.1303 |10223.08   |  319.45   | 3225.
        1|    3.94714| 8107.49   |  253.34   | 2041.
        2|    4.97722| 6429.58   |  200.91   | 1282.
        3|    6.2765 | 5098.61   |  159.32   |  800.
        4|    7.9141 | 4043.6    |  126.35   |  505.
        5|    9.97983| 3206.61   |  100.20   |  318.
        6|   12.5847 | 2542.89   |   79.462  |  200.
        7|   15.8696 | 2015.51   |   63.013  |  126.
        8|   20.0097 | 1599.3    |   49.976  |   80.
        9|   25.229  | 1268.44   |   39.636  |   50.
       10|   31.8212 | 1055.66   |   31.426  |   37.
       11|   40.1202 |  797.649  |   24.924  |   20.
       12|   50.5906 |  632.555  |   19.766  |   12.65
       13|   63.7948 |  501.63   |   15.674  |    7.87
       14|   80.4415 |  397.822  |   12.435  |    5.00
       15|  101.4365 |  315.482  |    9.859  |    3.12
       16|  127.12   |  250.184  |    7.819  |    1.95
       17|  161.29   |  198.409  |    6.199  |    1.23
       18|  203.374  |  157.35   |    4.916  |     .775
       19|  256.468  |  124.777  |    3.899  |     .473
       20|  323.399  |   98.9533 |    3.094  |     .305
       21|  407.815  |   78.473  |    2.452  |     .192
       22|  514.193  |   62.236  |    1.945  |     .119
       23|  648.452  |   49.3504 |    1.542  |     .075
       24|  817.688  |   39.1365 |    1.223  |     .047
       25| 1031.038  |   31.0381 |     .9699 |     .030
       26| 1300.180  |   24.6131 |     .7692 |     .0187
       27| 1639.49   |   19.5191 |     .6099 |     .0118
       28| 2067.364  |   15.4793 |     .4837 |     .0074
       29| 2606.959  |   12.2854 |     .3835 |     .0047
       30| 3287.084  |    9.7355 |     .3002 |     .0029
       31| 4414.49   |    7.72143|     .2413 |     .0018
       32| 5226.915  |    6.12243|     .1913 |     .0011
       33| 6590.41   |    4.85575|     .1517 |     .00076
       34| 8312.8    |    3.84966|     .1204 |     .00046
       35|10481.77   |    3.05305|     .0956 |     .00028
       36|13214.16   |    2.4217 |     .0757 |     .00018
       37|16659.97   |    1.92086|     .06003|     .00011
       38|21013.25   |    1.52292|     .04758|     .00007
       39|26496.237  |    1.20777|     .03755|     .00004
       40|33420.63   |    0.97984|     .02992|     .000029
    -----+-----------+-----------+-----------+------------

Comparative Table of the Fractional and Decimal Parts of an Inch.

    +-----------------+
    | 1/64  = .015625 |
    | 1/32  = .031250 |
    | 3/64  = .046875 |
    | 1/16  = .062500 |
    | 5/64  = .078125 |
    | 3/32  = .093750 |
    | 7/64  = .109375 |
    | 1/8   = .125000 |
    | 9/64  = .140625 |
    | 5/32  = .156250 |
    | 11/64 = .171875 |
    | 3/16  = .187500 |
    | 13/64 = .203125 |
    | 7/32  = .218750 |
    | 15/64 = .234375 |
    | 1/4   = .250000 |
    | 17/64 = .265625 |
    | 9/32  = .281250 |
    | 19/64 = .296875 |
    | 5/16  = .312500 |
    | 21/64 = .328125 |
    | 11/32 = .343750 |
    | 23/64 = .359375 |
    | 3/8   = .375000 |
    | 25/64 = .390625 |
    | 13/32 = .406250 |
    | 27/64 = .421875 |
    | 7/16  = .437500 |
    | 29/64 = .453125 |
    | 15/32 = .468750 |
    | 31/64 = .484375 |
    | 1/2   = .500000 |
    +-----------------+

Comparative Table of B. and S. and B. W. Gauges in Decimal Parts of an
Inch.

    +------------+--------------+-------------+
    |Birmingham  |  American    |   No. of    |
    |Wire Gauge. |  (B. and S.) | Wire Gauge. |
    |            |  Wire Gauge. |             |
    +------------+--------------+-------------+
    | 0000       |  .46         |  .454       |
    |  000       |  .40964      |  .425       |
    |   00       |  .3648       |  .38        |
    |    0       |  .32486      |  .34        |
    |    1       |  .2893       |  .3         |
    |    2       |  .25763      |  .284       |
    |    3       |  .22942      |  .259       |
    |    4       |  .20431      |  .238       |
    |    5       |  .18194      |  .22        |
    |    6       |  .16202      |  .203       |
    |    7       |  .14428      |  .18        |
    |    8       |  .12849      |  .165       |
    |    9       |  .11443      |  .148       |
    |   10       |  .10189      |  .134       |
    |   11       |  .090742     |  .12        |
    |   12       |  .080808     |  .109       |
    |   13       |  .071961     |  .095       |
    |   14       |  .064084     |  .083       |
    |   15       |  .057068     |  .072       |
    |   16       |  .05082      |  .065       |
    |   17       |  .045257     |  .058       |
    |   18       |  .040303     |  .049       |
    |   19       |  .03589      |  .042       |
    |   20       |  .031961     |  .035       |
    |   21       |  .028468     |  .032       |
    |   22       |  .025347     |  .028       |
    |   23       |  .022571     |  .025       |
    |   24       |  .0201       |  .022       |
    |   25       |  .0179       |  .02        |
    |   26       |  .01594      |  .018       |
    |   27       |  .014195     |  .016       |
    |   28       |  .012641     |  .014       |
    |   29       |  .011257     |  .013       |
    |   30       |  .010025     |  .012       |
    |   31       |  .008928     |  .01        |
    |   32       |  .00795      |  .009       |
    |   33       |  .00708      |  .008       |
    |   34       |  .006304     |  .007       |
    |   35       |  .005614     |  .005       |
    |   36       |  .005        |  .004       |
    |   37       |  .004453     |             |
    |   38       |  .003965     |             |
    |   39       |  .003531     |             |
    |   40       |  .003114     |             |
    +------------+--------------+-------------+




LIST OF APPARATUS

FOR

The Study of Elementary Electricity and Magnetism by Experiment.


The =100= pieces of apparatus in the following list are referred to,
by number, in the experiments contained in "The Study of Elementary
Electricity and Magnetism by Experiment." This list is furnished
to give those who wish to make their own apparatus an idea of the
approximate size, etc., of the various articles used. The author is
preparing a price catalogue of the articles included in this list,
and of odds and ends needed in the construction of simple, home-made
apparatus.

