Produced by Giovanni Fini and the Online Distributed
Proofreading Team at http://www.pgdp.net (This file was
produced from images generously made available by The
Internet Archive)






[Illustration: FIRST DIRECT-CONNECTED ELECTRIC GENERATOR UNIT OF
LARGE CAPACITY EVER CONSTRUCTED UP TO THE TIME IT WAS MADE BY THOMAS
A. EDISON IN JUNE, 1881. CAPACITY, 1200 INCANDESCENT LAMPS OF 16
CANDLE-POWER EACH]




                         A-B-C OF ELECTRICITY


                       BY WILLIAM H. MEADOWCROFT


            HARPER & BROTHERS PUBLISHERS NEW YORK & LONDON




                         A-B-C of Electricity

           COPYRIGHT, 1888, 1909, BY WILLIAM H. MEADOWCROFT


                 COPYRIGHT, 1915, BY HARPER & BROTHERS
                PRINTED IN THE UNITED STATES OF AMERICA
                          PUBLISHED MAY, 1915




FROM THE LABORATORY OF THOMAS A. EDISON
                          ORANGE, N. J.

  MR. W. H. MEADOWCROFT,
     _New York City_.

  _DEAR SIR_:

_I have read the MS. of your "A-B-C of Electricity," and find that the
statements you have made therein are correct. Your treatment of the
subject, and arrangement of the matter, have impressed me favorably.

                                 _Yours truly_,
                                       _THOS. A. EDISON_




CONTENTS


  CHAP.                                PAGE

        INTRODUCTION TO NEW EDITION    viii


        PREFACE                           x

     I.                                   1

    II. DEFINITIONS                       3

   III. MAGNETISM                        16

   IV. THE TELEGRAPH                     23

     V. WIRELESS TELEGRAPHY              33

    VI. THE TELEPHONE                    40

   VII. ELECTRIC LIGHT                   54

  VIII. ELECTRIC POWER                   87

    IX. BATTERIES                        95

     X. CONCLUSION                      127




INTRODUCTION TO NEW EDITION


The favor with which this book has been received has brought about the
preparation of this new edition. The present volume has been enlarged
by the addition of certain new material and it has been entirely
reset. Some new illustrations have been made, and in its new dress
the book, it is hoped, will be found to afford an even larger measure
of usefulness. The principles of the science remain the same, but the
author is glad of the opportunity to note certain developments in their
application.

                                                              W. H. M.
  EDISON LABORATORY, _April, 1915_.




PREFACE


While there is no lack of most excellent text-books for the study of
those branches of Electricity which are above the elementary stage,
there is a decided need of text-books which shall explain, in simple
language, to young people of, say, fourteen years and upward, a general
outline of the science, as well as the ground-work of those electrical
inventions which are to-day of such vast commercial importance.

There is also a need for such a book among a large part of the adult
population, for the reason that there have been great and radical
changes in this science since the time they completed their studies,
and they have not the time to follow up the subject in the advanced
books.

As instances of those changes just spoken of, the electric light,
telephone, and storage batteries may be mentioned, which have been
developed during the last ten or twelve years, with the result of
adding very many features that were entirely new to electricians.

With these ideas in view I have prepared this little volume. It is not
intended, in the slightest degree, to be put forward as a scientific
work, but it will probably give to many the information they desire
without requiring too great a research into books which treat more
extensively and deeply of this subject.

                                                              W. H. M.




                         A-B-C OF ELECTRICITY




                         A-B-C OF ELECTRICITY




I


We now obtain so many of our comforts and conveniences by the use of
electricity that all young people ought to learn something of this
wonderful force, in order to understand some of the principles which
are brought into practice.

You all know that we have the telegraph, the telephone, the electric
light, electric motors on street-cars, electric bells, etc., besides
many other conveniences which the use of electricity gives us.

Every one knows that, by the laws of multiplication, twice two makes
four, and that twice two can never make anything but four. Well, these
useful inventions have been made by applying the _laws of electricity_
in certain ways, just as well known, so as to enable us to send in a
few moments a message to our absent friends at any distance, to speak
with them at a great distance, to light our houses and streets with
electric light, and to do many other useful things with quickness and
ease.

But you must remember that we do not know what electricity itself
really is. We only know how to produce it by certain methods, and we
also know what we can do with it when we have obtained it.

In this little book we will try to explain the various ways by which
electricity is obtained, and how it is applied to produce the useful
results that we see around us.

We will try and make this explanation such that it will encourage many
of you to study this very important and interesting subject more deeply.

