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THINGS A BOY SHOULD KNOW ABOUT ELECTRICITY
[Illustration]
* * * * *
_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
[Illustration]
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.)