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THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY

_BY THE SAME AUTHOR._

  =FUN WITH MAGNETISM.= A book and complete outfit of apparatus
     for _Sixty-One Experiments_.

  =FUN WITH ELECTRICITY.= A book and complete outfit of
     apparatus for _Sixty Experiments_.

  =FUN WITH PUZZLES.= A book, key and complete outfit for _Four
     Hundred Puzzles_.

  =FUN WITH SOAP-BUBBLES.= A book and complete outfit of
     apparatus for _Fancy Bubbles and Films_.

  =FUN WITH SHADOWS.= Including book of instructions with one
     hundred illustrations and a complete outfit of apparatus
     for _Shadow Pictures, Pantomimes, Entertainments, etc.,
     etc._

  =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.

  =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.

  =ANS., OR ACCURACY, NEATNESS AND SPEED.= For teachers and
     pupils. Containing study-charts, practice devices and
     special methods for accurate, rapid work with figures.

    _Ask Your Bookseller, Stationer, or Toy Dealer for our
    Books, Games, Puzzles, Educational Amusements, Etc._

    CATALOGUE UPON APPLICATION

    Thomas M. St. John, 407 West 51st St., New York.

THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY

by

THOMAS M. ST. JOHN, Met. E.

Author of "Fun With Magnetism," "Fun With Electricity,"
"How Two Boys Made Their Own Electrical Apparatus,"
"The Study of Elementary Electricity
and Magnetism by Experiment," etc.

SECOND EDITION

New York
Thomas M. St. John
407 West 51st Street
1903

Copyright, 1900.
By Thomas M. St. John.

THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY

TABLE OF CONTENTS

  CHAPTER                                                   PAGE
      I. About Frictional Electricty                           7
     II. About Magnets and Magnetism                          21
    III. How Electricity is Generated by the Voltaic Cell,    32
     IV. Various Voltaic Cells,                               36
      V. About Push-Buttons, Switches and Binding-Posts,      43
     VI. Units and Apparatus for Electrical Measurements,     48
    VII. Chemical Effects of the Electric Current,            58
   VIII. How Electroplating and Electrotyping are Done,       60
     IX. The Storage Battery, and How it Works,               63
      X. How Electricity is Generated by Heat,                68
     XI. Magnetic Effects of the Electric Current,            71
    XII. How Electricity is Generated by Induction,           77
   XIII. How the Induction Coil Works,                        80
    XIV. The Electric Telegraph, and How it Sends Messages,   84
     XV. The Electric Bell and Some of its Uses,              91
    XVI. The Telephone and How it Transmits Speech,           95
   XVII. How Electricity is Generated by Dynamos,            101
  XVIII. How the Electric Current is Transformed,            109
    XIX. How Electric Currents are Distributed for Use,      114
     XX. How Heat is Produced by the Electric Current,       124
    XXI. How Light is Produced by the Incandescent Lamp,     129
   XXII. How Light is Produced by the Arc Lamp,              135
  XXIII. X-Rays, and How the Bones of the Human Body are
             Photographed,                                   141
   XXIV. The Electric Motor, and How it Does Work,           147
    XXV. Electric Cars, Boats and Automobiles,               154
   XXVI. A Word About Central Stations,                      162
  XXVII. Miscellaneous Uses of Electricity,                  165

TO THE READER

For the benefit of those who wish to make their own electrical
apparatus for experimental purposes, references have been made
throughout this work to the "Apparatus Book;" by this is meant the
author's "How Two Boys Made Their Own Electrical Apparatus."

For those who wish to take up a course of elementary electrical
experiments that can be performed with simple, home-made apparatus,
references have been made to "Study;" by this is meant "The Study of
Elementary Electricity and Magnetism by Experiment."

                                                       THE AUTHOR.

Things A Boy Should Know About Electricity

CHAPTER I.

ABOUT FRICTIONAL ELECTRICITY.

=1. Some Simple Experiments.= Have you ever shuffled your feet along
over the carpet on a winter's evening and then quickly touched your
finger to the nose of an unsuspecting friend? Did he jump when a bright
spark leaped from your finger and struck him fairly on the very tip of
his sensitive nasal organ?

[Illustration: Fig. 1.]

Did you ever succeed in proving to the pussy-cat, Fig. 1, that
something unusual occurs when you thoroughly rub his warm fur with your
hand? Did you notice the bright sparks that passed to your hand when it
was held just above the cat's back? You should be able to see, hear,
and feel these sparks, especially when the air is dry and you are in a
dark room.

Did you ever heat a piece of paper before the fire until it was real
hot, then lay it upon the table and rub it from end to end with your
hand, and finally see it cling to the wall?

Were you ever in a factory where there were large belts running rapidly
over pulleys or wheels, and where large sparks would jump to your hands
when held near the belts?

If you have never performed any of the four experiments mentioned, you
should try them the first time a chance occurs. There are dozens of
simple, fascinating experiments that may be performed with this kind of
electricity.

=2. Name.= As this variety of electricity is made, or generated, by
the friction of substances upon each other, it is called _frictional_
electricity. It is also called _static_ electricity, because it
generally stands still upon the surface of bodies and does not "flow in
currents" as easily as some of the other varieties. Static electricity
may be produced by induction as well as by friction.

[Illustration: Fig. 2.]

=3. History.= It has been known for over 2,000 years that certain
substances act queerly when rubbed. Amber was the first substance upon
which electricity was produced by friction, and as the Greek name for
amber is _elektron_, bodies so affected were said to be _electrified_.
When a body, like ebonite, is rubbed with a flannel cloth, we say that
it becomes _charged with electricity_. Just what happens to the ebonite
is not clearly understood. We know, however, that it will attract
light bodies, and then quickly repel them if they be conductors. Fig.
2 shows a piece of tissue-paper jumping toward a sheet of ebonite that
has been electrified with a flannel cloth.

=4. Conductors and Non-Conductors.= Electricity can be produced upon
glass and ebonite because they do not carry or conduct it away. If a
piece of iron be rubbed, the electricity passes from the iron into the
earth as fast as it is generated, because the iron is a _conductor_ of
electricity. Glass is an _insulator_ or _non-conductor_. Frictional
electricity resides upon the outside, only, of conductors. A hollow
tin box will hold as great a charge as a solid piece of metal having
the same outside size and shape. When frictional electricity passes
from one place to another, sparks are produced. Lightning is caused
by the passage of static electricity from a cloud to the earth, or
from one cloud to another. In this case air forms the conductor. (For
experiments, see "Study," Chapter VII.)

[Illustration: Fig. 3.]

=5. Electroscopes.= A piece of carbon, pith, or even a small piece of
damp tissue-paper will serve as an electroscope to test the presence of
static electricity. The pith is usually tied to a piece of silk thread
which is a non-conductor. Fig. 3 shows the ordinary form of _pith-ball
electroscope_.

The _leaf electroscope_ is a very delicate apparatus. Gold-leaf is
generally used, but aluminum-leaf will stand handling and will do for
all ordinary purposes. Fig. 4 shows a common form, the glass being
used to keep currents of air from the leaves and at the same time to
insulate them from the earth.

Electroscopes are used to show the presence, relative amount, or kind
of static electricity on a body. (See "Study," Chapter XI.)

[Illustration: Fig. 4.]

=6. Two Kinds of Electrification.= It can be shown that the
electrification produced on all bodies by friction is not the same;
for example, that generated with glass and silk is not the same as
that made with ebonite and flannel. It has been agreed to call that
produced by glass and silk _positive_, and that by ebonite and flannel
_negative_. The signs + and - are used for positive and negative.

=7. Laws of Electrification.= (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.

=8. Static Electric Machines.= In order to produce static electricity
in quantities for experiments, some device is necessary.

The _electrophorus_ (e-lec-troph´-o-rus) is about the simplest form
of machine. Fig. 5 shows a simple electrophorus in which are two
insulators and one conductor. The ebonite sheet E S is used with a
flannel cloth to generate the electricity. The metal cover E C is
lifted by the insulating handle E R. The cover E C is placed upon the
thoroughly charged sheet E S, and then it is touched for an instant
with the finger, before lifting it by E R. The charge upon E C can then
be removed by bringing the hand near it. The bright spark that passes
from E C to the hand indicates that E C has discharged itself into the
earth. The action of the electrophorus depends upon induction. (For
experiments, details of action, induced electrification, etc., see "The
Study of Elementary Electricity and Magnetism by Experiment," Chapters
VIII. and IX.)

[Illustration: Fig. 5.]

_The first electric machine_ consisted of a ball of sulphur fastened to
a spindle which could be turned by a crank. By holding the hands or a
pad of silk upon the revolving ball, electricity was produced.

[Illustration: Fig. 6.]

[Illustration: Fig. 7.]

=9. The Cylinder Electric Machine= consists, as shown in Fig. 6, of a
glass cylinder so mounted that it can be turned by a crank. Friction
is produced by a pad of leather C, which presses against the cylinder
as it turns. Electric sparks can be taken from the large "conductors"
which are insulated from the earth. The opposite electricities unite
with sparks across D and E. If use is to be made of the electricity,
either the rubber or the prime conductor must be connected with the
ground. In the former case positive electricity is obtained; in the
latter, negative.

=10. The Plate Electrical Machine.= Fig. 7 also shows an old form of
machine. Such machines are made of circular plates of glass or ebonite,
two rubbing pads being usually employed, one on each side of the plate.
One operator is seen on an insulated stool (Fig. 7), the electricity
passing through him before entering the earth by way of the body of the
man at the right.

[Illustration: Fig. 8.]

=11. The Toepler-Holtz Machine=, in one form, is shown in Fig. 8. The
electricity is produced by the principle of induction, and not by mere
friction. This machine, used in connection with condensers, produces
large sparks.

=12. The Wimshurst Machine= is of recent date, and not being easily
affected by atmospheric changes, is very useful for ordinary laboratory
work. Fig. 9 shows one form of this machine.

=13. Influence Machines for Medical Purposes= are made in a large
variety of forms. A Wimshurst machine is generally used as an exciter
to charge the plates of the large machine when they lose their charge
on account of excessive moisture in the atmosphere. Fig. 10 shows a
large machine.

[Illustration: Fig. 9.]

=14. Uses of Electrical Machines.= Static electricity has been used for
many years in the laboratory for experimental purposes, for charging
condensers, for medical purposes, etc. It is now being used for X-ray
work, and considerable advancement has been made within a few years in
the construction and efficiency of the machines.

[Illustration: Fig. 10.]

With the modern machines large sparks are produced by merely turning
a crank, enough electricity being produced to imitate a small
thunderstorm. The sparks of home-made lightning will jump several
inches.

Do not think that electricity is generated in a commercial way by
static electric machines. The practical uses of static electricity are
very few when compared with those of current electricity from batteries
and dynamos.

=15. Condensation of Static Electricity.= By means of apparatus called
_condensers_, a terrific charge of static electricity may be stored.
Fig. 11 shows the most common form of condenser, known as the _Leyden
jar_. It consists of a glass jar with an inside and outside coating of
tin-foil.

[Illustration: Fig. 11.]

[Illustration: Fig. 12.]

_To charge_ the jar it is held in the hand so that the outside coating
shall be connected with the earth, the sparks from an electric machine
being passed to the knob at the top, which is connected by a chain to
the inside coating.

_To discharge_ the jar, Fig. 12, a conductor with an insulating handle
is placed against the outside coat; when the other end of the conductor
is swung over towards the knob, a bright spark passes between them.
This device is called a discharger. Fig. 13 shows a discharge through
ether which the spark ignites.

[Illustration: Fig. 13.]

=16. The Leyden Battery=, Fig. 14, consists of several jars connected
in such a way that the area of the inner and outer coatings is greatly
increased. The battery has a larger capacity than one of its jars. (For
Experiments in Condensation, see "Study," Chapter X.)

[Illustration: Fig. 14.]

=17. Electromotive Force of Static Electricity.= Although the sparks
of static electricity are large, the _quantity_ of electricity is very
small. It would take thousands of galvanic cells to produce a spark
an inch long. While the quantity of static electricity is small, its
potential, or electromotive force (E. M. F.), is very high. We say that
an ordinary gravity cell has an E. M. F. of a little over one volt.
Five such cells joined in the proper way would have an E. M. F. of a
little over five volts. You will understand, then, what is meant when
we say that the E. M. F. of a lightning flash is millions of volts.

=18. Atmospheric Electricity.= The air is usually electrified, even
in clear weather, although its cause is not thoroughly understood. In
1752 it was proved by Benjamin Franklin (Fig. 15), with his famous
kite experiment, that atmospheric and frictional electricities are
of the same nature. By means of a kite, the string being wet by the
rain, he succeeded, during a thunderstorm, in drawing sparks, charging
condensers, etc.

[Illustration: Fig. 15.]

[Illustration: Fig. 16.]

=19. Lightning= may be produced by the passage of electricity between
clouds, or between a cloud and the earth (Fig. 16), which, with the
intervening air, have the effect of a condenser. When the attraction
between the two electrifications gets great enough, a spark passes.
When the spark has a zigzag motion it is called _chain lightning_.
In hot weather flashes are often seen which light 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 lightning flash represents billions of
volts.