=No. 1.= A package of 25 steel sewing-needles. To be suitable for
experiments in magnetism, these should be of good, hard steel, and not
too thick.

=No. 2.= A flat cork, about 1 in. in diameter and 3/8 in. thick.

=No. 3.= A candle for annealing steel.

=No. 4-15.= One dozen assorted annealed iron wires, from 1 in. to 6 in.
in length. The iron should be very soft.

=No. 16.= One English horseshoe magnet, 2-1/2 in. long, best quality.

=No. 17.= A small box of iron filings from soft iron.

=No. 18.= A compass (Fig. 5). The needle swings very freely; it is
enclosed in a wooden pill box, the cover of which forms the support.

=No. 19, 20.= Two soft steel wire nails, 2 in. long.

=No. 21, 22.= Two pieces of spring steel, about 3 in. long and 3/8 in.
wide, to be magnetized by the student and used as bar magnets.

=No. 23.= An iron ring, or washer, about 7/8 in. in diameter.

=No. 24.= A sifter for iron filings. This consists of a pasteboard pill
box: Prick holes through the bottom with a pin.

=No. 25.= A thin, flexible piece of spring steel, about 3 in. long and
1/8 in. wide.

=No. 26, 27.= Two ebonite sheets (E S, Fig. 34), each 4 in. square.
These are made with a special surface. They are very much better than
the ordinary smooth ebonite.

=No. 28.= One ebonite rod (E R, Fig. 34), 3-1/2 in. long, with special
surface.

=No. 29.= One ebonite rod, 1-3/4 in. long, with special surface, used
to support the insulating table, No. 43 (I T, Fig. 32).

=No. 30.= One piece of flannel cloth, 7 in. square.

=No. 31.= Six sheets of tissue-paper, each 4 in. square.

=No. 32.= A few feet of white cotton thread.

=No. 33.= A few feet of black silk thread.

=No. 34.= One support base (S B, Fig. 56). This is of thin wood, about
3-3/4 in. by 6-1/2 in., to one end of which is fastened a spool for
holding the support rod (No. 35).

=No. 35.= One support rod (S R, Fig. 56), 7 in. long and 5/16 in.
in diameter. This rod has a hole in each end. The small hole is for
holding the support wire (No. 36); the large hole is for the ebonite
rod (No. 29).

=No. 36.= One support wire (S W, Fig. 144).

=No. 37.= One wire swing (W S, Fig. 29).

=No. 38.= One sheet of glass, 4 in. square.

=No. 39.= One bent hairpin (H P, Fig. 32).

=No. 40.= Bottom of flat box (B F B, Fig. 32), 3-5/8 in. in diameter.

=No. 41.= Top of flat box (T F B, Fig. 33).

=No. 42.= One electrophorus cover (E C, Fig. 34), 3-5/8 in. in
diameter. This has rounded edges, and a small tube is riveted into the
top of it to hold the insulating handle, E R.

=No. 43.= One insulating table (I T, Fig. 32). This is made the same as
No. 42, and is supported by No. 29.

=No. 44.= One insulated copper wire, 2-1/2 feet long.

=No. 45.= One rubber band (R B, Fig. 33).

=No. 46.= Six bent wire clamps (B C, Fig. 37).

=No. 47.= One tin box conductor (T B, Fig. 42). This cylindrical
conductor is about the size of an ordinary baking powder box.

=No. 48.= One hairpin discharger for the condenser.

=No. 49.= Two sheets of aluminum-leaf for the leaf electroscope (Fig.
57) and other experiments.

=No. 50.= One bent wire (H P, Fig. 57) used in connection with the leaf
electroscope.

=No. 51.= A dry cell, ordinary size about 7 in. high and 2-1/2 in. in
diameter.

=No. 52.= Enough mercury to amalgamate battery zincs. A wooden pill box
containing about half a thimbleful will do.

=No. 53.= A coil containing 25 feet of No. 24 insulated copper wire for
connections.

=No. 54.= One dozen spring connectors (Fig. 61) for making connections.
These are made of brass, nickel plated, and do not affect the
compass-needle.

=No. 55.= A telegraph key (Fig. 68) without switch. The metal straps
are made of aluminum; they are 1/2 in. wide, and are fastened to a neat
wooden base.

=No. 56.= Three metal plates, each about 2 in. by 3/4 in., on which
spring connectors (No. 54) are to be pushed in order to join two wires.

=No. 57.= A current reverser (Fig. 69). The straps are made of aluminum
and are fastened to a neat wooden base.

=No. 58.= A galvanoscope (Fig. 72) including a degree-card (No. 99).
The cardboard coil-support, C S, is 5 × 6-1/4 in., and the hole in it
is 3-3/8 in. in diameter. The coil is 4-1/4 in. in diameter, made of
No. 24 insulated copper wire.

=No. 59.= An astatic galvanoscope (Fig. 77). The whole may be taken
apart and mailed in the containing box, B, which is 4-3/8 × 3-1/8 × 1
in. The coil is made of No. 31 wire, and has a resistance of about 5
ohms. Spring connectors are used to join a wire to the apparatus by
pushing the connectors into the tubular binding-posts, L and R.

=No. 60-63.= Four strips of sheet zinc, 4 in. by 1/2 in., not
amalgamated.

=No. 64.= A carbon rod, 4 in. long (Fig. 81).

=No. 65, 66.= Two glass tumblers (Fig. 81).

=No. 67, 68.= Two strips of sheet copper, 4 in. by 1/2 in. (Fig. 85).

=No. 69.= One galvanized iron nail.

=No. 70, 71.= Two wooden cross-pieces (Fig. 85).

=No. 72.= One dozen brass screws, 5/8 in. long, size No. 5, with round
heads.

=No. 73.= A porous cup (P C, Fig. 87) that will stand inside of the
tumblers (No. 65).

=No. 74.= A zinc rod, about 3/8 in. in diameter, like those used in
Leclanché cells.

=No. 75.= A sheet copper plate for the two-fluid cell (C, Fig. 87).
This is 2 in. wide; it nearly surrounds the porous cup, and is
supported upon the edge of the tumbler by a narrow strip, A, with which
connections are made by spring connectors (No. 54).

=No. 76.= One strip of sheet iron, 4 in. by 1/2 in.

=No. 77, 78.= Two strips of sheet lead, 4 in. by 1/2 in.

=No. 79.= A resistance coil (Fig. 94). The coil is made of No. 24
insulated copper wire; it has a resistance of 2 ohms (nearly) and is
fastened to a cardboard base. It is so arranged that either one or two
ohms can be used at will.

=No. 80.= A Wheatstone's bridge (Fig. 103), including a scale (No.
100). The aluminum straps, 1, 2, 3, are fastened to a neat wooden base,
10 in. long by 2 in. wide. A No. 28 German-silver wire is used for the
bridge.