In the advanced books on electricity there are many technical terms
which are somewhat difficult to understand, but in this book it will
only be necessary to use a few of the more simple ones, which it will
be well for you to learn and understand before going further.




II

DEFINITIONS


The three measurements most frequently used in electricity are

  The Volt,
  The Ampère,
  The Ohm.

We will explain these in their order.

[Illustration: Fig. 1]

_The Volt._--This term may be better understood by making a comparison
with something you all know of. Suppose we have a tank containing
one hundred gallons of water, and we want to discharge it through a
half-inch pipe at the bottom of the tank. Suppose, further, that we
wanted to make the water spout upward, and for this purpose the pipe
was bent upward as in Fig. 1.

If you opened the tap the water would spout out and upward as in Fig. 1.

[Illustration: Fig. 2]

The cause of its spouting upward would be the _weight_ or _pressure_ of
the water in the tank. This pressure is reckoned as so many _pounds_ to
the square inch of water.

Now, if the tank were placed on the roof of the house and the pipe
brought to the ground as shown in Fig. 2, the water would spout up very
much higher, because there would be _many more pounds_ of pressure on
account of the height of the pipe.

So, you see, the force or pressure of water is measured in pounds, and,
therefore, a pound is the unit of pressure, or force, of water. Now, in
electricity the unit of pressure, or force, is called a volt.

This word "volt" does not mean any weight, as the word "pound"
weight does. You all know that if you have a pound of water you must
have something to hold it, because it has weight, and, consequently,
occupies some space. But _electricity itself has no weight_ and
therefore cannot occupy any space.

When we desire to carry water into a house or other building we do so
by means of hollow pipes, which are usually made of iron. This is the
way that water is brought into houses in cities and towns, so that it
may be drawn and used in any part of a dwelling. Now, the principal
supply usually comes from a reservoir which is placed up on high ground
so as to give the necessary pounds of pressure to force the water up to
the upper part of the houses. If some arrangement of this kind were not
made we could get no water in our bedrooms, because, as you know, water
will not rise above its own level unless by force.

The water cannot escape as long as there are no holes or leaks in the
iron pipes, but if there should be the slightest crevice in them the
water will run out.

In electricity we find similar effects.

The electricity is carried into houses by means of wires which are
covered, or _insulated_, with various substances, such, for instance,
as rubber. Just as the iron of the pipes prevents the water from
escaping, the insulation of the wire prevents the escape of the
electricity.

Now, if we were to cause the pounds of pressure of water, in pipes of
ordinary thickness, to be very greatly increased, the pipes could not
stand the strain and would burst and the water escape. So it is with
electricity. If there were too many volts of pressure the insulation
would not be sufficient to hold it and the electricity would escape
through the covering, or insulation, of the wire.

It is a simple and easy matter to stop the flow of water from an
ordinary faucet by placing your finger over the opening. As the water
cannot then flow, your finger is what we will call a non-conductor and
the water will be retained in the pipe.

We have just the same effects in electricity. If we place some
substance which is practically a non-conductor, or insulator, such as
rubber, around an electric wire, or in the path of an electric current,
the electricity, acted upon by the volts of pressure, cannot escape,
because the insulation keeps it from doing so, just as the iron of the
pipe keeps the water from escaping. Thus, you see, the volt does not
itself represent electricity, but only the pressure which forces it
through the wire.

There are other words and expressions in electricity which are
sometimes used in connection with the word "volt." These words are
"pressure" and "intensity." We might say, for instance, that a certain
dynamo machine had an electromotive force of 110 volts; or that the
intensity of a cell of a battery was 2 volts, etc.

We might mention, as another analogy, the pressure of steam in a
boiler, which is measured or calculated in pounds, just as the pressure
of water is measured. So, we might say that 100 pounds steam pressure
used through the medium of a steam-engine to drive a dynamo could thus
be changed to electricity at 100 volts pressure.

_The Ampère._--Now, in comparing the pounds pressure of water with the
volts of pressure of electricity we used as an illustration a tank of
water containing 100 gallons, and we saw that this water had a downward
force or pressure in pounds. Let us now see what this pressure was
acting upon.

It was forcing the quantity of water to spout upward through the end of
the pipe. Now, as the quantity of water was 100 gallons, it could not
all be forced at once out of the end of the pipe. The pounds pressure
of water acting on the 100 gallons would force it out at a _certain
rate_, which, let us say, would be one gallon per minute.

This would be the _rate of the flow_ of water out of the tank.

Thus, you see, we find a second measurement to be considered in
discharging the water-tank. The first was the force, or pounds of
pressure, and the second the _rate_ at which the quantity of water was
being discharged per minute by that pressure.