[Illustration: Fig. 17.]

=20. Thunder= is caused by the violent disturbances produced in the
air by lightning. Clouds, hills, etc., produce echoes, which, with the
original sound, make the rolling effect.

=21. Lightning-Rods=, when well constructed, often prevent violent
discharges. Their pointed prongs at the top allow the negative
electricity of the earth to pass quietly into the air to neutralize
the positive in the cloud above. In case of a discharge, or stroke of
lightning, the rods aid in conducting the electricity to the earth. The
ends of the rods are placed deep in the earth, Fig. 17.

=22. St. Elmo's Fire.= Electrification from the earth is often drawn up
from the earth through the masts of ships, Fig. 18, to neutralize that
in the clouds, and, as it escapes from the points of the masts, light
is produced.

[Illustration: Fig. 18.]

=23. Aurora Borealis=, also called Northern Lights, are luminous
effects, Fig. 19, often seen in the north. They often occur at the
same time with magnetic storms, when 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.

[Illustration: Fig. 19.]

CHAPTER II.

ABOUT MAGNETS AND MAGNETISM.

=24. Natural Magnets.= Hundreds of years ago it was discovered that
a certain ore of iron, called lodestone, had the power of picking up
small pieces of iron. It was used to indicate the north and south
line, and it was discovered later that small pieces of steel could be
permanently magnetized by rubbing them upon the lodestone.

=25. Artificial Magnets.= Pieces of steel, when magnetized, are called
artificial magnets. They are made in many forms. The electromagnet is
also an artificial magnet; this will be treated separately.

[Illustration: Fig. 20]

=26. The Horseshoe Magnet=, Fig. 20, is, however, the one with which we
are the most familiar. They are always painted red, but the red paint
has nothing to do with the magnetism.

The little end-piece is called the keeper, or armature; it should
always be kept in place when the magnet is not in use. The magnet
itself is made of steel, while the armature is made of soft iron. Steel
retains magnetism for a long time, while soft iron loses it almost
instantly. The ends of the magnet are called its _poles_, and nearly
all the strength of the magnet seems to reside at the poles, the curved
part having no attraction for outside bodies. One of the poles of the
magnet is marked with a line, or with the letter N. This is called the
north pole of the magnet, the other being its south pole.

[Illustration: Fig. 21.]

=27. Bar Magnets= are straight magnets. Fig. 21 shows a round bar
magnet. The screw in the end is for use in the telephone, described
later.

=28. Compound Magnets.= When several thin steel magnets are riveted
together, a compound magnet is formed. These can be made with
considerable strength. Fig. 22 shows a compound horseshoe magnet. Fig.
23 shows a form of compound bar magnet used in telephones. The use of
the coil of wire will be explained later. A thick piece of steel can
not be magnetized through and through. In the compound magnet we have
the effect of a thick magnet practically magnetized through and through.

[Illustration: Fig. 22.]

[Illustration: Fig. 23.]

=29. Magnetic and Diamagnetic Bodies.= Iron, and substances containing
iron, are the ones most readily attracted by a magnet. Iron is said to
be _magnetic_. Some substances, like nickel, for example, are visibly
attracted by very strong magnets only. Strange as it may seem, some
substances are actually repelled by strong magnets; these are called
_diamagnetic_ bodies. Brass, copper, zinc, etc., are not visibly
affected by a magnet. Magnetism will act through paper, glass, copper,
lead, etc.

[Illustration: Fig. 24.]

=30. Making Magnets.= One of the strangest properties that a magnet
has is its power to give magnetism to another piece of steel. If
a sewing-needle be properly rubbed upon one of the poles of a
magnet, it will become strongly magnetized and will retain its
magnetism for years. Strong permanent magnets are made with the aid
of electromagnets. Any number of little magnets may be made from a
horseshoe magnet without injuring it.

[Illustration: Fig. 25.]

31. Magnetic Needles and Compasses. If a bar magnet be suspended
by a string, or floated upon a cork, which can easily be done with
the magnet made from a sewing-needle, Fig. 24, it will swing around
until its poles point north and south. Such an arrangement is called
a _magnetic needle_. In the regular _compass_, a magnetic needle is
supported upon a pivot. Compasses have been used for many centuries
by mariners and others. Fig. 25 shows an ordinary pocket compass, and
Fig. 26 a form of mariner's compass, in which the small bar magnets are
fastened to a card which floats, the whole being so mounted that it
keeps a horizontal position, even though the vessel rocks.

[Illustration: Fig. 26.]

32. Action of Magnets Upon Each Other. By making two small
sewing-needle magnets, you can easily study the laws of attraction and
repulsion. By bringing the two north poles, or the two south poles,
near each other, a repulsion will be noticed. Unlike poles attract each
other. The attraction between a magnet and iron is mutual; that is,
each attracts the other. Either pole of a magnet attracts soft iron.

In magnetizing a needle, either end may be made a north pole at will;
in fact, the poles of a weak magnet can easily be reversed by properly
rubbing it upon a stronger magnet.

=33. Theory of Magnetism.= Each little particle of a piece of steel or
iron is supposed to be a magnet, even before it touches a magnet. When
these little magnets are thoroughly mixed up in the steel, they pull in
all sorts of directions upon each other and tend to keep the steel from
attracting outside bodies. When a magnet is properly rubbed upon a bar
of steel, the north poles of the little molecular magnets of the steel
are all made to point in the same direction. As the north poles help
each other, the whole bar can attract outside bodies.

By jarring a magnet its molecules are thoroughly shaken up; in fact,
most of the magnetism can be knocked out of a weak magnet by hammering
it.

=34. Retentivity.= The power that a piece of steel has to hold
magnetism is called _retentivity_. Different kinds of steel have
different retentivities. A sewing-needle of good steel will retain
magnetism for years, and it is almost impossible to knock the magnetism
out by hammering it. Soft steel has very little retentivity, because
it does not contain much carbon. Soft iron, which contains less
carbon than steel, holds magnetism very poorly; so it is not used for
permanent magnets. A little magnetism, however, will remain in the
soft iron after it is removed from a magnet. This is called _residual
magnetism_.

=35. Heat and Magnetism.= Steel will completely lose its magnetism
when heated to redness, and a magnet will not attract red-hot iron.
The molecules of a piece of red-hot iron are in such a state of rapid
vibration that they refuse to be brought into line by the magnet.

=36. Induced Magnetism.= A piece of soft iron may be induced to become
a magnet by holding it near a magnet, absolute contact not being
necessary. When the soft iron is removed, again, from the influence of
the magnet, its magnetism nearly all disappears. It is said to have
_temporary_ magnetism; it had _induced_ magnetism. If a piece of soft
iron be held near the north pole of a magnet, as in Fig. 27, poles will
be produced in the soft iron, the one nearest the magnet being the
south pole, and the other the north pole.

[Illustration: Fig. 27.]

[Illustration: Fig. 28.]

=37. Magnetic Field.= If a bar magnet be laid upon the table, and a
compass be moved about it, the compass-needle will be attracted by the
magnet, and it will point in a different direction for every position
given to the compass. This strange power, called magnetism, reaches out
on all sides of a magnet. The magnet may be said to act by induction
upon the compass-needle. The space around the magnet, in which this
inductive action takes place, is called the _magnetic field_. Fig. 28
shows some of the positions taken by a compass-needle when moved about
on one side of a bar magnet.

[Illustration: Fig. 29.]

[Illustration: Fig. 30.]

=38. Magnetic Figures= can be made by sprinkling iron filings upon a
sheet of paper under which is placed a magnet. Fig. 29 shows a magnetic
figure made with an ordinary bar magnet. The magnet was placed upon the
table and over this was laid a piece of smooth paper. Fine iron filings
were sifted upon the paper, which was gently tapped so that the filings
could arrange themselves. As each particle of iron became a little
magnet, by induction, its poles were attracted and repelled by the
magnet; and when the paper was tapped they swung around to their final
positions. Notice that the filings have arranged themselves in lines.
These lines show the positions of some of the _lines of magnetic force_
which surrounded the magnet.

These lines of force pass from the north pole of a magnet through the
air on all sides to its south pole.

[Illustration: Fig. 31.]

Fig. 30 shows a magnetic figure made from two bar magnets placed side
by side, their unlike poles being next to each other. Fig. 31 shows
the magnetic figure of a horseshoe magnet with round poles, the poles
being uppermost.

=39. The Use of Armatures.= A magnet attracts iron most strongly at its
poles, because it is at the poles that the greatest number of lines
of force pass into the air. Lines of force pass easily through soft
iron, which is said to be a good conductor of them. Air is not a good
conductor of the lines of force; in order, then, for the lines of force
to pass from the north pole of a magnet to its south pole, they must
overcome this resistance of the air, unless the armature is in place. A
magnet will gradually grow weaker when its armature is left off.

=40. Terrestrial Magnetism.= As the compass-needle points to the north
and south, the earth must act like a magnet. There is a place very far
north, about a thousand miles from the north pole of the earth, which
is called the earth's north magnetic pole. Compass-needles point to
this place, and not to the earth's real north pole. You can see, then,
that if a compass be taken north of this magnetic pole, its north pole
will point south. Lines of force pass from the earth's north magnetic
pole through the air on all sides of the earth and enter the earth's
south magnetic pole. The compass-needle, in pointing toward the north
magnetic pole, merely takes the direction of the earth's lines of
force, just as the particles of iron filings arrange themselves in the
magnetic figures.

=41. Declination.= As the magnetic needle does not point exactly to the
north, an angle is formed between the true north and south line and the
line of the needle. In Fig. 32 the line marked N S is the true north
and south line. The _angle of variation_, or the declination, is the
angle A between the line N S and the compass-needle.

[Illustration: Fig. 32.]

[Illustration: Fig. 33.]

=42. Dip or Inclination.= If a piece of steel be carefully balanced
upon a support, and then magnetized, it will be found that it will no
longer balance. The north pole will _dip_ or point downward. Fig. 33
shows what happens to a needle when it is held in different positions
over a bar magnet. It simply takes the directions of the lines of
force as they pass from the north to the south pole of the magnet.
As the earth's lines of force pass in curves from the north to the
south magnetic pole, you can see why the magnetic needle dips, unless
its south pole is made heavier than its north. Magnetic needles are
balanced after they are magnetized.

[Illustration: Fig. 34.]

Fig. 34 shows a simple form of dipping needle. These are often used
by geologists and miners. In the hands of the prospector, the
miner's compass, or dipping needle, proves a serviceable guide to the
discovery and location of magnetic iron ore. In this instrument the
magnetic needle is carefully balanced upon a horizontal axis within a
graduated circle, and in which the needle will be found to assume a
position inclined to the horizon. This angle of deviation is called the
_inclination_ or _dip_, and varies in different latitudes, and even at
different times in the same place.

=43. The Earth's Inductive Influence.= The earth's magnetism acts
inductively upon pieces of steel or iron upon its surface. If a piece
of steel or iron, like a stove poker, for example, be held in a north
and south line with its north end dipping considerably, it will be
in the best position for the magnetism of the earth to act upon it;
that is, it will lie in the direction taken by the earth's lines of
force. If the poker be struck two or three times with a hammer to
shake up its molecules, we shall find, upon testing it, that it has
become magnetized. By this method we can pound magnetism right out of
the air with a hammer. If the magnetized poker be held level, in an
east and west direction, it will no longer be acted upon to advantage
by the inductive influence of the earth, and we can easily hammer the
magnetism out of it again. (For experiments on magnets and magnetism
see "Study," Part I.)

CHAPTER III.

HOW ELECTRICITY IS GENERATED BY THE VOLTAIC CELL.

=44. Early Experiments.= In 1786 Galvani, an Italian physician, made
experiments to study the effect of static electricity upon the nervous
excitability of animals, and especially upon the frog. He found that
electric machines were not necessary to produce muscular contractions
or kicks of the frog's legs, and that they could be produced when two
different metals, Fig. 35, like iron and copper, for example, were
placed in proper contact with a nerve and a muscle and then made to
touch each other. Galvani first thought that the frog generated the
electricity instead of the metals.

[Illustration: Fig. 35.]

Volta proved that the electricity was caused by the contact of the
metals. He used the condensing electroscope as one means of proving
that two dissimilar metals become charged differently when in contact.
Volta also carried out his belief by constructing what is called a
_Voltaic Pile_. He thought that by making several pairs of metals so
arranged that all the little currents would help each other, a strong
current could be generated. Fig. 36 shows a _pile_, it being made by
placing a pair of zinc and copper discs in contact with one another,
then laying on the copper disc a piece of flannel soaked in brine, then
on top of this another pair, etc., etc. By connecting the first zinc
and the last copper, quite a little current was produced. This was a
start from which has been built our present knowledge of electricity.
Strictly speaking, electricity is not generated by combinations of
metals or by cells; they really keep up a difference of potential, as
will be seen.

[Illustration: Fig. 36.]

[Illustration: Fig. 37.]

[Illustration: Fig. 38.]