=No. 81.= A piece of No. 30 uncovered German-silver wire, 2.1 meters
long, used for resistance (Fig. 96).

=No. 82.= A piece of No. 28 uncovered German-silver wire, 2.1 meters
long.

=No. 83-85.= Three plate binding-posts, consisting of bent straps of
sheet aluminum (X, Y, Z, Fig. 96).

=No. 86.= Two ounces of copper sulphate, commonly called bluestone. The
crystals may be kept in a large wooden pill box.

=No. 87.= One dozen copper washers.

=No. 88.= One combination rule, 1 ft. long, marked with English and
metric systems.

=No. 89.= A hollow coil of No. 24 insulated copper wire (Fig. 130). The
spool, on which the wire is wound, has a hole for a five-sixteenths
inch core. It is turned down thin, so that the wire is near the
core. The coil is about 1-1/8 in. long and 1 in. in diameter. Spring
connectors are joined to the ends of the coil.

=No. 90.= A hollow coil of No. 25 insulated copper wire, similar to No.
89, with spring connectors attached to its ends.

=No. 91.= Carbon rod for electroplating.

=No. 92, 93.= Two soft iron cores, with screws (I C, Fig. 130). These
cores are 5/16 in. in diameter, and have a threaded hole in one end for
fastening them to No. 94.

=No. 94.= A tin box with three holes punched in its top (Fig. 132).
This serves as a base, as well as a yoke, for the two electromagnets,
A, B, shown in plan.

=No. 95.= Combination connecting plates (Fig. 132). Three aluminum
straps are fastened to a wooden base. They are turned up at their ends
so that spring connectors can be easily pushed upon them.

=No. 96.= One long iron core (L I C, Fig. 140). This is of soft iron,
5/16 in. in diameter, and long enough to pass through both coils (No.
89, 90).

=No. 97.= Bar magnet, about 4 in. long and 5/16 in. in diameter.

=No. 98.= Coil of insulated wire wound on a soft iron core, to act as
a primary coil for induction experiments. This coil fits inside of the
hollow coils (Nos. 89, 90).

=No. 99.= A printed degree-card for the galvanoscope (No. 58). This is
printed on stiff cardboard, about 3 in. in diameter.

=No. 100.= A printed scale for the Wheatstone's bridge (No. 80). This
is printed on stiff paper. The scale is 8 in. long, and is divided into
10 large divisions, each of which is subdivided into 10 parts, thus
making 100 parts in all.




INDEX.

Numbers refer to paragraphs. See Table of Contents for the various
experiments.


  Abreast, arrangement of cells, 365.

  Accumulators. (See storage cells.)

  Action, local, 273.

  Air, as insulator, 144, _a_.

  Alternating currents, 443.

  Amalgamating, 257, 274.

  Amber, 107.

  Ammeter, 353.

  Ampere, the, 351, 357.

  Ampere's rule, 386.

  Annealing, 6.

  Annunciators, 471.

  Anode, 373, 378.

  Applications of electricity, Chap. XXVII.

  Arc lamp, 485.

  Arrangement, of cells, 363 to 368.

  Armature, the, 11, 78, 476.

  Astatic needles, 251, 253, 254;
    galvanoscope, 252, 256.

  Atmospheric electricity, Chap. XIII., 217;
    causes of, 221.

  Attraction, mutual, 111;
    and repulsion, laws of magnetic, 29;
    electric, laws of, 121.

  Aurora borealis, 223.


  Batteries, Chap. XV.;
    storage, 382.

  Bell, electric, 468.

  Bell telephone, 480.

  Bichromate cell, 289.

  Bound electrification, 162, 191.

  Breaking a magnet, 51.

  Bridge, Wheatstone's, 324 to 330.

  Brushes, 476.

  Buzzers, electric, 468.


  Cable, submarine, as condenser, 182.

  Capacity, inductive, 169;
    electrical, 176, 178.

  Carbon, transmitter, 482;
    electroscope, 114.

  Cathode, 373.

  Cell, galvanic, Chap. XV.;
    arrangement of, 363, 364, 365, 368;
    chemical action in, 270, 271;
    direction of current in, 268;
    local action in, 273;
    open and closed circuit, 286;
    polarization of, 278;
    poles of, 269;
    simple, 275;
    secondary, 382;
    single-fluid, 275;
    two-fluid, 281, 285;
    various galvanic, 286 to 291.

  Charge, in condenser, 195;
    residual of condensers, 197.

  Charging conductors, Chap. VIII.

  Chemical action, 369.

  Chemical effects of current, Chap. XXI.

  Circuit, electric, 266;
    divided, 293;
    short, 295.

  Coercive force, 44, 46.

  Coils, 390;
    induction, 473, 474;
    method of joining, 408;
    polarity of, 392;
    resistance, 309;
    simple resistance, 310.

  Commutator, 476.

  Compass, 26;
    our, 32;
    needle, 243, 249.

  Compound magnets, 73.

  Condensation of electrification, Chap. X., 178.

  Condensers, 178;
    action of, 186, 191;
    induction coil, 181;
    submarine cables, 182.

  Conductive discharge, 149, 184.

  Conductors, 126, 129, 133;
    hollow and solid, 153;
    and insulators, relation between, 133;
    and non-conductors, 312.

  Connections, electrical, 226 to 230.

  Contact breaker, Exp. 195;
    § 466.

  Convective discharge, 149.

  Copper sulphate solution, 283.

  Cores, of electromagnets, 397.

  Coulomb, the, 354.

  Current electricity, Part III.

  Current, 144, _a_, 264;
    detectors, 232, 239;
    direction of in cell, 268;
    direct and alternating, 443;
    extra, 444;
    interrupters, 466;
    primary and secondary, 441;
    measurement of, 352;
    reverser, 235, 237;
    strength of, Chap. XX., 350, 358, 362 to 365;
    unit of, 351.


  Daniell cell, 290.

  Declination, 84.

  Depolarizers, 280, 282.

  Detectors, current, 232, 239.

  Diamagnetic bodies, 15.

  Dielectric, 184, 191, 195.

  Dielectrics, 166.

  Dip, 86.

  Direct currents, 428, 443.

  Dischargers, 188.

  Discharges, kinds of, 149.

  Divided circuits, 293, 323.

  Dry cells, 288.

  Dynamo, 435, 476.


  Earth's magnetism, 83, 92, 93.

  Electric, bells, 468;
    chime, 193;
    circuit, 266, Chap. XVI.;
    current, 144, _a_, 264;
    density, 155;
    field, 156;
    horse-power, 355;
    lighting, 484;
    machines, static, 216;
    motor, 477;
    polarization, 159;
    resistance, Chap. XVIII.;
    wind, 155.