=45. The Simple Cell.= It has been stated that two different kinds of
electrifications may be produced by friction; one positive, the other
negative. Either can be produced, at will, by using proper materials.
Fig. 37 shows a section of a _simple cell_; Fig. 38 shows another view.
Cu is a piece of copper, and Zn a piece of zinc. When they are placed
in dilute sulphuric acid, it can be shown by delicate apparatus that
they become charged differently, because the acid acts differently
upon the plates. They become charged by chemical action, and not by
friction. The zinc is gradually dissolved, and it is this chemical
burning of the zinc that furnishes energy for the electric current in
the simple cell. The electrification, or charge, on the plates tends to
flow from the place of higher to the place of lower potential, just as
water tends to flow down hill. If a wire be joined to the two metals, a
constant current of electricity will flow through it, because the acid
continues to act upon the plates. The simple cell is a _single-fluid_
cell, as but one liquid is used in its construction.

=45a. Plates and Poles.= The metal strips used in voltaic cells are
called _plates_ or _elements_. The one most acted upon by the acid
is called the positive (+) plate. In the simple cell the zinc is the
+ plate, and the copper the negative (-) plate. The end of a wire
attached to the - plate is called the + pole, or electrode. Fig. 37
shows the negative (-) electrode as the end of the wire attached to the
+ plate.

=46. Direction of Current.= In the cell the current passes from the
zinc to the copper; that is, from the positive to the negative plate,
where bubbles of hydrogen gas are deposited. In the wire connecting the
plates, the current passes from the copper to the zinc plate. In most
cells, carbon takes the place of copper. (See "Study," § 268.)

=47. Local Currents; Amalgamation.= Ordinary zinc contains impurities
such as carbon, iron, etc., and when the acid comes in contact with
these, they form with the zinc a small cell. This tends to eat away the
zinc without producing useful currents. The little currents in the cell
from this cause are called _local currents_. (See "Study," Exp. 111, §
273.) This is largely overcome by coating the zinc with mercury. This
process is called _amalgamation_. It makes the zinc act like pure zinc,
which is not acted upon by dilute sulphuric acid when the current does
not pass. (See "Study," § 257, 274.)

=48. Polarization of Cells.= Bubbles of hydrogen gas are formed when
zinc is dissolved by an acid. In the ordinary simple cell these bubbles
collect on the copper plate, and not on the zinc plate, as might be
expected. The hydrogen is not a conductor of electricity, so this film
of gas holds the current back. The hydrogen acts like a metal and sets
up a current that opposes the zinc to the copper current. Several
methods are employed to get rid of the hydrogen. (See "Study," § 278,
279, 280.)

CHAPTER IV.

VARIOUS VOLTAIC CELLS.

=49. Single-Fluid and Two-Fluid Cells.= The simple cell (§ 45) is a
single-fluid cell. The liquid is called the _electrolyte_, and this
must act upon one of the plates; that is, chemical action must take
place in order to produce a current. The simple cell polarizes rapidly,
so something must be used with the dilute sulphuric acid to destroy the
hydrogen bubbles. This is done in the _bichromate of potash cell_.

In order to get complete depolarization--that is, to keep the carbon
plate almost perfectly free from hydrogen, it is necessary to use
_two-fluid cells_, or those to which some solid depolarizer is added to
the one fluid.

=50. Open and Closed Circuit Cells.= If we consider a voltaic cell, the
wires attached to it, and perhaps some instrument through which the
current passes, we have an _electric circuit_. When the current passes,
the circuit is _closed_, but when the wire is cut, or in any way
disconnected so that the current can not pass, the circuit is _open_ or
_broken_. (See "Study," § 266.)

_Open Circuit Cells_ are those which can give momentary currents at
intervals, such as are needed for bells, telephones, etc. These must
have plenty of time to rest, as they polarize when the circuit is
closed for a long time. The _Leclanché_ and _dry_ cells are the most
common open circuit cells.

_Closed Circuit Cells._ For telegraph lines, motors, etc., where a
current is needed for some time, the cell must be of such a nature
that it will not polarize quickly; it must give a strong and constant
current. The _bichromate_ and _gravity cells_ are examples of this
variety. (See "Study," § 286.)

=51. Bichromate of Potash Cells= are very useful for general laboratory
work. They are especially useful for operating induction coils, small
motors, small incandescent lamps, for heating platinum wires, etc.
These cells have an E.M.F. of about 2 volts. Dilute sulphuric acid is
used as the exciting fluid, and in this is dissolved the bichromate of
potash which keeps the hydrogen bubbles from the carbon plate. (See
"Apparatus Book," § 26.) Zinc and carbon are used for the plates, the +
pole being the wire attached to the carbon.

[Illustration: Fig. 39.]

Fig. 39 shows one form of bichromate cell. It furnishes a large
quantity of current, and as the zinc can be raised from the fluid, it
may be kept charged ready for use for many months, and can be set in
action any time when required by lowering the zinc into the liquid. Two
of these cells will burn a one candle-power miniature incandescent lamp
several hours. The carbon is indestructible.

    =Note.= For various forms of home-made cells, see "Apparatus
    Book," Chapter I., and for battery fluids see Chapter II.

=52. The Grenet Cell.= Fig. 40 is another form of bichromate cell. The
carbon plates are left in the fluid constantly. The zinc plate should
be raised when the cell is not in use, to keep it from being uselessly
dissolved.

[Illustration: Fig. 40.]

[Illustration: Fig. 41.]

=53. Plunge Batteries.= Two or more cells are often arranged so that
their elements can be quickly lowered into the acid solution. Such a
combination, Fig. 41, is called a _plunge battery_. The binding-posts
are so arranged that currents of different strengths can be taken from
the combination. The two binding-posts on the right of the battery
will give the current of one cell; the two binding-posts on the left
of the battery will give the current of two cells, and the two end
binding-posts will give the current of all three cells. When not in
use the elements must always be hung on the hooks and kept out of the
solution.

=54. Large Plunge Batteries=. Fig. 42, are arranged with a winch and
a bar above the cells; these afford a ready and convenient means of
lifting or lowering the elements and avoiding waste. In the battery
shown, Fig. 42, the zincs are 4×6 inches; the carbons have the same
dimensions, but there are two carbon plates to each zinc, thus giving
double the carbon surface.

[Illustration: Fig. 42.]

=55. The Fuller Cell=, Fig. 43, is another type of bichromate cell,
used largely for long-distance telephone service, for telephone
exchange and switch service, for running small motors, etc. It consists
of a glass jar, a carbon plate, with proper connections, a clay porous
cup, containing the zinc, which is made in the form of a cone. A little
mercury is placed in the porous cup to keep the zinc well amalgamated.
Either bichromate of potash or bichromate of soda can be used as a
depolarizer.

[Illustration: Fig. 43.]

[Illustration: Fig. 44.]

=56. The Gravity Cell=, sometimes called the _bluestone_ or _crowfoot_
cell, is used largely for telegraph, police, and fire-alarm signal
service, laboratory and experimental work, or whenever a closed circuit
cell is required. The E.M.F. is about one volt. This is a modified form
of the Daniell cell. Fig. 44 shows a home-made gravity cell.

A copper plate is placed at the bottom of the glass jar, and upon
this rests a solution of copper sulphate (bluestone). The zinc plate
is supported about four inches above the copper, and is surrounded
by a solution of zinc sulphate which floats upon the top of the blue
solution. An insulated wire reaches from the copper to the top of the
cell and forms the positive pole. (See "Apparatus Book," § 11 to 15,
for home-made gravity cell, its regulation, etc. For experiments with
two-fluid Daniell cell, see "Study," Exp. 113, § 281 to 286.)

[Illustration: Fig. 45.]

=56a. Bunsen Cells,= Fig. 45, are used for motors, small incandescent
lamps, etc. A carbon rod is inclosed in a porous cup, on the outside of
which is a cylinder of zinc that stands in dilute sulphuric acid, the
carbon being in nitric acid.

=57. The Leclanché Cell= is an open circuit cell. Sal ammoniac is used
as the exciting fluid, carbon and zinc being used for plates. Manganese
dioxide is used as the depolarizer; this surrounds the carbon plate,
the two being either packed together in a porous cup or held together
in the form of cakes. The porous cup, or pressed cake, stands in the
exciting fluid. The E. M. F. is about 1.5 volts.

[Illustration: Fig. 46.]

[Illustration: Fig. 47.]

[Illustration: Fig. 48.]

[Illustration: Fig. 49.]

Fig. 46 shows a form with porous cup. The binding-post at the top of
the carbon plate forms the + electrode, the current leaving the cell at
this point.

_The Gonda Prism Cell_ (Fig. 47), is a form of Leclanché in which the
depolarizer is in the form of a cake.

=58. Dry Cells= are open circuit cells, and can be carried about,
although they are moist inside. The + pole is the end of the carbon
plate. Zinc is used as the outside case and + plate. Fig. 48 shows the
ordinary forms.

Fig. 49 shows a number of dry cells arranged in a box with switch in
front, so that the current can be regulated at will.

[Illustration: Fig. 50.]

=59. The Edison-Lelande Cells=, Fig. 50, are made in several sizes and
types. Zinc and copper oxide, which is pressed into plates, form the
elements. The exciting fluid consists of a 25 per cent. solution of
caustic potash in water. They are designed for both open and closed
circuit work.

CHAPTER V.

ABOUT PUSH-BUTTONS, SWITCHES AND BINDING-POSTS.

=60. Electrical Connections.= In experimental work, as well as in
the everyday work of the electrician, electrical connections must
constantly be made. One wire must be joined to another, just for a
moment, perhaps, or one piece of apparatus must be put in an electric
circuit with other apparatus, or the current must be turned on or off
from motors, lamps, etc. In order to conveniently and quickly make such
connections, apparatus called push-buttons, switches and binding-posts
are used.

[Illustration: Fig. 51.]

[Illustration: Fig. 52.]

=61. Push-Buttons.= The simple act of pressing your finger upon a
movable button, or knob, may ring a bell a mile away, or do some other
equally wonderful thing. Fig. 51 shows a simple push-button, somewhat
like a simple key in construction. If we cut a wire, through which a
current is passing, then join one of the free ends to the screw A and
the other end to screw C, we shall be able to let the current pass at
any instant by pressing the spring B firmly upon A.

Push-buttons are made in all sorts of shapes and sizes. Fig. 52 gives
an idea of the general internal construction. The current enters A by
one wire, and leaves by another wire as soon as the button is pushed
and B is forced down to A. The bottom of the little button rests upon
the top of B.

Fig. 53 shows a _Table Clamp-Push_ for use on dining-tables,
card-tables, chairs, desks, and other movable furniture. Fig. 54 shows
a combination of push-button, speaking-tube, and letter-box used in
city apartment houses. Fig. 55 shows an _Indicating Push_. The buzzer
indicates, by the sound, whether the call has been heard; that is, the
person called answers back.

[Illustration: Fig. 53.]

[Illustration: Fig. 54.]

_Modifications_ of ordinary push-buttons are used for floor
push-buttons, on doors, windows, etc., for burglar-alarms, for turning
off or on lights, etc., etc. (See "Apparatus Book," Chapter III., for
home-made push-buttons.)

[Illustration: Fig. 55.]

=62. Switches= have a movable bar or plug of metal, moving on a pivot,
to make or break a circuit, or transfer a current from one conductor to
another.

Fig. 56 shows a _single point switch_. The current entering the pivoted
arm can go no farther when the switch is open, as shown. To close
the circuit, the arm is pushed over until it presses down upon the
contact-point. For neatness, both wires are joined to the under side of
the switch or to binding-posts.

[Illustration: Fig. 56.]

Fig. 57 shows a _knife switch_. Copper blades are pressed down between
copper spring clips to close the circuit. The handle is made of
insulating material.

_Pole-changing switches_, Fig. 58, are used for changing or reversing
the poles of batteries, etc.

Fig. 59 shows a home-made switch, useful in connection with resistance
coils. By joining the ends of the coils A, B, C, D, with the
contact-points 1, 2, 3, etc., more or less resistance can be easily
thrown in by simply swinging the lever E around to the left or right.
If E be turned to 1, the current will be obliged to pass through all
the coils A, B, etc., before it can pass out at Y. If E be moved to
3, coils A and B will be cut out of the circuit, thus decreasing the
resistance to the current on its way from X to Y. Current regulators
are made upon this principle. (See "Apparatus Book," Chapter IV., for
home-made switches.)

[Illustration: Fig. 57.]

[Illustration: Fig. 58.]

[Illustration: Fig. 59.]

_Switchboards_ are made containing from two or three to hundreds of
switches, and are used in telegraph and telephone work, in electric
light stations, etc., etc. (See Chapter on Central Stations.) Fig. 60
shows a switch used for incandescent lighting currents.

[Illustration: Fig. 60.]

[Illustration: Fig. 61.]

=63. Binding-Posts= are used to make connections between two pieces of
apparatus, between two or more wires, between a wire and any apparatus,
etc., etc. They allow the wires to be quickly fastened or unfastened
to the apparatus. A large part of the apparatus shown in this book has
binding-posts attached. Fig. 61 shows a few of the common forms used.
(See "Apparatus Book," Chapter V., for home-made binding-posts.)

CHAPTER VI.

UNITS AND APPARATUS FOR ELECTRICAL MEASUREMENTS.

=64. Electrical Units.= In order to measure electricity for
experimental or commercial purposes, standards or units are just as
necessary as the inch or foot for measuring distances.

=65. Potential; Electromotive Force.= If water in a tall tank be
allowed to squirt from two holes, one near the bottom, the other near
the top, it is evident that the force of the water that comes from the
hole at the bottom will be the greater. The pressure at the bottom is
greater than that near the top, because the "head" is greater.

When a spark of static electricity jumps a long distance, we say that
the charge has a high _potential_; that is, it has a high electrical
pressure. Potential, for electricity, means the same as pressure, for
water. The greater the potential, or _electromotive force_ (E.M.F.) of
a cell, the greater its power to push a current through wires. (See
"Study," § 296 to 305, with experiments.)

=66. Unit of E.M.F.; the Volt.=--In speaking of water, we say that its
pressure is so many pounds to the square inch, or that it has a fall,
or head, of so many feet. We speak of a current as having so many
volts; for example, we say that a wire is carrying a 110-volt current.
The volt is the unit of E.M.F. An ordinary gravity cell has an E.M.F.
of about one volt. This name was given in honor of Volta.

=67. Measurement of Electromotive Force.= There are several ways by
which the E.M.F. of a cell, for example, can be measured. It is usually
measured _relatively_, by comparison with the E. M. F. of some standard
cell. (See "Study," Exp. 140, for measuring the E. M. F. of a cell by
comparison with the two-fluid cell.)

[Illustration: Fig. 62.]

_Voltmeters_ are instruments by means of which E. M. F. can be read on
a printed scale. They are a variety of galvanometer, and are made with
coils of such high resistance, compared with the resistance of a cell
or dynamo, that the E. M. F. can be read direct. The reason for this
will be seen by referring to Ohm's law ("Study," § 356); the resistance
is so great that the strength of the current depends entirely upon the
E. M. F.

[Illustration: Fig. 63.]

Voltmeters measure electrical pressure just as steam gauges measure
the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63
shows a voltmeter with illuminated dial. An electrical bulb behind the
instrument furnishes light so that the readings can be easily taken.

=68. Electrical Resistance.= Did you ever ride down hill on a
hand-sled? How easily the sled glides over the snow! What happens,
though, when you strike a bare place, or a place where some evil-minded
person has sprinkled ashes? Does the sled pass easily over bare ground
or ashes? Snow offers very little _resistance_ to the sled, while ashes
offer a great resistance.

[Illustration: Fig. 64.]

All substances do not allow the electric current to pass through
them with the same ease. Even the liquid in a cell tends to hold the
current back and offers _internal resistance_. The various wires and
instruments connected to a cell offer _external resistance_. (See
"Study," Chapter XVIII., for experiments, etc.)

=69. Unit of Resistance.= =The Ohm= is the name given to the unit of
resistance. About 9 ft. 9 in. of No. 30 copper wire, or 39 feet 1 in.
of No. 24 copper wire, will make a fairly accurate ohm.

_Resistance coils_, having carefully measured resistances, are made
for standards. (See "Apparatus Book," Chapter XVII., for home-made
resistance coils.) Fig. 64 shows a commercial form of a standard
resistance coil. The coil is inclosed in a case and has large wires
leading from its ends for connections. Fig. 65 gives an idea of
the way in which coils are wound and used with plugs to build up
_resistance boxes_, Fig. 66.

=70. Laws of Resistance.= 1. The resistance of a wire is directly
proportional to its length, provided its cross-section, material, etc.,
are uniform.

2. The resistance of a wire is inversely proportional to its area of
cross-section; or, in other words, inversely proportional to the square
of its diameter, other things being equal.

[Illustration: Fig. 65.]

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

4. The resistance of a wire increases as its temperature rises. (See
"Study," Chapters XVIII. and XIX., for experiments on resistance, its
measurement, etc.)

[Illustration: Fig. 66.]

=71. Current Strength.= The strength of a current at the end of a
circuit depends not only upon the _electrical pressure_, or E. M. F.,
which drives the current, but also upon the _resistance_ which has to
be overcome. The greater the resistance the weaker the current at the
end of its journey.

=72. Unit of Current Strength; The Ampere.= A current having an E. M.
F. of _one volt_, pushing its way through a resistance of _one ohm_,
would have a unit of strength, called _one ampere_. This current, one
ampere strong, would deposit, under proper conditions, .0003277 gramme
of copper in _one second_ from a solution of copper sulphate.

=73. Measurement of Current Strength.= A magnetic needle is deflected
when a current passes around it, as in instruments like the
galvanometer. The _galvanoscope_ merely indicates the presence of a
current. _Galvanometers_ measure the strength of a current, and they
are made in many forms, depending upon the nature and strength of the
currents to be measured. Galvanometers are standardized, or calibrated,
by special measurements, or by comparison with some standard
instrument, so that when the deflection is a certain number of degrees,
the current passing through it is known to be of a certain strength.

[Illustration: Fig. 67.]

Fig. 67 shows an _astatic galvanometer_. Fig. 68 shows a _tangent
galvanometer_, in which the strength of the current is proportional
to the tangent of the angle of deflection. Fig. 69 shows a _D'Arsonval
galvanometer_, in which a coil of wire is suspended between the poles
of a permanent horseshoe magnet. The lines of force are concentrated
by the iron core of the coil. The two thin suspending wires convey the
current to the coil. A ray of light is reflected from the small mirror
and acts as a pointer as in other forms of reflecting galvanometers.

[Illustration: Fig. 68.]

=74. The Ammeter=, Fig. 70, is a form of galvanometer in which the
strength of a current, in amperes, can be read. In these the strength
of current is proportional to the angular deflections. The coils are
made with a small resistance, so that the current will not be greatly
reduced in strength in passing through them.

[Illustration: Fig. 69.]

=75. Voltameters= measure the strength of a current by chemical means,
the quantity of metal deposited or gas generated being proportional
to the time that the current flows and to its strength. In the _water
voltameter_, Fig. 71, the hydrogen and oxygen produced in a given time
are measured. (See "Study," Chapter XXI.)

[Illustration: Fig. 70.]

The _copper voltameter_ measures the amount of copper deposited in a
given time by the current. Fig. 72 shows one form. The copper cathode
is weighed before and after the current flows. The weight of copper
deposited and the time taken are used to calculate the current strength.

[Illustration: Fig. 71.]

=76. Unit of Quantity=; =The Coulomb= is the quantity of electricity
given, in _one second_, by a current having a strength of one ampere.
Time is an important element in considering the work a current can do.

[Illustration: Fig. 72.]

=77. Electrical Horse-power=; =The Watt= is the unit of electrical
power. A current having the strength of one ampere, and an E. M.
F. of one volt has a unit of power. 746 watts make one electrical
horse-power. Watts = amperes × volts. Fig. 73 shows a direct reading
wattmeter based on the international volt and ampere. They save taking
simultaneous ammeter and voltmeter readings, which are otherwise
necessary to get the product of volts and amperes, and are also used on
alternating current measurements.

[Illustration: Fig. 73.]

There are also forms of wattmeters, Fig. 74, in which the watts are
read from dials like those on an ordinary gas-meter, the records being
permanent.

Fig. 75 shows a voltmeter V, and ammeter A, so placed in the circuit
that readings can be taken. D represents a dynamo. A is placed so that
the whole current passes through it, while V is placed between the main
wires to measure the difference in potential. The product of the two
readings in volts and amperes gives the number of watts.

[Illustration: Fig. 74.]

=78. Chemical Meters= also measure the quantity of current that is
used; for example, one may be placed in the cellar to measure the
quantity of current used to light the house.

[Illustration: Fig. 75.]

Fig. 76 shows a chemical meter, a part of the current passing through
a jar containing zinc plates and a solution of zinc sulphate. Metallic
zinc is dissolved from one plate and deposited upon the other. The
increase in weight shows the amount of chemical action which is
proportional to the ampere hours. Knowing the relation between the
quantity of current that can pass through the solution to that which
can pass through the meter by another conductor, a calculation can be
made which will give the current used. A lamp is so arranged that it
automatically lights before the meter gets to the freezing-point; this
warms it up to the proper temperature, at which point the light goes
out again.

[Illustration: Fig. 76.]

CHAPTER VII.

CHEMICAL EFFECTS OF THE ELECTRIC CURRENT.

=79. Electrolysis.= It has been seen that in the voltaic cell
electricity is generated by chemical action. Sulphuric acid acts upon
zinc and dissolves it in the cell, hydrogen is produced, etc. When
this process is reversed, that is, when the electric current is passed
through some solutions, they are decomposed, or broken up into their
constituents. This process is called _electrolysis_, and the compound
decomposed is the _electrolyte_. (See "Study," § 369, etc., with
experiments.)

[Illustration: Fig. 77.]

Fig. 77 shows how water can be decomposed into its two constituents,
hydrogen and oxygen, there being twice as much hydrogen formed as
oxygen.

Fig. 78 shows a glass jar in which are placed two metal strips, A and
C, these being connected with two cells. In this jar may be placed
various conducting solutions to be tested. If, for example, we use
a solution of copper sulphate, its chemical formula being CuSO_{4},
the current will break it up into Cu (copper) and SO_{4}. The Cu will
be deposited upon C as the current passes from A to C through the
solution. A is called the _anode_, and C the _cathode_.

[Illustration: Fig. 78.]

Fig. 79 shows another form of jar used to study the decomposition of
solutions by the electric current.

[Illustration: Fig 79.]

=80. Ions.= When a solution is decomposed into parts by a current, the
parts are called the _Ions_. When copper sulphate (Cu SO_{4}) is used,
the ions are Cu, which is a metal, and SO_{4}, called an acid radical.
When silver nitrate (Ag NO_{3}) is used, Ag and NO_{3} are the ions.
The metal part of the compound goes to the cathode.

CHAPTER VIII.

HOW ELECTROPLATING AND ELECTROTYPING ARE DONE.

=81. Electricity and Chemical Action.= We have just seen, Chapter VII.,
that the electric current has the power to decompose certain compounds
when they are in solution. By choosing the right solutions, then, we
shall be able to get copper, silver, and other metals set free by
electrolysis.

=82. Electroplating= consists in coating substances with metal with
the aid of the electric current. If we wish to electroplate a piece
of metal with copper, for example, we can use the arrangement shown
in Fig. 78, in which C is the cathode plate to be covered, and A is
a copper plate. The two are in a solution of copper sulphate, and,
as explained in § 79, the solution will be decomposed. Copper will
be deposited upon C, and the SO_{4} part of the solution will go to
the anode A, which it will attack and gradually dissolve. The SO_{4},
acting upon the copper anode, makes CuSO_{4} again, and this keeps the
solution at a uniform strength. The amount of copper dissolved from the
copper anode equals, nearly, the amount deposited upon the cathode. The
metal is carried in the direction of the current.

If we wish to plate something with silver or gold, it will be necessary
to use a solution of silver or gold for the electrolyte, a plate of
metallic silver or gold being used for the anode, as the case may be.

Great care is used in cleaning substances to be plated, all dirt and
grease being carefully removed.

Fig. 80 shows a plating bath in which several articles can be plated
at the same time by hanging them upon a metal bar which really forms a
part of the cathode. If, for example, we wish to plate knives, spoons,
etc., with silver, they would be hung from the bar shown, each being a
part of the cathode. The vat would contain a solution of silver, and
from the other bar would be hung a silver plate having a surface about
equal to that of the combined knives, etc.

[Illustration: Fig. 80.]

Most metals are coated with copper before they are plated with silver
or gold. When plating is done on a large scale, a current from a dynamo
is used. For experimental purposes a Gravity cell will do very well.
(See "Study," § 374 to 380 with experiments.)

=83. Electrotyping.= It was observed by De La Rue in 1836 that in the
Daniell cell an even coating of copper was deposited upon the copper
plate. From this was developed the process of electrotyping, which
consists in making a copy in metal of a wood-cut, page of type, etc.
A mould or impression of the type or coin is first made in wax, or
other suitable material. These moulds are, of course, the reverse
of the original, and as they do not conduct electricity, have to be
coated with graphite. This thin coating lines the mould with conducting
material so that the current can get to every part of the mould.
These are then hung upon the cathode in a bath of copper sulphate
as described in § 82. The electric current which passes through the
vat deposits a thin layer of metallic copper next to the graphite.
When this copper gets thick enough, the wax is melted away from it,
leaving a thin shell of copper, the side next to the graphite being
exactly alike in shape to the type, but made of copper. These thin
copper sheets are too thin to stand the pressure necessary on printing
presses, so they are strengthened by backing them with soft metal which
fills every crevice, making solid plates about ¼ in. thick. These
plates or _electrotypes_ are used to print from, the original type
being used to set up another page.

CHAPTER IX.

THE STORAGE BATTERY, AND HOW IT WORKS.