  Electricity, static, Part II.;
    Current, Part III.;
    Applications of, Chap. XXVII., 456;
    kinds of, 100;
    derivation of name, 107;
    Atmospheric, Chap. XIII., 217.

  Electrification, Chap. VI., 103, 116, 132, 134;
    and heat, 104;
    condensation of, Chap. X.;
    escape of, 155;
    free and bound, 162;
    induced, Chap. IX.;
    kinds of, 120;
    of earth, 222;
    source of in cells, 265;
    theories about, 145;
    two kinds of, 120, 211.

  Electrics and non-electrics, 134.

  Electrified bodies, 102, 107.

  Electrodes, 269.

  Electromagnetism, Chap. XXII., 383.

  Electromagnets, Chap. XXIII., 396;
    cores of, 397;
    horseshoe, 405.

  Electromotive force, 144, Chap. XVII., 296, 300, 303;
    measurement of, Exp. 140;
    of polarization, 373, 382;
    series, 301;
    unit of, 297.

  Electrophorus, 138;
    action of, 171, 172;
    our, 139.

  Electrolysis, 370.

  Electrolyte, 370.

  Electroplating, 376, 378.

  Electroscope, action of, 206, 208;
    carbon, 114;
    pith-ball, 200;
    our leaf, 201, 202.

  Electroscopes, Chap. XI.

  Electrotyping, 379.

  Equator of magnet, 13.

  Equipotential points, 326.

  External resistance, 307, 368.

  Extra current, 444;
    Exp. 199.


  Field, electric, 156;
    magnetic, Chap. IV., 62, 80;
    magnets, 476.

  Figures, magnetic, 64;
    permanent, 417.

  Force, 103;
    lines of magnetic, 64, 73, 80, 156;
    lines of electric, 156;
    lines of about a wire, 385.

  Franklin, Benjamin, 218.

  Free electrification, 162.

  Frictional electricity, Part II.

  Fulminating panes, 180.


  Galvanic cells, Chap. XV., 265;
    chemical action in, 270;
    various kinds of, 286 to 291.

  Galvanoscope, 240 to 249.

  Glass, as insulator, 136.

  Gold-leaf electroscope, 200, 209.

  Gravity cell, 291.


  Hardening steel, 8, 10.

  Heat, effect on resistance, 343;
    effect on magnet, 49.

  Horse-power, electric, 355.

  Horseshoe magnet, 11;
    advantages of, 82;
    electromagnets, 405.

  Hydrogen, 260, 262, 271, 278, 279, 373.


  Inclination of needle, 86.

  Induced currents, Chap. XXV.;
    and work, 429;
    and lines of force, 432, 435, 438;
    direction of, 439.

  Induced magnetism, Chap. III.

  Induction coils, 473;
    action of, 474;
    condensers of, 181;
    with telephone, 483.

  Induction, electromagnetic, 426;
    laws of, 440;
    static, theory of, 159;
    successive, 168.

  Inductive capacity, 169.

  Insulators, Chap. VII., 125.

  Internal resistance, 307, 314, 358, 362, 368.

  Iron and steel, Chap. I.

  Iron, hardening properties of,
    Exp. 4;
    impurities of, 1;
    kinds of, 2;
    soft, 10.


  Jar, Leyden, 179.


  Key, 233, 234.


  Lamp, arc, 485;
    incandescent, 486.

  Laws, of electrification, 121;
    of induction, 440;
    of magnetism, 29;
    of resistance, 349.

  Leclanché cell, 287.

  Leyden jar, 179.

  Lighting, 484, 485, 486.

  Lightning, 144, _a_; 218;
    rods, 220.

  Lines of force, about a wire, 385, 388;
    electric, 156;
    and induced currents, 432, 438;
    magnetic, 64, 73, 74, 80, 156;
    resistance to, 78, 397.

  Local action, 273.

  Local currents, 273.

  Lodestone, 93.


  Magnetic, bodies, 15;
    circuits, closed, 420;
    field, 62, 80;
    figures, permanent, 417;
    figures, 64, Exp. 161, 162, Exps. 168 to 171;
    force, lines of, 64, 80, 156;
    induction of the earth, 92;
    needle, 26;
    needles, balancing of, 88;
    needle, dip of, 86;
    problems, 33;
    saturation, 42;
    screens, 18;
    tick, Exp. 160;
    transparency, 18.

  Magnetism, Part I.;
    induced, Chap. III., 53;
    residual, 44, 53;
    temporary, 53;
    terrestrial, Chap. V.;
    theory of, 42;
    of earth, 83;
    laws of, 29.

  Magnets, bar, 21;
    compound, 73;
    effect of breaking, 51;
    equator of, 13, 51;
    experimental, 407;
    kinds of, 11;
    natural, 93;
    poles of, 13, 25;
    practical uses of, 16.

  Mercury, 274.

  Motion, production of, Chap. XXVI., 445, 452, 455.

  Motors, electric, 477.

  Mutual attractions, 111.


  Natural magnets, 93.

  Needle, astatic, 251, 253, 254;
    magnetic, 26, 32.

  Negative electrification, 120.

  Neutral bodies, 102.

  Non-conductors, 312.

  Non-electrics, 134.

  North-seeking poles, 25.


  Ohm, the, 308.

  Ohm's law, 356.

  One-fluid theory, 145.

  Open and closed circuits, 266;
    cells, 286.

  Oxygen, 372, 373, 382.


  Peltier effect, 424.

  Pith-ball electroscope, 200.

  Plates or elements, 267.

  Polarization of cells, 278;
    effects of, 279;
    electric, 159, 164;
    electromotive force of, 373;
    magnetic, 56;
    remedies for, 280.

  Poles, 13, 25, 64, 92;
    consequent, 39;
    of coils, 392;
    of electrodes, 269;
    reversal of, 35;
    rule for, 31.

  Pole pieces, 56.

  Positive electrification, 120.

  Potential, 133,144;
    energy, 103.

  Primary current, 441.

  Proof-plane, 209.


  Quantity, unit of, 354.


  Recorder, Exp. 197.

  Relay, telegraph, 462.

  Repulsion, laws of electrostatic, 121;
    laws of magnetic, 29.

  Residual, charge in condenser, 197;
    magnetism, 44;
    magnetism of core, Exp. 159.

  Resistance, coils, 309;
    effect of heat on, 343;
    electrical, Chap. XVIII., 305, 319, 321;
    external and internal, 307, 362, 368;
    internal, 314, 358;
    laws of, 349;
    to lines of force, 78;
    measurement of, Chap. XIX.;
    unit of, 308.

  Retentivity, 44, 46.

  Reverser, current, 235, 237.