=84. Polarization.= It has been stated that a simple cell polarizes
rapidly on account of hydrogen bubbles that form upon the copper plate.
They tend to send a current in the opposite direction to that of the
main current, which is thereby weakened.

[Illustration: Fig. 81.]

=85. Electromotive Force of Polarization.= It has been shown, Fig. 71,
that water can be decomposed by the electric current. Hydrogen and
oxygen have a strong attraction or chemical affinity for each other, or
they would not unite to form water. This attraction has to be overcome
before the water can be decomposed. As soon as the decomposing current
ceases to flow, the gases formed try to rush together again; in fact,
if the water voltameter be disconnected from the cells and connected
with a galvanoscope, the presence of a current will be shown. This
voltameter will give a current with an E. M. F. of nearly 1.5 volts; so
it is evident that we must have a current with a higher voltage than
this to decompose water. This E. M. F., due to polarization, is called
the E. M. F. of polarization.

=86. Secondary or Storage Batteries=, also called _accumulators_, do
not really store electricity. They must be charged by a current before
they can give out any electricity. Chemical changes are produced in the
storage cells by the charging current just as they are in voltameters,
electroplating solutions, etc.; so it is potential chemical energy
that is really stored. When the new products are allowed to go back to
their original state, by joining the electrodes of the charged cell, a
current is produced.

Fig. 81 shows two lead plates, A and B, immersed in dilute sulphuric
acid, and connected with two ordinary cells. A strong current will pass
through the liquid between A and B at first, but it will quickly become
weaker, as chemical changes take place in the liquid. This may be shown
by a galvanometer put in the circuit before beginning the experiment.
By disconnecting the wires from the cells and joining them to the
galvanometer, it will be shown that a current comes from the lead
plates. This arrangement may be called a simple storage cell. Regular
storage cells are charged with the current from a dynamo. (See "Study,"
Exp. 151.)

[Illustration: Fig. 82.]

The first storage cells were made of plain lead plates, rolled up in
such a way that they were close to each other, but did not touch. These
were placed in dilute sulphuric acid. They were charged in alternate
directions several times, until the lead became properly acted upon, at
which time the cell would furnish a current.

A great improvement was made in 1881, by Faure, who coated the plates
with red lead.

[Illustration: Fig. 83.]

The method now generally practiced is to cast a frame of lead, with
raised right-angled ribs on each side, thus forming little depressed
squares, or to punch a lead plate full of holes, which squares or holes
are then filled with a pasty mixture of red oxide of lead in positive
plates, and with litharge in negatives. In a form called the chloride
battery, instead of cementing lead oxide paste into or against a lead
framing in order to obtain the necessary active material, the latter is
obtained by a strictly chemical process.

Fig. 82 shows a storage cell with plates, etc., contained in a glass
jar. Fig. 83 shows a cell of 41 plates, set up in a lead-lined wood
tank. Fig. 84 shows three cells joined in series. Many storage cells
are used in central electric light stations to help the dynamos during
the "rush" hours at night. They are charged during the day when the
load on the dynamos is not heavy.

Fig. 85 shows another form of storage cell containing a number of
plates.

[Illustration: Fig. 84.]

=87. The Uses of Storage Batteries= are almost numberless. The current
can be used for nearly everything for which a constant current is
adapted, the following being some of its applications: Carriage
propulsion; electric launch propulsion; train lighting; yacht lighting;
carriage lighting; bicycle lighting; miners' lamps; dental, medical,
surgical, and laboratory work; phonographs; kinetoscopes; automaton
pianos; sewing-machine motors; fan motors; telegraph; telephone;
electric bell; electric fire-alarm; heat regulating; railroad switch
and signal apparatus.

By the installing of a storage plant many natural but small sources
of power may be utilized in furnishing light and power; sources which
otherwise are not available, because not large enough to supply maximum
demands. The force of the tides, of small water powers from irrigating
ditches, and even of the wind, come under this heading.

[Illustration: Fig. 85.]

As a regulator of pressure, in case of fluctuations in the load, the
value of a storage plant is inestimable. These fluctuations of load are
particularly noticeable in electric railway plants, where the demand is
constantly rising and falling, sometimes jumping from almost nothing to
the maximum, and _vice versa_, in a few seconds. If for no other reason
than the prevention of severe strain on the engines and generators,
caused by these fluctuations of demand, a storage plant will be
valuable.

CHAPTER X.

HOW ELECTRICITY IS GENERATED BY HEAT.

=88. Thermoelectricity= is the name given to electricity that is
generated by heat. If a strip of iron, I, be connected between two
strips of copper, C C, these being joined by a copper wire, C W, we
shall have an arrangement that will generate a current when heated at
either of the junctions between C and I. When it is heated at A the
current will flow as shown by arrows, from C to I. If we heat at B,
the current will flow in the opposite direction through the metals,
although it will still go from C to I as before. Such currents are
called _thermoelectric currents_.

[Illustration: Fig. 86.]

Different pairs of metals produce different results. Antimony and
bismuth are generally used, because the greatest effect is produced
by them. 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 bismuth to antimony across the joint. By
cooling the juncture below the temperature of the rest of the circuit,
a current will be produced in the opposite direction to the above. The
energy of the current is kept up by the heat absorbed, just as it is
kept up by chemical action in the voltaic cell.

=89. Peltier Effect.= If an electric current be passed through pairs of
metals, the parts at the junction become slightly warmer or cooler than
before, depending upon the direction of the current. This action is
really the reverse of that in which currents are produced by heat.

[Illustration: Fig. 87.]

=90. Thermopiles.= As the E.M.F. of the current produced by a single
pair of metals is very small, several pairs are usually joined in
series, so that the different currents will help each other by flowing
in the same direction. Such combinations are called thermoelectric
piles, or simply _thermopiles_.

Fig. 87 shows such an arrangement, in which a large number of elements
are placed in a small space. The junctures are so arranged that the
alternate ones come together at one side.

Fig. 88 shows a thermopile connected with a galvanometer. The heat of
a match, or the cold of a piece of ice, will produce a current, even if
held at some distance from the thermopile. The galvanometer should be
a short-coil astatic one. (See "Study," Chapter XXIV., for experiments
and home-made thermopile.)

[Illustration: Fig. 88.]

CHAPTER XI.

MAGNETIC EFFECTS OF THE ELECTRIC CURRENT.

=91. Electromagnetism= is the name given to magnetism that is developed
by electricity. We have seen that if a magnetic needle be placed in the
field of a magnet, its N pole will point in the direction taken by the
lines of force as they pass from the N to the S pole of the magnet.

[Illustration: Fig. 89.]

=92. Lines of Force about a Wire.= When a current passes through a
wire, the magnetic needle placed over or under it tends to take a
position at right angles to the wire. Fig. 89 shows such a wire and
needle, and how the needle is deflected; it twists right around from
its N and S position as soon as the current begins to flow. 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 toward the N.

Fig. 90 shows a bent wire through which a current passes from C to Z.
If you look along the wire from C toward the points A and B, you will
see that _under_ the wire the lines of force pass to the left. Looking
along the wire from Z toward D you will see that the lines of force
pass opposite to the above, as the current comes _toward_ you. This is
learned by experiment. (See "Study," Exp. 152, § 385, etc.)

[Illustration: Fig. 90.]

[Illustration: Fig. 91.]

_Rule._ Hold the right hand with the thumb extended (Fig. 89) and with
the fingers pointing in the direction of the current, the palm being
toward the needle and on the opposite side of the wire from the needle.
The north-seeking pole will then be deflected in the direction in which
the thumb points.

=93. Current Detectors.= As there is a magnetic field about a wire when
a current passes through it, and as the magnetic needle is affected, we
have a means of detecting the presence of a current. When the current
is strong it is simply necessary to let it pass once over or under a
needle; when it is weak, the wire must pass several times above and
below the needle, Fig. 91, to give the needle motion. (See "Apparatus
Book," Chapter XIII., for home-made detectors.)

[Illustration: Fig. 92.]

=94. Astatic Needles and Detectors.= By arranging two magnetized
needles with their poles opposite each other, Fig. 92, an _astatic
needle_ is formed. The pointing-power is almost nothing, although
their magnetic fields are retained. This combination is used to detect
feeble currents. In the ordinary detector, the tendency of the needle
to point to the N and S has to be overcome by the magnetic field about
the coil before the needle can be moved; but in the _astatic detector_
and _galvanoscope_ this pointing-power is done away with. Fig. 93 shows
a simple _astatic galvanoscope_. Fig. 67 shows an astatic galvanometer
for measuring weak currents.

[Illustration: Fig. 93.]

=95. Polarity of Coils.= When a current of electricity passes through
a coil of wire, the coil acts very much like a magnet, although no
iron enters into its construction. The coil becomes magnetized by the
electric current, lines of force pass from it into the air, etc. Fig.
94 shows a coil connected to copper and zinc plates, so arranged with
cork that the whole can float in a dish of dilute sulphuric acid. The
current passes as shown by the arrows, and when the N pole of a magnet
is brought near the right-hand end, there is a repulsion, showing that
that end of the coil has a N pole.

_Rule._ 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. When the current passes in a clockwise direction a S pole is
produced.

[Illustration: Fig. 94.]

=96. Electromagnets.= A coil of wire has a stronger field than a
straight wire carrying the same current, because each turn adds its
field to the fields of the other turns. By having the central part of
the coil made of iron, or by having the coil of insulated wire wound
upon an iron _core_, the strength of the magnetic field of the coil is
greatly increased.

Lines of force do not pass as readily through air as through iron;
in fact, lines of force will go out of their way to go through iron.
With a coil of wire the lines of force pass from its N pole through
the air on all sides of the coil to its S pole; they then pass through
the inside of the coil and through the air back to the N pole. When
the resistance to their passage through the coil is decreased by the
core, the magnetic field is greatly strengthened, and we have an
_electromagnet_.

The coil of wire temporarily magnetizes the iron core; it can
permanently magnetize a piece of steel used as a core. (See "Study,"
Chapter XXII., for experiments.)

[Illustration: Fig. 95.]

=97. Forms of Electromagnets.= Fig. 95 shows a _straight, or
bar electromagnet_. Fig. 96 shows a simple form of _horseshoe
electromagnet_. As this form is not easily wound, the coils are
generally wound on two separate cores which are then joined by a
_yoke_. The yoke merely takes the place of the curved part shown
in Fig. 96. In Fig. 97 is shown the ordinary form of horseshoe
electromagnet used for all sorts of electrical instruments. (See
"Apparatus Book," Chapter IX., for home-made electromagnets.)

=98. Yokes and Armatures.= In the horseshoe magnet 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, instead of going through
the air. This reduces the resistance to them. If we had no yoke we
should simply have two straight electromagnets, and the resistance to
the lines of force would be so great that the total strength would
be much reduced. Yokes are made of soft iron, as well as the cores
and armature. The _armature_, as with permanent horseshoe magnets, is
strongly drawn toward the poles. As soon as the current ceases to flow,
the attraction also ceases.

[Illustration: Fig. 96.]

[Illustration: Fig. 97.]

[Illustration: Fig. 98.]

Beautiful magnetic figures can be made with horseshoe magnets. Fig. 98
shows that the coils must be joined so that the current can pass around
the cores in opposite directions to make unlike poles. (See "Study,"
Exp. 164 to 173.)

CHAPTER XII.

HOW ELECTRICITY IS GENERATED BY INDUCTION.

=99. Electromagnetic Induction.= We have seen that a magnet has the
power to act through space and induce another piece of iron or steel
to become a magnet. A charge of static electricity can induce a
charge upon another conductor. We have now to see how a _current_ of
electricity in one conductor can induce a current in another conductor,
not in any way connected with the first, and how a magnet and a coil
can generate a current.

[Illustration: Fig. 99.]

[Illustration: Fig. 100.]

=100. Current from Magnet and Coil.= If a bar magnet, Fig. 99, be
suddenly thrust into a hollow coil of wire, a momentary current of
electricity will be generated in the coil. No current passes when the
magnet and coil are still; at least one of them must be in motion. Such
a current is said to be _induced_, and is an _inverse_ one when the
magnet is inserted, and a _direct_ one when the magnet is withdrawn
from the coil.

=101. Induced Currents and Lines of Force.= Permanent magnets are
constantly sending out thousands of lines of force. Fig. 100 shows
a bar magnet entering a coil of wire; the number of lines of force
is increasing, 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 an iron core be used in the coil,
the induced current will be greatly strengthened.

[Illustration: Fig. 101.]

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. We have, in
this simple experiment, the key to the action of the dynamo and other
electrical machines.

=102. Current from two Coils.= Fig. 101 shows two coils of wire, the
smaller being connected to a cell, the larger to a galvanometer.
By moving the small coil up and down inside of the large one,
induced currents are generated, first in one direction and then in
the opposite. We have here two entirely separate circuits, in no
way connected. The _primary_ current comes from the cell, while the
_secondary_ current is an induced one. By placing a core in the small
coil of Fig. 101, the induced current will be greatly strengthened.

It is not necessary to have the two coils so that one or both of them
can move. They may be wound on the same core, or otherwise arranged as
in the induction coil. (See "Study," Chapter XXV., for experiments on
induced currents.)