  Rheostat, simple, 344.


  Saturation, magnetic, 42.

  Secondary cells, 382;
   current, 441.

  Self-induction, 444.

  Series arrangement of cells, 364.

  Shocks, 188.

  Short circuits, 295.

  Shunts, 293.

  Silk, as insulator, 136.

  Single-fluid cell, 275.

  Single needle telegraph, Exp. 194.

  Sounder, telegraph, 458.

  Spark, 144, _a_.

  Static electricity, Part II.

  Static electric machines, 216.

  Steel, Chap. I.;
    kinds of, 2;
    magnetism of, 42, 46, 49.

  St. Elmo's fire, 222.

  Storage cells, 382.

  Successive, induction, 168;
    condensation, 199.

  Sulphuric acid, 258, 262, 314.


  Tangent galvanometer, 352.

  Telegraph, line, 459, 460;
    relay, 462;
    single needle instrument, Exp. 194;
    sounder, 458;
    static, 130.

  Telephone, the, 479;
    Bell, 480;
    carbon transmitter, 482;
    with induction coils, 483;
    receiver, 481.

  Tempering steel, 8.

  Temporary magnetism, 53.

  Terrestrial magnetism, Chap. V.

  Thermoelectricity, Chap. XXIV., 423.

  Thermopile, 425;
    home-made, 421.

  Thunder, 219.

  Transformers, 475.

  Transmitters, 480, 482.

  Two-fluid cell, 280, 281;
    care of, 282;
    chemical action in, 285.

  Two-fluid theory of electrification, 146.


  Unit of, current strength, 351;
    E. M. F., 297;
    of power, 355;
    quantity, 354;
    resistance, 308.


  Variation, angle of, 84.

  Varieties of electricity, 100.

  Volt, the, 297.

  Voltameters, 297, 353, 380.


  Water, composition of, 372.

  Watt, the, 355.

  Wheatstone's bridge, 324 to 330.

  Wind, electric, 155.

  Wire tables, Chap. XXVIII.


  Yoke, use of, 406.


  Zero, potential, 144, _a_.

  Zinc, chemical action with, 271;
    with commercial, 273.

  Zinc plates, reasons for amalgamating, 274.




Notes.




Notes.




                           ELECTRICAL BOOKS
                         ELECTRICAL APPARATUS
                            GAMES PUZZLES
                              EDUCATIONAL
                              AMUSEMENTS

[Illustration]

                      THOMAS M. ST. JOHN, Met. E.




A Word to Parents About Games and Educational Amusements.


Systematic play is as important as systematic work. The best games
and home amusements are as valuable to a child as school-studies; in
fact, they bring out and stimulate qualities in a child, which no
school-study can. Fascinating home amusements are as necessary as
school-books.

Boys and girls like to be busy. Their amusements should be entered
into as heartily, chosen as carefully, and purchased as willingly as
school-books.

=Games.=--JINGO and HUSTLE-BALL are good games.

They are interesting and full of action.

They arouse a child's common-sense.

They cultivate an ability to think rapidly, judge correctly, and decide
quickly.

They educate the eye and hand at the same time.

They are very simple, and may be played at once.

=Educational Amusements.=--There is a peculiar fascination about
Electricity and Magnetism, which makes these subjects appeal to every
boy and girl.

There is nothing better than science studies, to teach children
to observe and to see what they look at; besides, it is _fun_ to
experiment. "Fun With Electricity" and "Fun With Magnetism" are
educational amusements. They contain fascinating experiments and are
systematically arranged.




Juvenile Work in Electricity.


_From The Electrical Engineer, May 19, 1898._

The position that Young America is now taking in the electric and
magnetic field is very clearly shown at the Electrical Show now being
held at Madison Square Garden, by an exhibit of simple experimental
apparatus made by young boys from the Browning School, of this city.

The models shown cover every variety of apparatus that is dear to
the heart of a boy, and yet, along the whole line from push-buttons
to motors, one is struck by the extreme simplicity of design and the
ingenious uses made of old tin tomato cans, cracker boxes, bolts,
screws, wire, and the wood that a boy can get from a soap box.

The apparatus in this exhibit was made by boys 13, 14 and 15 years
of age, from designs made by Mr. Thomas M. St. John, of the Browning
School. It clearly shows that good, practical apparatus can be made
from cheap materials by an average boy. The whole exhibit is wired and
in working order, and it attracts the attention of a large number of
parents and boys who hover around to see, in operation, the telegraph
instruments, buzzers, shocking coils, current detectors, motors, etc.

Mr. St. John deserves the thanks of every boy who wants to build his
own electrical apparatus for amusement or for experimental purposes, as
he has made the designs extremely simple, and has kept constantly in
mind the fact that the average boy has but a limited supply of pocket
money, and an equally limited supply of tools.




How Two Boys Made Their Own Electrical Apparatus.


    =CONTENTS:= _Chapter_ I. Cells and Batteries.--II. Battery
    Fluids and Solutions.--III. Miscellaneous Apparatus and Methods
    of Construction.--IV. Switches and Cut-Outs.--V. Binding-Posts
    and Connectors.--VI. Permanent Magnets.--VII. Magnetic
    Needles and Compasses.--VIII. Yokes and Armatures.--IX.
    Electro-Magnets.--X. Wire-Winding Apparatus.--XI. Induction
    Coils and Their Attachments.--XII. Contact Breakers
    and Current Interrupters.--XIII. Current Detectors and
    Galvanometers.--XIV. Telegraph Keys and Sounders.--XV.
    Electric Bells and Buzzers.--XVI. Commutators and Current
    Reversers.--XVII. Resistance Coils.--XVIII. Apparatus for
    Static Electricity.--XIX. Electric Motors.--XX. Odds and
    Ends.--XXI. Tools and Materials.