CHAPTER XIII.

HOW THE INDUCTION COIL WORKS.

=103. The Coils.= We saw, § 102, that an induced current was generated
when a current-carrying coil, Fig. 101, was thrust into another coil
connected with a galvanometer. The galvanometer was used merely to show
the presence of the current. The _primary coil_ is the one connected
with the cell; the other one is called the _secondary coil_.

[Illustration: Fig. 102.]

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. It is evident, then, that we can keep the small coil of
Fig. 101 with its core inside of the large coil, and generate induced
currents by merely making and breaking the primary circuit.

We may consider that when the primary circuit is closed, the lines of
force shoot out through the turns of the secondary coil just as they
do when a magnet or a current-carrying coil is thrust into it. Upon
opening the circuit, the lines of force cease to exist; that is, we may
imagine them drawn in again.

=104. Construction.= Fig. 102 shows one form of home-made induction
coil, given here merely to explain the action and connections. Nearly
all induction coils have some form of automatic current interrupter,
placed in the primary circuit, to rapidly turn the current off and on.

_Details of Figs. 102 and 103._ Wires 5 and 6 are the ends of the
primary coil, while wires 7 and 8 are the terminals of the secondary
coil. The primary coil is wound on a bolt which serves as the core, and
on this coil is wound the secondary which consists of many turns of
fine wire. The wires from a battery should be joined to binding-posts W
and X, and the handles, from which the shock is felt, to Y and Z. Fig.
103 shows the details of the interrupter.

[Illustration: Fig. 103.]

If the current from a cell 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 SI, as A is no longer attracted; the circuit being closed, the
operation is rapidly repeated.

A _condenser_ is usually connected to commercial forms. It is placed
under the wood-work and decreases sparking at the interrupter. (See
"Apparatus Book," Chapter XI., for home-made induction coils.)

[Illustration: Fig. 104.]

Fig. 104 shows one form of coil. The battery wires are joined to the
binding-posts at the left. The secondary coil ends in two rods, and the
spark jumps from one to the other. The interrupter and a switch are
shown at the left.

Fig. 105 shows a small coil for medical purposes. A dry cell is placed
under the coil and all is included in a neat box. The handles form the
terminals of the secondary coil.

=105. The Currents.= It should be noted that the current from the
cell does not get into the secondary coil. The coils are thoroughly
insulated from each other. The secondary current is an induced one,
its voltage depending upon the relative number of turns of wire there
are in the two coils. (See Transformers.) The secondary current is
an alternating one; that is, it flows in one direction for an instant
and then immediately reverses its direction. The rapidity of the
alternations depends upon the speed of the interrupter. Coils are made
that give a secondary current with an enormous voltage; so high, in
fact, that the spark will pass many inches, and otherwise act like
those produced by static electric machines.

[Illustration: Fig. 105.]

=106. Uses of Induction Coils.= Gas-jets can be lighted at a distance
with the spark from a coil, by extending wires from the secondary
coil to the jet. Powder can be fired at a distance, and other things
performed, when a high voltage current is needed. Its use in medicine
has been noted. It is largely used in telephone work. Of late, great
use has been made of the secondary current in experiments with
vacuum-tubes, X-ray work, etc.

CHAPTER XIV.

THE ELECTRIC TELEGRAPH, AND HOW IT SENDS MESSAGES.

=107. The Complete Telegraph Line= consists of several instruments,
switches, etc., etc., but its essential parts are: The _Line_, or wire,
which connects the different stations; the _Transmitter_ or _Key_; the
_Receiver_ or _Sounder_, and the _Battery_ or _Dynamo_.

=108. The Line= is made of strong copper, iron, or soft steel wire. To
keep the current in the line it is insulated, generally upon poles, by
glass insulators. For very short lines two wires can be used, the line
wire and the return; but for long lines the earth is used as a return,
a wire from each end being joined to large metal plates sunk in the
earth.

[Illustration: Fig. 106.]

=109. Telegraph Keys= are merely instruments by which the circuit
can be conveniently and rapidly opened or closed at the will of the
operator. An ordinary push-button may be used to turn the current off
and on, but it is not so convenient as a key.

Fig. 106 shows a side view of a simple key which can be put anywhere
in the circuit, one end of the cut wire being attached to X and the
other to Y. By moving the lever C up and down according to a previously
arranged set of signals, a current will be allowed to pass to a
distant station. As X and Y are insulated from each other, the current
can pass only when C presses against Y.

Fig. 107 shows a regular key, with switch, which is used to allow the
current to pass through the instrument when receiving a message.

[Illustration: Fig. 107.]

=110. Telegraph Sounders= receive the current from some distant
station, and with its electromagnet produce sounds that can be
translated into messages.

[Illustration: Fig. 108.]

Fig. 108 shows simply an electromagnet H, the coil being connected in
series with a key K and a cell D C. The key and D C are shown by a top
view. The lever of K does not touch the other metal strap until it is
pressed down. A little above the core of H is held a strip of iron, on
armature I. As soon as the circuit is closed at K, the current rushes
through the circuit, and the core attracts I making a distinct _click_.
As soon as K is raised, I springs away from the core, if it has been
properly held. In regular instruments a click is also made when the
armature springs back again.

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 home-made keys,
sounders, etc., see "Apparatus Book," Chapter XIV.)

[Illustration: Fig. 109.]

[Illustration: Fig. 110.]

Fig. 109 shows a form of home-made sounder. Fig. 110 shows one form of
telegraph sounder. Over the poles of the horseshoe electromagnet is an
armature fixed to a metal bar that can rock up and down. The instant
the current passes through the coils the armature comes down until a
stop-screw strikes firmly upon the metal frame, making the down click.
As soon as the distant key is raised, the armature is firmly pulled
back and another click is made. The two clicks differ in sound, and can
be readily recognized by the operator.

=111. Connections for Simple Line.= Fig. 111 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. 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 in the same room, 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 the wire, R W, or through the
earth; but for short lines a wire is best.

[Illustration: Fig. 111.]

=112. Operation of Simple Line.= Suppose two boys, R (right) and L
(left) have a line. Fig. 111 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. As soon as the key is raised, the armatures lift and
make the up-click. As soon as R has finished, he closes his switch E.
As the armatures are then held down, L knows that R has finished, so
he opens his switch 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 must be used for such lines. On
very large lines dynamos are used to furnish the current.

=113. The Relay.= 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. To get around this, relays
are used; these are very delicate instruments that replace the sounder
in the line wire circuit. Their coils are usually wound with many turns
of fine wire, so that a feeble current will move its nicely adjusted
armature. The relay armature merely acts as an automatic key to open
and close a local circuit which includes a battery and sounder. The
line current does not enter the sounder; it passes back from the relay
to the sending station through the earth.

[Illustration: Fig. 112.]

Fig. 112 gives an idea of simple relay connections. The key K, and
cell D C, represent a distant sending station. E is the electromagnet
of the relay, and R A is its armature. L W and R W represent the line
and return wires. R A will vibrate toward E every time K is pressed,
and close the local circuit, which includes a local battery, L B, and
a sounder. It is evident that as soon as K is pressed the sounder will
work with a good strong click, as the local battery can be made as
strong as desired.

Fig. 113 shows a regular instrument which opens and closes the local
circuit at the top of the armature.

[Illustration: Fig. 113.]

=114. Ink Writing Registers= are frequently used instead of sounders.
Fig. 114 shows a writing register that starts itself promptly at the
opening of the circuit, and stops automatically as soon as the circuit
returns to its normal condition. A strip of narrow paper is slowly
pulled from the reel by the machine, a mark being made upon it every
time the armature of an inclosed electromagnet is attracted. When the
circuit is simply closed for an instant, a short line, representing a
_dot_, is made.

Registers are built both single pen and double pen. In the latter case,
as the record of one wire is made with a fine pen, and the other with
a coarse pen, they can always be identified. The record being blocked
out upon white tape in solid black color, in a series of clean-cut dots
and dashes, it can be read at a glance, and as it is indelible, it may
be read years afterward. Registers are made for local circuits, for
use in connection with relays, or for direct use on main lines, as is
usually desirable in fire-alarm circuits.

[Illustration: Fig. 114.]

CHAPTER XV.

THE ELECTRIC BELL AND SOME OF ITS USES.

[Illustration: Fig. 115.]

[Illustration: Fig. 116.]

=115. Automatic Current Interrupters= are used on most common bells,
as well as on induction coils, etc. (See § 104.) Fig. 115 shows a
simple form of interrupter. The wire 1, from a cell D C, is joined to
an iron strip I a short distance from its end. The other wire from D C
passes to one end of the electromagnet coil H. The remaining end of H
is placed in contact with I as shown, completing the circuit. As soon
as the current passes, I is pulled down and away from the upper wire
2, breaking the circuit. I, being held by its left-hand end firmly in
the hand, immediately springs back to its former position, closing the
circuit again. This action is repeated, the rapidity of the vibrations
depending somewhat upon the position of the wires on I. In regular
instruments a platinum point is used where the circuit is broken; this
stands the sparking when the armature vibrates.

=116. Electric Bells= may be illustrated by referring to Fig. 116,
which shows a circuit similar to that described in § 115, but which
also contains a key K, in the circuit. This allows the circuit to
be opened and closed at a distance from the vibrating armature. The
circuit must not be broken at two places at the same time, so wires
should touch at the end of I before pressing K. Upon pressing K the
armature I will vibrate rapidly. By placing a small bell near the end
of the vibrating armature, so that it will be struck by I at each
vibration, we should have a simple electric bell. This form of electric
bell is called a _trembling_ bell, on account of its vibrating armature.

[Illustration: Fig. 117.]

[Illustration: Fig. 118.]

Fig. 117 shows a form of trembling bell with cover removed. Fig. 118
shows a _single-stroke_ bell, used for fire-alarms and other signal
work. In this the armature is attracted but once each time the current
passes. As many taps of the bell can be given as desired by pressing
the push-button. Fig. 119 shows a gong for railway crossings, signals,
etc. Fig. 120 shows a circuit including cell, push-button, and bell,
with extra wire for lengthening the line.

[Illustration: Fig. 119.]

_Electro-Mechanical Gongs_ are used to give loud signals for special
purposes. The mechanical device is started by the electric current when
the armature of the electromagnet is attracted. Springs, weights, etc.,
are used as the power. Fig. 121 shows a small bell of this kind.

[Illustration: Fig. 120.]

=117. Magneto Testing Bells=, Fig. 122, are really small hand-power
dynamos. The armature is made to revolve between the poles of strong
permanent magnets, and it is so wound that it gives a current with a
large E. M. F., so that it can ring through the large resistance of a
long line to test it.

_Magneto Signal Bells_, Fig. 123, are used as generator and bell in
connection with telephones. The generator, used to ring a bell at a
distant station, stands at the bottom of the box. The bell is fastened
to the lid, and receives current from a distant bell.

[Illustration: Fig. 121.]

[Illustration: Fig. 122.]

[Illustration: Fig. 123.]

[Illustration: Fig. 124.]

=118. Electric Buzzers= have the same general construction as electric
bells; 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. Fig. 124 shows the usual form of
buzzers, the cover being removed.

CHAPTER XVI.

THE TELEPHONE, AND HOW IT TRANSMITS SPEECH.

=119. 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 sound out again is called the _receiver_. Sound
itself does not pass over the line. While the same apparatus can be
used for both transmitter and receiver, they are generally different in
construction to get the best results.

[Illustration: Fig. 125.]

[Illustration: Fig. 126.]

[Illustration: Fig. 127.]

=120. The Bell or Magneto-transmitter= generates its own current, and
is, strictly speaking, a dynamo that is run by the voice. It depends
upon induction for its action.

[Illustration: Fig. 128.]

Fig. 125 shows a coil of wire, H, with soft iron core, the ends of the
wires being connected to a delicate galvanoscope. If one pole of the
magnet H M be suddenly moved up and down near the core, an alternating
current will be generated in the coil, the circuit being completed
through the galvanoscope. As H M approaches the core the current will
flow in one direction, and as H M is withdrawn it will pass in the
opposite direction. The combination makes a miniature alternating
dynamo.

[Illustration: Fig. 129.]

If we imagine the soft iron core of H, Fig. 125, taken out, and one
pole of H M, or preferably that of a bar magnet stuck through the coil,
a feeble current will also be produced by moving the soft iron back and
forth near the magnet's pole. This is really what is done in the Bell
transmitter, soft iron in the shape of a thin disc (D, Fig. 126) being
made to vibrate by the voice immediately in front of a coil having
a permanent magnet for a core. The disc, or _diaphragm_, as it is
called, is fixed near, but it does not touch, the magnet. It is under
a constant strain, being attracted by the magnet, so its slightest
movement changes the strength of the magnetic field, causing more or
less lines of force to shoot through the turns of the coil and induce a
current. The coil consists of many turns of fine, insulated wire. The
current generated is an alternating one, and although exceedingly small
can force its way through a long length of wire.

[Illustration: Fig. 130.]