"The author of this book is a teacher and writer of great ingenuity,
and we imagine that the effect of such a book as this falling into
Juvenile hands must be highly stimulating and beneficial. It is
full of explicit details and instructions in regard to a great
variety of apparatus, and the materials required are all within the
compass of very modest pocket-money. Moreover, it is systematic and
entirely without rhetorical frills, so that the student can go right
along without being diverted from good helpful work that will lead
him to build useful apparatus and make him understand what he is
about. The drawings are plain and excellent. We heartily commend the
book."--_Electrical Engineer._

"Those who visited the electrical exhibition last May cannot have
failed to notice on the south gallery a very interesting exhibit,
consisting, as it did, of electrical apparatus made by boys. The
various devices there shown, comprising electro-magnets, telegraph keys
and sounders, resistance coils, etc., were turned out by boys following
the instructions given in the book with the above title, which is
unquestionably one of the most practical little works yet written that
treat of similar subjects, for with but a limited amount of mechanical
knowledge, and by closely following the instructions given, almost any
electrical device may be made at very small expense. That such a book
fills a long-felt want may be inferred from the number of inquiries
we are constantly receiving from persons desiring to make their own
induction coils and other apparatus."--_Electricity._

"At the electrical show in New York last May one of the most
interesting exhibits was that of simple electrical apparatus made by
the boys in one of the private schools in the city. This apparatus,
made by boys of thirteen to fifteen years of age, was from designs
by the author of this clever little book, and it was remarkable to
see what an ingenious use had been made of old tin tomato-cans,
cracker-boxes, bolts, screws, wire, and wood. With these simple
materials telegraph instruments, coils, buzzers, current detectors,
motors, switches, armatures, and an almost endless variety of apparatus
were made. In his book Mr. St. John has given directions in simple
language for making and using these devices, and has illustrated
these directions with admirable diagrams and cuts. The little volume
is unique, and will prove exceedingly helpful to those of our young
readers who are fortunate enough to possess themselves of a copy. For
schools where a course of elementary science is taught, no better
text-book in the first-steps in electricity is obtainable."--_The Great
Round World._




    Exhibit of Experimental Electrical Apparatus AT THE ELECTRICAL
    SHOW, MADISON SQUARE GARDEN, NEW YORK.

    While only 40 pieces of simple apparatus were shown in this
    exhibit, it gave visitors something of an idea of what young
    boys can do if given proper designs.

[Illustration:
      "HOW TWO BOYS MADE THEIR OWN ELECTRICAL APPARATUS"
       Gives Proper Designs--Designs for over 150 Things.]




JUST PUBLISHED.

How Two Boys Made Their Own Electrical Apparatus.


    Containing complete directions for making all kinds of simple
    electrical apparatus for the study of elementary electricity.
    By PROFESSOR THOMAS M. ST. JOHN, New York City.

    The book measures 5 × 7-1/2 in., and is beautifully bound in
    cloth. It contains 141 pages and 125 illustrations. Complete
    directions are given for making 152 different pieces of
    Apparatus for the practical use of students, teachers, and
    others who wish to experiment.

PRICE, POST-PAID, $1.00.


The shocking coils, telegraph instruments, batteries, electromagnets,
motors, etc., etc., are so simple in construction that any boy of
average ability can make them; in fact, the illustrations have been
made directly from apparatus constructed by young boys.

The author has been working along this line for several years, and he
has been able, _with the help of boys_, to devise a complete line of
simple electrical apparatus.


_THE APPARATUS IS SIMPLE because the designs and methods of
construction have been worked out practically in the school-room,
absolutely no machine-work being required._

_THE APPARATUS IS PRACTICAL because it has been designed for real use
in the experimental study of elementary electricity._

_THE APPARATUS IS CHEAP because most of the parts can be made of old
tin cans and cracker boxes, bolts, screws, wires and wood._


  Address, THOMAS M. ST. JOHN,
               407 West 51st Street,
                             New York.




Fun With Magnetism.

BOOK AND COMPLETE OUTFIT FOR SIXTY-ONE EXPERIMENTS IN MAGNETISM....

[Illustration]


Children like to do experiments; and in this way, better than in any
other, _a practical knowledge of the elements of magnetism_ may be
obtained.

These experiments, although arranged to amuse boys and girls, have been
found to be very _useful in the class-room_ to supplement the ordinary
exercises given in text-books of science.

To secure the best _possible quality of apparatus_, the horseshoe
magnets were made at Sheffield, England, especially for these sets.
They are new and strong. Other parts of the apparatus have also been
selected and made with great care, to adapt them particularly to these
experiments.--_From the author's preface._

=CONTENTS.=--Experiments With Horseshoe Magnet.--Experiments With
Magnetized Needles.--Experiments With Needles, Corks, Wires, Nails,
etc.--Experiments With Bar Magnets.--Experiments With Floating
Magnets.--Miscellaneous Experiments.--Miscellaneous Illustrations
showing what very small children can do with the Apparatus.--Diagrams
showing how Magnetized Needles may be used by little children to make
hundreds of pretty designs upon paper.

=AMUSING EXPERIMENTS.=--Something for Nervous People to Try.--The
Jersey Mosquito.--The Stampede.--The Runaway.--The Dog-fight.--The
Whirligig.--The Naval Battle.--A String of Fish.--A Magnetic Gun.--A
Top Upsidedown.--A Magnetic Windmill.--A Compass Upsidedown.--The
Magnetic Acrobat.--The Busy Ant-hill.--The Magnetic Bridge.--The
Merry-go-Round.--The Tight-rope Walker.--A Magnetic Motor Using
Attractions and Repulsions.

_The Book and Complete Outfit will be sent, Post-paid, upon receipt of
35 Cents, by_

THOMAS M. ST. JOHN, 407 W. 51st St., New York.




A Few Off-Hand Statements


that have been made about "Fun With Magnetism" and "Fun With
Electricity" in letters of inquiry to the author. (These statements
were absolutely unsolicited.)

"My little boy has your 'Fun With Magnetism' and enjoys it so much that
if the 'Fun With Electricity' is ready I would like to have it for him.
Please let me know," etc.

"I have had much fun with 'Fun With Magnetism.'"

"My boy has 'Fun With Magnetism' and has enjoyed it very much and would
like the other. Will you," etc.

"Please let me know when 'Fun With Electricity' is upon the market, for
if it is as good as this, I shall certainly want it."

"I have just received 'Fun With Magnetism' and am delighted with it.
Please send me 12 sets," etc.

"I have 'Fun With Magnetism' and 'Fun With Electricity' and have
enjoyed them very much. Please send," etc.

"I am much pleased with 'Fun With Electricity' and would like to have,"
etc.

"'Fun With Electricity' is fine and I have had lots of fun with it.
Please send," etc.

"Having experimented with both of your apparatus 'Fun With Magnetism'
and 'Fun With Electricity,' and having found them both amusing and
instructive, I wish to ask," etc.

"I have purchased your outfits 'Fun With Electricity' and 'Fun With
Magnetism,' and though they are designed for amusement, I find them a
great help in my studies. Will you please," etc.

"I have one of your outfits of 'Fun With Electricity,' and I enjoy it
very much, some of the experiments being very astonishing. Will you
please," etc.

"I have enjoyed 'Fun With Magnetism' and 'Fun With Electricity' very
much."

"My little boy has your book 'Fun With Electricity,' which has given
him much amusement. He would like to have," etc.

"I am very much pleased with both outfits. I am very much in favor of
such things for boys; it keeps them occupied with something that is
both amusing and instructive. Send me," etc.




Fun With Electricity.

BOOK AND COMPLETE OUTFIT FOR SIXTY EXPERIMENTS IN ELECTRICITY....