Fig. 127 shows a section of a regular transmitter, and Fig. 128 a form
of compound magnet frequently used in the transmitter. Fig. 129 shows a
transmitter with cords which contain flexible wires.

[Illustration: Fig. 131.]

=121. The Receiver=, for short lines, may have the same construction as
the Bell transmitter. Fig. 130 shows a diagram of two Bell receivers,
either being used as the transmitter and the other as the receiver.
As the alternating current goes to the distant receiver, it flies
through the coil first in one direction and then in the other. This
alternately strengthens and weakens the magnetic field near the
diaphragm, causing it to vibrate back and forth as the magnet pulls
more or less. The receiver diaphragm repeats the vibrations in the
transmitter. Nothing but the induced electric current passes over the
wires.

[Illustration: Fig. 132.]

=122. The Microphone.= If a current of electricity be allowed to
pass through a circuit like that shown in Fig. 131, which includes a
battery, a Bell receiver, and a microphone, any slight sound near the
microphone will be greatly magnified in the receiver. The microphone
consists of pieces of carbon so fixed that they form loose contacts.
Any slight movement of the carbon causes the resistance to the current
to be greatly changed. The rapidly varying resistance allows more or
less current to pass, the result being that this pulsating current
causes the diaphragm to vibrate. The diaphragm has a constantly varying
pull upon it when the carbons are in any way disturbed by the voice, or
by the ticking of a watch, etc. This principle has been made use of in
carbon transmitters, which are made in a large variety of forms.

[Illustration: Fig. 133.]

=123. 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. 132, X and Y are two carbon buttons, X
being attached to the diaphragm D. Button Y presses gently against X,
allowing a little current to pass through the circuit which includes
a battery, D C, and a receiver, R. 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. 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. In
regular lines, of course, a receiver and transmitter are connected at
each end, together with bells, etc., for signaling.

[Illustration: Fig. 134.]

=124. Induction Coils in Telephone Work.= As the resistance of long
telephone lines is great, a high electrical pressure, or E.M.F. is
desired. While the current from one or two cells is sufficient to work
the transmitter properly, and cause undulating currents in the short
line, it does not have power enough to force its way over a long line.

To get around this difficulty, an induction coil, Fig. 133, is used
to transform the battery current, that flows through the carbon
transmitter and primary coil, into a current with a high E. M. F. The
battery current in the primary coil is undulating, but always passes in
the same direction, making the magnetic field around the core weaker
and stronger. This causes an alternating current in the secondary coil
and main line. In Fig. 133 P and S represent the primary and secondary
coils. P is joined in series with a cell and carbon transmitter; S
is joined to the distant 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.

[Illustration: Fig. 135.]

=125. Various forms= of telephones are shown in Figs. 134, 135, 136.
Fig. 134 shows a form of desk telephone; Fig. 135 shows a common form
of wall telephone; Fig. 136 shows head-telephones for switchboard
operators.

[Illustration: Fig. 136.]

CHAPTER XVII.

HOW ELECTRICITY IS GENERATED BY DYNAMOS.

=126. The Dynamo=, _Dynamo-Electric Machine_ or _Generator_, is a
machine for converting mechanical energy into an electric current,
through electromagnetic induction. The dynamo is a machine that will
convert steam power, for example, into an electric current. Strictly
speaking, a dynamo creates electrical pressure, or electromotive force,
and not electricity, just as a force-pump creates water-pressure, and
not water. They are generally run by steam or water power.

[Illustration: Fig. 137.]

=127. Induced Currents.= We have already spoken about currents being
induced by moving a coil of wire in a magnetic field. We shall now
see how this principle is used in the dynamo which is a generator of
induced currents.

[Illustration: Fig. 138.]

Fig. 137 shows how a current can be generated by a bar magnet and
a coil of wire. Fig. 138 shows how a current can be generated by a
horseshoe magnet and a coil of wire having an iron core. The ends of
the coil are to be connected to an astatic galvanoscope; this forms a
closed circuit. The coil may be moved past the magnet, or the magnet
past the coil.

[Illustration: Fig. 139.]

[Illustration: Fig. 140.]

[Illustration: Fig. 141.]

[Illustration: Fig. 142.]

Fig. 139 shows how a current can be generated by two coils, H being
connected to an astatic galvanoscope and E to a battery. By suddenly
bringing E toward H or the core of E past that of H, a current is
produced. We have in this arrangement the main features of a dynamo.
We can reverse the operation, holding E in one position and moving H
rapidly toward it. In this case H would represent the armature and E
the field-magnet. When H is moved toward E, the induced current in H
flows in one direction, and when H is suddenly withdrawn from E the
current is reversed in H. (See "Study," Chapter XXV., for experiments.)

[Illustration: Fig. 143.]

=128. Induced Currents by Rotary Motion.= The motions of the coils in
straight lines are not suitable for producing currents strong enough
for commercial purposes. In order to generate currents of considerable
strength and pressure, the coils of wire have to be pushed past
magnets, or electromagnets, with great speed. In the dynamo the coils
are so wound 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. 144.]

=129. Field-Magnets; Armature; Commutator.= What we need then, to
produce an induced current by a rotary motion, is a strong magnetic
field, a rotating coil of wire properly placed in the field, and some
means of leading the current from the machine.

[Illustration: Fig. 145.]

[Illustration: Fig. 146.]

If a loop of wire, Fig. 140, 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. This agrees with the experiments suggested in § 127, when
the current generated in a coil passed in one direction during its
motion _toward_ the strongest part of the field, and in the opposite
direction when the coil passed _out_ of it. A coil must be cut by
lines of force to generate a current. A current inside of the machine,
as in Fig. 140, would be of no value; it must be led out to external
conductors where it can do work. Some sort of sliding contact is
necessary to connect a revolving conductor with outside stationary
ones. The magnet, called the _field-magnet_, is merely to furnish lines
of magnetic force. The one turn of wire represents the simplest form of
_armature_.

Fig. 141 shows the ends of a coil joined to two rings, X, Y, insulated
from each other, and rotating with the coil. The 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. 147.]

In Fig. 142 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, to do the 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 toward 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.

[Illustration: Fig. 148.]

In regular machines, the field-magnets are electromagnets, the whole
or a part of the current from the dynamo passing around them on its
way out, to excite them and make a powerful field between the poles.
To lessen the resistance to the lines of force on their way from the
N to the S pole of the field-magnets, the armature coils are wound on
an iron core; this greatly increases the strength of the field, as
the lines of force have to jump across but two small air-gaps. There
are many loops of wire on regular armatures, and many segments to the
commutator, carefully insulated from each other, each getting its
current from the coil attached to it.

=130. Types of Dynamos.= While there is an almost endless number of
different makes and shapes of dynamos, they may be divided into two
great types; the _continuous_ or _direct current_, and the _alternating
current_ dynamo. Direct current machines give out a current which
constantly flows in one direction, and this is because a commutator is
used. Alternating currents come from collectors or rings, as shown in
Fig. 141; and as an alternating current cannot be used to excite the
fields, an outside current from a small direct current machine must be
used. These are called exciters.

[Illustration: Fig. 149.]

In direct current machines enough residual magnetism is left in the
field to induce a slight current in the armature when the machine is
started. This immediately adds strength to the field-magnets, which, in
turn, induce a stronger current in the armature.

=131. Winding of Dynamos.= There are several ways of winding dynamos,
depending upon the special uses to be made of the current.

The _series wound_ dynamo, Fig. 143, is so arranged that the entire
current passes around the field-magnet cores on its way from the
machine. In the _shunt wound_ dynamo, Fig. 144, a part, only, of the
current from the machine is carried around the field-magnet cores
through many turns of fine wire. The _compound wound_ dynamo is really
a combination of the two methods just given. In _separately-excited_
dynamos, the current from a separate machine is used to excite the
field-magnets.

=132. Various Machines.= Fig. 145 shows a hand power dynamo
which produces a current for experimental work. Fig. 146 shows a
magneto-electrical generator which produces a current for medical use.
Figs. 147, 148 show forms of dynamos, and Fig. 149 shows how arc lamps
are connected in series to dynamos.

CHAPTER XVIII.

HOW THE ELECTRIC CURRENT IS TRANSFORMED.

=133. Electric Current and Work.= The amount of work a current can do
depends upon two factors; the strength (amperes), and the pressure,
or E. M. F. (volts). A current of 10 amperes with a pressure of 1,000
volts = 10 × 1,000 = 10,000 watts. This furnishes the same amount of
energy as a current of 50 amperes at 200 volts; 50 × 200 = 10,000 watts.

=134. Transmission of Currents.= It is often necessary to carry a
current a long distance before it is used. A current of 50 amperes
would need a copper conductor 25 times as large (sectional area) as one
to carry the 10 ampere current mentioned in § 133. As copper conductors
are very expensive, electric light companies, etc., generally try to
carry the current on as small a wire as possible. To do this, the
voltage is kept high, and the amperage low. Thus, as seen in § 133,
the current of 1,000 volts and 10 amperes could be carried on a much
smaller wire than the other current of equal energy. A current of
1,000 volts, however, is not adapted for lights, etc., so it has to be
changed to lower voltage by some form of transformer before it can be
used.

=135. 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
one core; in fact, an induction coil may be considered a transformer,
but in this a direct current has to be interrupted. If the secondary
coil has 100 times as many turns of wire as the primary, a current of
100 volts can be taken from the secondary coil when the primary current
is but 1 volt; but the _strength_ (amperes) of this new current will be
but one-hundredth that of the primary current.

By using the coil of fine wire as the primary, we can lower the voltage
and increase the strength in the same proportion.

[Illustration: Fig. 150.]

[Illustration: Fig. 151.]

Fig. 150 shows about the simplest form of transformer with a solid iron
core, on which are wound two coils, the one, P, being the primary, and
the other, S, the secondary. Fig. 151 shows the general appearance of
one make of transformer. The operation of this apparatus, as already
mentioned, is to reduce the high pressure alternating current sent out
over the conductors from the dynamo, to a potential at which it can
be employed with convenience and safety, for illumination and other
purposes. They consist of two or more coils of wire most carefully
insulated from one another. A core or magnetic circuit of soft iron,
composed of very thin punchings, is then formed around these coils,
the purpose of the iron core being to reduce the magnetic resistance
and increase the inductive effect. One set of these coils is connected
with the primary or high-pressure wires, while the other set, which are
called the secondary coils, is connected to the house or low-pressure
wires, or wherever the current is required for use. The rapidly
alternating current impulses in the primary or high-pressure wires
induce secondary currents similar in form but opposite in direction
in the secondary coils. These current impulses are of a much lower
pressure, depending upon the ratio of the number of turns of wire
in the respective coils, it being customary to wind transformers in
such a manner as to reduce from 1,000 or 2,000-volt primaries to 50
or 100-volt secondaries, at which voltage the secondary current is
perfectly harmless.

[Illustration: Fig. 152.]

=136. Motor-Dynamos.= Fig. 152. These consist essentially of two
belt-type machines on a common base, direct coupled together, one
machine acting as a motor to receive current at a certain voltage,
and the other acting as a dynamo to give out the current usually
at a different voltage. As they transform current from one voltage
to another, motor-dynamos are sometimes called Double Field Direct
Current Transformers. The larger sizes have three bearings, one bearing
being between the two machines, while the smaller sizes have but two
bearings, the two armatures being fastened to a common spider.

[Illustration: Fig. 153.]

_Applications._ The uses to which motor-dynamos are put are very
various. They are extensively used in the larger sizes as "Boosters,"
for giving the necessary extra force on long electric supply circuits
to carry the current to the end with the same pressure as that which
reaches the ends of the shorter circuits from the station.

Motor-dynamos have the advantage over dynamotors, described later, of
having the secondary voltage easily and economically varied over wide
ranges by means of a regulator in the dynamo field.

=137. Dynamotors.= Fig. 153. In Dynamotors the motor and dynamo
armatures are combined in one, thus requiring a single field only.
The primary armature winding, which operates as a motor to drive the
machine, and the secondary or dynamo winding, which operates as a
generator to produce a new current, are upon the same armature core,
so that the armature reaction of one winding neutralizes that of the
other. They therefore have no tendency to spark, and do not require
shifting of the brushes with varying load. Having but one field and two
bearings, they are also more efficient than motor-dynamos.

_Applications._ They have largely displaced batteries for telegraph
work. The size shown, occupying a space of about 8-inch cube, and
having an output of 40 watts, will displace about 800 gravity cells,
occupying a space of about 10 feet cube. The cost of maintenance of
such a battery per year, exclusive of rent, is about $800, whereas the
1-6 dynamotor can be operated at an annual expense of $150.

Dynamotors are largely used by telephone companies for charging storage
batteries, and for transforming from direct to alternating current, for
ringing telephone bells. Electro-cautery, electroplating, and electric
heating also give use to dynamotors.

CHAPTER XIX.

HOW ELECTRIC CURRENTS ARE DISTRIBUTED FOR USE.

[Illustration: Fig. 154.]

[Illustration: Fig. 155.]

[Illustration: Fig. 156.]