[Illustration]


Enough of the principles of electricity are brought out to make the
book instructive as well as amusing. The experiments are systematically
arranged, and make a fascinating science course. No chemicals, no
danger.

The book is conversational and not at all "schooly," Harry and Ned
being two boys who perform the experiments and talk over the results as
they go along.

"The book reads like a story."--"An appropriate present for a
boy or girl."--"Intelligent parents will appreciate 'Fun With
Electricity.'"--"Very complete, because it contains both book and
apparatus."--"There is no end to the fun which a boy or girl can have
with this fascinating amusement."

=THERE IS FUN IN THESE EXPERIMENTS.=--Chain Lightning.--An Electric
Whirligig.--The Baby Thunderstorm.--A Race with Electricity.--An
Electric Frog Pond.--An Electric Ding-Dong.--The Magic
Finger.--Daddy Long-Legs.--Jumping Sally.--An Electric Kite.--Very
Shocking.--Condensed Lightning.--An Electric Fly-Trap.--The Merry
Pendulum.--An Electric Ferry-Boat.--A Funny Piece of Paper.--A Joke on
the Family Cat.--Electricity Plays Leap-Frog.--Lightning Goes Over a
Bridge.--Electricity Carries a Lantern.--And _=40 Others=_.

The _=OUTFIT=_ contains 20 different articles. The _=BOOK OF
INSTRUCTION=_ measures 5 × 7-1/2 inches, and has 38 illustrations, 55
pages, good paper and clear type.

_The Book and Complete Outfit will be sent, by mail or express, Charges
Prepaid, upon receipt of 65 Cents, by_

THOMAS M. ST. JOHN, 407 W. 51st St., New York.




Fun With Puzzles.

BOOK, KEY, AND COMPLETE OUTFIT FOR FOUR HUNDRED PUZZLES....


The BOOK measures 5 × 7-1/2 inches. It is well printed, nicely bound,
and contains 15 chapters, 80 pages, and 128 illustrations. The KEY is
illustrated. It is bound with the book, and contains the solution of
every puzzle. The COMPLETE OUTFIT is placed in a neat box with the
book. It consists of numbers, counters, figures, pictures, etc., for
doing the puzzles.

=CONTENTS=: _Chapter_ (1) Secret Writing. (2) Magic Triangles, Squares,
Rectangles, Hexagons, Crosses, Circles, etc. (3) Dropped Letter and
Dropped Word Puzzles. (4) Mixed Proverbs, Prose and Rhyme. (5) Word
Diamonds, Squares, Triangles, and Rhomboids. (6) Numerical Enigmas. (7)
Jumbled Writing and Magic Proverbs. (8) Dissected Puzzles. (9) Hidden
and Concealed Words. (10) Divided Cakes, Pies, Gardens, Farms, etc.
(11) Bicycle and Boat Puzzles. (12) Various Word and Letter Puzzles.
(13) Puzzles with Counters. (14) Combination Puzzles. (15) Mazes and
Labyrinths.

"Fun With Puzzles" is a book that every boy and girl should have. It
is amusing, instructive,--educational. It is just the thing to wake up
boys and girls and make them think. They like it, because it is real
fun. This sort of educational play should be given in every school-room
and in every home.

"Fun With Puzzles" will puzzle your friends, as well as yourself; it
contains some real brain-splitters. Over 300 new and original puzzles
are given, besides many that are hundreds of years old.

=Secret Writing.= Among the many things that "F. W. P." contains, is
the key to _secret writing_. It shows you a very simple way to write
letters to your friends, and it is simply impossible for others to read
what you have written, unless they know the secret. This, alone is a
valuable thing for any boy or girl who wants to have some fun.

_The Book, Key, and Complete Outfit will be sent, postpaid, upon
receipt of 35 cents, by_

THOMAS M. ST. JOHN, 407 West 51st St., New York City.




Fun With Soap-Bubbles.

BOOK AND COMPLETE OUTFIT FOR FANCY BUBBLES AND FILMS....

[Illustration]


=THE OUTFIT= contains everything necessary for thousands of beautiful
bubbles and films. All highly colored articles have been carefully
avoided, as cheap paints and dyes are positively dangerous in
children's mouths. The outfit contains the following articles:

One Book of Instructions, called "Fun With Soap-Bubbles," 1 Metal Base
for Bubble Stand, 1 Wooden Rod for Bubble Stand, 8 Large Wire Rings for
Bubble Stand, 1 Small Wire Ring, 3 Straws, 1 Package of Prepared Soap,
1 Bubble Pipe, 1 Water-proof Bubble Horn. The complete outfit is placed
in a neat box with the book. (Extra Horns, Soap, etc., furnished at
slight cost.)

=CONTENTS OF BOOK.=--Twenty-one Illustrations.--Introduction.--The
Colors of Soap-bubbles.--The Outfit.--Soap Mixture.--Useful
Hints.--Bubbles Blown With Pipes.--Bubbles Blown With Straws.--Bubbles
Blown With the Horn.--Floating Bubbles.--Baby Bubbles.--Smoke
Bubbles.--Bombshell Bubbles.--Dancing Bubbles.--Bubble
Games.--Supported Bubbles.--Bubble Cluster.--Suspended Bubbles.--Bubble
Lamp Chimney.--Bubble Lenses.--Bubble Basket.--Bubble Bellows.--To
Draw a Bubble Through a Ring.--Bubble Acorn.--Bubble Bottle.--A
Bubble Within a Bubble.--Another Way.--Bubble Shade.--Bubble
Hammock.--Wrestling Bubbles.--A Smoking Bubble.--Soap Films.--The
Tennis Racket Film.--Fish-net Film.--Pan-shaped Film.--Bow and Arrow
Film.--Bubble Dome.--Double Bubble Dome.--Pyramid Bubbles.--Turtle-back
Bubbles.--Soap-bubbles and Frictional Electricity.

"There is nothing more beautiful than the airy-fairy soap-bubble with
its everchanging colors."

_=THE BEST POSSIBLE AMUSEMENT FOR OLD AND YOUNG.=_

_The Book and Complete Outfit will be sent, POST-PAID, upon receipt of
35 cents, by_

THOMAS M. ST. JOHN, 407 West 51st St., New York City.




Things A Boy Should Know About Electricity.

(In Preparation.)


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.

It is free from technical language and rhetorical frills, but it tells
how things work, and why they work.

It is brimful of illustrations--the best that can be had--illustrations
that are taken directly from apparatus and machinery, and that show
what they are intended to show.

This book does not contain experiments, or tell how to make apparatus;
our other books do that. After explaining the simple principles of
electricity, it shows how these principles are used and combined to
make electricity do every-day work. The following are


  _Some of the Things Electricity Can Do:_

  It signals without wires.

  It drills rock, coal, and teeth.

  It cures diseases and kills criminals.

  It protects, heats, and ventilates houses.

  It photographs the bones of the human body.

  It rings church bells and plays church organs.

  It lights streets, cars, boats, mines, houses, etc.

  It pumps water, cooks food, and fans you while eating.

  It runs all sorts of machinery, elevators, cars, boats, and
  wagons.

  It sends messages with the telegraph, telephone, telautograph,
  and search-light.

  It cuts cloth, irons clothes, washes dishes, blackens boots,
  welds metals, prints books, etc., etc.

_Everyone Should Know About Electricity._

=Things A Boy Should Know About Electricity= will interest _you_. We
shall be glad to send you complete information as soon as it is ready.
Send us your address now.




Dewey Flag Poles

=ARE LITTLE MODELS OF REAL FLAG POLES....=

[Illustration]

They are appropriate for any occasion, and suitable for any kind of
decoration. They should stand on tables, mantels, pianos, etc.; in
fact, there is no better ornament for general use.

"They should be in every home and in every school-room in the United
States."

"No toy fort complete without a Dewey Flag Pole."

"The children can fasten them on the windowsill and watch them flutter
by the hour."

"They hoist like big flags, at half-mast, etc."

"Invaluable for store-window decoration."

[Illustration]

PRICES

  SMALL SIZE: height 18 inches, fitted with
  United States or Cuban Silk Flag (4×6 in.)
  post-paid,                                           30c.

  LARGE SIZE: height 24 inches, fitted with
  United States Silk Flag (7 × 10 in.), post-paid,     40c.

  LARGE SIZE: fitted with Cuban or British Silk
  Flag (8×12 in.), post-paid,                          50c.

_DEWEY FLAG POLES are beautifully made of hard wood, and fitted with
best Silk Flags_.




GAMES.


Hustle=Ball.

A quick, sharp, decisive game that is thoroughly American.

Played by means of Magic Wands, and Polished Balls of Steel.

_=Needed:= "A quick eye and a nimble hand."_--_Shakespeare_.

_The Rule: Just keep cool--and hustle._

Four Games with One Outfit:

    "Hustle Ball,"
    "Leap-Frog,"
    "Cross-Country Race,"
    "Magnetic Potato-Race."

The game-board is new and original, as well as the methods of playing.
The game is put up in a strong box having a beautifully lithographed
cover, and measures 8 × 15-1/2 inches. With the game-board are
furnished, magic wands, polished steel balls, an "extra strength"
horseshoe magnet, and a complete set of illustrated rules and
directions for playing.

  "Unlike all other games."
  "Any one can play Hustle-Ball."
  "Just the thing for progressive parties."
  "Hustle-Ball games are intensely exciting."
  "No waiting for some one to play."
  "You win or lose a point in a few seconds."
  "By handicapping the best players, all games are made equally
  interesting and exciting."
  "A game for Grandparents, as well as for Grandchildren."

A Brand-New Idea in Games.

[Illustration]

[Illustration]

'_A HUSTLE FROM THE WORD GO._'

    _=This Exciting Game=_ will be sent, _=Charges Prepaid=_, by mail
    or express, upon receipt of 65 Cents.

  Address THOMAS M. ST. JOHN,
                407 West 51st Street,
                        New York City.


JINGO

THE GREAT WAR GAME

Social Exciting Interesting Simple.

A Thorough War Game:--Infantry Against Infantry, Cavalry Against
Cavalry, Etc.

Jingo is really a great war contest between England and America. Upon
the game-board are 14 beautiful war scenes, each lithographed in 8
colors. American and English flags, coats of arms, cannon, torpedoes,
etc., aid in making this game artistic, handsome and attractive. The
following companies, ships, etc., are shown:--_American_, 12th U. S.
Infantry,--6th U. S. Cavalry,--2d U. S. Light Artillery,--U. S. Mortar
Battery,--U. S. Monitor "Miantonomoh,"--U. S. Ram "Katahdin,"--U.
S. Battleship "Indiana,"--U. S. Torpedo Boat "Cushing,"--U. S
Dynamite Cruiser "Vesuvius." _English_, 30th East Lancashire,--1st
Royal Dragoons,--Royal Horse Artillery,--Royal Artillery,--H. M.
S. "Thunderer,"--H. M. S. "Seagull,"--H. M. S. "Nile,"--H. M. S.
"Australia,"--H. M. S. "Dart."

The game board is over 16 inches square when opened. Jingo is made and
finished in a manner which makes it the most beautiful, artistic, and
practical game ever published.

  "Just what every boy likes."
  "A good idea well carried out."
  "The Game-board is a work of art."
  "Any child can play Jingo at once."
  "It is the handsomest game on the market."

JINGO JUNIOR is the Greatest Game ever Invented for Little Folks.

It is played upon the Jingo board with the extra ammunition furnished.

These Two Great Games make a most complete and beautiful outfit for
home amusement.


JINGO AND JINGO JUNIOR.

Two Fascinating and Entirely Different Games, Played with One Outfit,
and Complete in One Box.

[Illustration]

    _THIS HANDSOME OUTFIT_ for playing the _TWO GREAT WAR GAMES_
    will be sent _CHARGES PREPAID_ upon receipt of _=$1.00=_.

  Address THOMAS M. ST. JOHN,
                407 West 51st Street,
                        New York City.


Transcriber's Notes

In the text version, Italic text is denoted by _underscores_ and bold
text by =equal signs=.

Obvious punctuation errors have been repaired.

The book contains some inconsistent hyphenation which has been left as
printed.

p. xi. (TOC) "constructiou" changed to "construction"

p. xiv. (TOC) "The Prodution of Motion" changed to "The Production of
Motion"

p. 27. para. 73. "thick permament magnets" changed to "thick permanent
magnets"

p. 99. para 253. "wabble" may be a typo for wobble but has been left on
the off chance that this could be what was intended.

p. 118. Fig 91. The final column has been scored through but appears to
read "CU to ZN"

p. 131. para. 324, 325. German-silver Wire, G-s W used here but
previously G S W used.

p. 164. para 395. "circuit in closed" changed to "circuit is closed"

p. 166. para 398. "core inside of the c  l" changed to "core inside of
the coil" after checking original scans.

p. 169-170. It appears that a word has been omitted across the page
break. "The copper washer, C W, be used." has been changed to "The
copper washer, C W, should be used.". (Alternative words are possible!)

p. 211. No. 35. 5-16 in. changed to 5/16 in.

p. 213. No. 92, 93. 5-16 in. changed to 5/16 in.

p. 214. No. 96. and No. 97. 5-16 in. changed to 5/16 in.

p. 216. Entry for Coulomb moved from end of "C" to above Current.