=138. Conductors and Insulators.= To carry the powerful current from
the generating station to distant places where it is to give heat,
power, or light, or even to carry the small current of a single cell
from one room to another, _conductors_ must be used. To keep the
current from passing into the earth before it reaches its destination
_insulators_ must be used. The form of conductors and insulators used
will depend upon the current and many other conditions. It should be
remembered that the current has to be carried to the lamp or motor,
through which it passes, and then back again to the dynamo, to form a
complete circuit. A break anywhere in the circuit stops the current.
Insulators are as important as conductors.

[Illustration: Fig. 157.]

[Illustration: Fig. 158.]

=139. Mains, Service Wires, etc.= From the switchboard the current
flows out through the streets in large conductors, or _mains_, the
supply being kept up by the dynamos, just as water-pressure is kept up
by the constant working of pumps. Branches, called _service wires_, are
led off from the mains to supply houses or factories, one wire leading
the current into the house from one main, and a similar one leading it
out of the house again to the other main.

[Illustration: Fig. 159.]

[Illustration: Fig. 160.]

In large buildings, pairs of wires, called _risers_, branch out from
the service wires and carry the current up through the building. These
have still other branches--_floor mains_, _etc._, that pass through
halls, etc., smaller branches finally reaching the lamps. The sizes of
all of these wires depend upon how much current has to pass through
them. The mains in large cities are usually placed underground. In some
places they are carried on poles.

[Illustration: Fig. 161.]

=140. Electric Conduits= are underground passages for electric wires,
cables, etc. There are several ways of insulating the conductors.
Sometimes they are placed in earthenware or iron tubes, or in wood that
has been treated to make it water-proof. At short distances are placed
man-holes, where the different lengths are joined, and where branches
are attached.

[Illustration: Fig. 162.]

Fig. 154 shows creosoted wooden pipes; Fig. 155 shows another form of
wooden pipe. Fig. 156 shows a coupling-box used to join Edison tubes.
The three wires, used in the three-wire system, are insulated from each
other, the whole being surrounded by an iron pipe of convenient length
for handling. Fig. 157 shows sections of man-holes and various devices
used in conduit work.

[Illustration: Fig. 163.]

=141. Miscellaneous Appliances.= When the current enters a house for
incandescent lighting purposes, for example, quite a number of things
are necessary. To measure the current a meter is usually placed in the
cellar. In new houses the insulated conductors are usually run through
some sort of tube which acts as a double protection, all being hidden
from view. Fig. 158 shows a short length of iron tube with a lining of
insulating material. Wires are often run through tubes made of rubber
and various other insulating materials.

Where the current is to be put into houses after the plastering has
been done, the wires are usually run through _mouldings_ or supported
by _cleats_. Fig. 159 shows a cross-section of moulding. The insulated
wires are placed in the slots, which are then covered.

[Illustration: Fig. 164.]

[Illustration: Fig. 165.]

[Illustration: Fig. 166.]

[Illustration: Fig. 167.]

Fig. 160 shows a form of porcelain cleat. These are fastened to
ceilings or walls, and firmly hold the insulated wires in place. Fig.
161 shows a wood cleat. Fig. 162 shows small porcelain _insulators_.
These may be screwed to walls, etc., the wire being then fastened to
them. Fig. 163 shows how telegraph wires are supported and insulated.
Fig. 164 shows how wires may be carried by tree and insulated from them.

[Illustration: Fig. 168.]

[Illustration: Fig. 169.]

[Illustration: Fig. 170.]

=142. Safety Devices.= We have seen that when too large a current
passes through a wire, the wire becomes heated and may even be melted.
Buildings are wired to use certain currents, and if from any cause much
more current than the regular amount should suddenly pass through the
service wires into the house, the various smaller wires would become
overheated, and perhaps melt or start a fire. An accidental short
circuit, for example, would so reduce resistance that too much current
would suddenly rush through the wires. There are several devices by
which the over-heating of wires is obviated.

[Illustration: Figs. 171 to 175.]

Fig. 165 shows a _safety fuse_, or _safety cut-out_, which consists of
a short length of easily fusible wire, called _fuse wire_, placed in
the circuit and supported by a porcelain block. These wires are tested,
different sizes being used for different currents. As soon as there
is any tendency toward over-heating, the fuse _blows_; that is, it
promptly melts and opens the circuit before any damage can be done to
the regular conductors. Fig. 166 shows a cross-section of a _fuse plug_
that can be screwed into an ordinary socket. The fuse wire is shown
black.

Fig. 167 shows a _fuse link_. These are also of fusible material, and
so made that they can be firmly held under screw-heads. For heavy
currents _fuse ribbons_ are used, or several wires or links may be
used side by side. Fig. 168 shows a _fusible rosette_. Fig. 169 shows
two fuse wires fixed between screw-heads, the current passing through
them in opposite directions, both sides of the circuit being included.
Fig. 170 shows various forms of cut-outs.

[Illustration: Fig. 176.]

=143. Wires and Cables= are made in many sizes. Figs. 171 to 175 show
various ways of making small conductors. They are made very flexible,
for some purposes, by twisting many small copper wires together, the
whole being then covered with insulating material.

[Illustration: Fig. 177.]

Figs. 176, 177, show sections of submarine cables. Such cables consist
of copper conductors insulated with pure gutta-percha. These are then
surrounded by hempen yarn or other elastic material, and around the
whole are placed galvanized iron armor wires for protection. Each core,
or conductor, contains a conductor consisting of a single copper wire
or a strand of three or more twisted copper wires.

=144. Lamp Circuits.= As has been noted before, in order to have the
electric current do its work, we must have a complete circuit. The
current must be brought back to the dynamo, much of it, of course,
having been used to produce light, heat, power, etc. For lighting
purposes this is accomplished in two principal ways.

[Illustration: Fig. 178.]

Fig. 178 shows a number of lamps so arranged, "in series," that the
same current passes through them all, one after the other. The total
resistance of the circuit is large, as all of the lamp resistances are
added together.

[Illustration: Fig. 179.]

Fig. 179 shows lamps arranged side by side, or "in parallel," between
the two main wires. The current divides, a part going through each lamp
that operates. The total resistance of the circuit is not as large
as in the series arrangement, as the current has many small paths in
going from one main wire to the other. Fig. 179 also shows the ordinary
_two-wire system_ for incandescent lighting, the two main wires having
usually a difference of potential equal to 50 or 110 volts. These
comparatively small pressures require fairly large conductors.

_The Three-Wire System_, Fig. 180, uses the current from two dynamos,
arranged with three main wires. While the total voltage is 220, one of
the wires being neutral, 110 volts can be had for ordinary lamps. This
voltage saves in the cost of conductors.

[Illustration: Fig. 180.]

[Illustration: Fig. 181.]

_The Alternating System_, Fig. 181, uses transformers. The high
potential of the current allows small main wires, from which branches
can be run to the primary coil of the transformer. The secondary coil
sends out an induced current of 50 or 110 volts, while that in the
primary may be 1,000 to 10,000 volts.

CHAPTER XX.

HOW HEAT IS PRODUCED BY THE ELECTRIC CURRENT.

=145. Resistance and Heat.= We have seen that all wires and conductors
offer resistance to the electric current. The smaller the wire the
greater its resistance. Whenever resistance is offered to the current,
heat is produced. By proper appliances, the heat of resistance can be
used to advantage for many commercial enterprises. Dynamos are used to
generate the current for heating and lighting purposes.

[Illustration: Fig. 182.]

Fig. 182 shows how the current from two strong cells can be used to
heat a short length of very fine platinum or German-silver wire.
The copper conductors attached to the cells do not offer very much
resistance.

It will be seen from the above that in all electrical work the sizes
of the wires used have to be such that they do not overheat. The coils
of dynamos, motors, transformers, ampere-meters, etc., etc., become
somewhat heated by the currents passing through them, great care being
taken that they are properly designed and ventilated so that they will
not burn out.

[Illustration: Fig. 183.]

[Illustration: Fig. 184.]

=146. Electric Welding.= Fig. 183 shows one form of electric welding
machine. The principle involved in the art of electric welding is
that of causing currents of electricity to pass through the abutting
ends of the pieces of metal which are to be welded, thereby generating
heat at the point of contact, which also becomes the point of greatest
resistance, while at the same time mechanical pressure is applied
to force the parts together. As the current heats the metal at the
junction to the welding temperature, the pressure follows up the
softening surface until a complete union or weld is effected; and, as
the heat is first developed in the interior of the parts to be welded,
the interior of the joint is as efficiently united as the visible
exterior. With such a method and apparatus, it is found possible to
accomplish not only the common kinds of welding of iron and steel, but
also of metals which have heretofore resisted attempts at welding, and
have had to be brazed or soldered.

[Illustration: Figs. 185 to 189.]

The introduction of the electric transformer enables enormous currents
to be so applied to the weld as to spend their energy just at the point
where heating is required. They need, therefore, only to be applied
for a few seconds, and the operation is completed before the heat
generated at the weld has had time to escape by conduction to any other
part.

Although the quantity of the current so employed in the pieces to be
welded is enormous, the potential at which it is applied is extremely
low, not much exceeding that of the batteries of cells used for ringing
electric bells in houses.

[Illustration: Fig. 190.]

=147. Miscellaneous Applications.= Magneto Blasting Machines are now
in very common use for blasting rocks, etc. Fig. 184 shows one, it
being really a small hand dynamo, occupying less than one-half a cubic
foot of space. The armature is made to revolve rapidly between the
poles of the field-magnet by means of a handle that works up and down.
The current is carried by wires from the binding-posts to fuses. The
heat generated by resistance in the fuse ignites the powder or other
explosive.

_Electric soldering irons_, _flat-irons_, _teakettles_, _griddles_,
_broilers_, _glue pots_, _chafing-dishes_, _stoves_, etc., etc., are
now made. Figs. 185 to 189 show some of these applications. The coils
for producing the resistance are inclosed in the apparatus.

[Illustration: Fig. 191.]

Fig. 190 shows a complete electric kitchen. Any kettle or part of the
outfit can be made hot by simply turning a switch. Fig. 191 shows an
electric heater placed under a car seat. Many large industries that
make use of the heating effects of the current are now being carried
on.

CHAPTER XXI.

HOW LIGHT IS PRODUCED BY THE INCANDESCENT LAMP.

[Illustration: Fig. 192.]

[Illustration: Fig. 193.]

=148. Incandescence.= We have just seen that the electric current
produces heat when it flows through a conductor that offers
considerable resistance to it. As soon as this was discovered men
began to experiment to find whether a practical light could also be
produced. It was found that a wire could be kept hot by constantly
passing a current through it, and that the light given out from it
became whiter and whiter as the wire became hotter. The wire was said
to be _incandescent_, or glowing with heat. As metal wires are good
conductors of electricity, they had to be made extremely fine to offer
enough resistance; too fine, in fact, to be properly handled.

=149. The Incandescent Lamp.= Many substances were experimented upon
to find a proper material out of which could be made a _filament_
that would give the proper resistance and at the same time be strong
and lasting. It was found that hair-like pieces of carbon offered the
proper resistance to the current. When heated in the air, however,
carbon burns; so it became necessary to place the carbon filaments in a
globe from which all the air had been pumped before passing the current
through them. This proved to be a success.

[Illustration: Fig. 194.]

[Illustration: Fig. 195.]

[Illustration: Fig. 196.]

Fig. 192 shows the ordinary form of lamp. The _carbon filament_ is
attached, by carbon paste, to short platinum wires that are sealed in
the glass, their lower ends being connected to short copper wires that
are joined to the terminals of the lamp. When the lamp is screwed
into its socket, the current can pass up one side of the filament
and down the other. The filaments used have been made of every form
of carbonized vegetable matter. Bamboo has been largely used, fine
strips being cut by dies and then heated in air-tight boxes containing
fine carbon until they were thoroughly carbonized. This baking of the
bamboo produces a tough fiber of carbon. Various forms of thread have
been carbonized and used. Filaments are now made by pressing finely
pulverized carbon, with a binding material, through small dies. The
filaments are made of such sizes and lengths that will adapt them to
the particular current with which they are to be used. The longer the
filament, the greater its resistance, and the greater the voltage
necessary to push the current through it.

[Illustration: Fig. 197.]

[Illustration: Fig. 198.]

After the filaments are properly attached, the air is pumped from the
bulb or globe. This is done with some form of mercury pump, and the air
is so thoroughly removed from the bulb that about one-millionth only of
the original air remains. Before sealing off the lamp, a current is
passed through the filament to drive out absorbed air and gases, and
these are carried away by the pump. By proper treatment the filaments
have a uniform resistance throughout, and glow uniformly when the
current passes.

[Illustration: Fig. 199.]

[Illustration: Fig. 200.]

=150. Candle-Power.= A lamp is said to have 4, 8, 16 or more
candle-power. A 16-candle-power lamp, for example, means one that will
give as much light as sixteen standard candles. A standard sperm candle
burns two grains a minute. The candle-power of a lamp can be increased
by forcing a strong current through it, but this shortens its life.

_The Current_ used for incandescent lamps has to be strong enough to
force its way through the filament and produce a heat sufficient to
give a good light. The usual current has 50 or 110 volts, although