This second measurement teaches us that a _certain quantity_ will pass
out of the pipe in a _certain time_ if the pressure is steady, such
quantity depending, of course, on the size or friction resistance of
the pipe.

In electricity the volts of pressure act so as to force the quantity
of current to _flow through the wires at a certain rate_ per second,
and the rate at which it flows is measured in ampères. For instance,
let us suppose that an electric lamp required a pressure of 100 volts
and a current of one ampère to light it up, we should have to supply a
current of electricity flowing at the rate of one ampère, acted upon by
an electromotive force of 100 volts.

You will see, therefore, that while the volt does not represent any
electricity, but only its pressure, the ampère represents the _rate of
flow_ of the current itself.

You should remember that there are several words sometimes used in
connection with the word "ampère"--for instance, we might say that a
lamp required a "current" of one ampère or that a dynamo would give a
"quantity" of 20 ampères.

_The Ohm._--You have learned that the _pressure_ would discharge the
_quantity_ of water at a certain rate through the pipe. Now, suppose we
were to fix _two_ discharge-pipes to the tank, the water would run away
very much quicker, would it not? If we try to find a reason for this,
we shall see that a pipe can only, at a given pressure, admit so much
water through it at a time.

Therefore, you see, this pipe would present a certain amount of
_resistance_ to the passage of the total quantity of water, and would
only allow a limited quantity at once to go through. But, if we were to
attach two or more pipes to the tank, or one large pipe, we should make
it easier for the water to flow, and, therefore, the total amount of
resistance to the passage of the water would be very much less, and the
tank would quickly be emptied.

Now, as you already know, water has substance and weight and therefore
occupies some space, but electricity has neither substance nor
weight, and therefore cannot occupy any space; consequently, to carry
electricity from one place to another we do not need to use a pipe,
which is hollow, but we use a solid wire.

These solid wires have a certain amount of _resistance_ to the passage
of the electricity, just as the water-pipe has to the water, and (as
it is in the case of the water) the effect of the resistance to the
passage of electricity is greater if you pass a larger quantity through
than a smaller quantity.

If you wanted to carry a quantity of electricity to a certain distance,
and for that purpose used a wire, there would be a certain amount of
resistance in that wire to the passage of the current through it; but
if you used two or more wires of the same size, or one large wire, the
resistance would be very much less and the current would flow more
easily.

Suppose that, instead of emptying the water-tank from the roof through
the pipe, we had just turned the tank over and let the water all pour
out at once down to the ground. That would dispose of the water very
quickly and by a short way, would it not? That is very easy to be
seen, because there would be _no resistance_ to its passage to the
ground.

Well, suppose we had an electric battery giving a certain quantity of
current, say five ampères, and we should take a large wire that would
offer no resistance to that quantity and put it from one side of the
battery to the other, a large current would flow at once and tend to
exhaust the battery. This is called a _short circuit_ because there is
little or no resistance, and it provides the current with an easy path
to escape. Remember this, that _electricity always takes the easiest
path_. It will take as many paths as are offered, but the largest
quantity will always take the easiest.

As the subject of resistance is one of the most important in
electricity, we will give you one more example, because if you can
obtain a good understanding of this principle it will help you to
comprehend the whole subject more easily in your future studies.

We started by comparison with a tank holding 100 gallons of water,
discharging through a half-inch pipe, and showed you that the pounds
of pressure would force the quantity of gallons through the pipe. When
the tap was first opened the water would spout up very high, but as
the water in the tank became lower the pressure would be less, and,
consequently, the water would not spout so high.

So, if it were desired to keep the water spouting up to the height
it started with, we should have to keep the tank full, so as to have
the same pounds of pressure all the time. But, if we wanted the water
to spout still higher we should have to use other means, such as a
force-pump, to obtain a greater pressure.

Now, if we should use too many pounds pressure it would force the
quantity of water more rapidly through the pipe and would cause the
water to become heated because of the resistance of the pipe to the
passage of that quantity acted upon by so great a pressure.

This is just the same in electricity, except that the wire itself would
become heated, some of the electricity being turned into heat and lost.
If a wire were too small for the volts pressure and ampères of current
of electricity the resistance of such wire would be overcome, and it
would become red-hot and perhaps melt. Electricians are therefore
very careful to calculate the resistance of the wires they use before
putting them up, especially when they are for electric lighting, in
order to make allowances for the ampères of current to flow through
them, so that but little of the electricity will be turned into heat
and thus rendered useless for their purpose.

The unit of resistance is called the _ohm_ (pronounced like "home"
without the "h").

All wires have a certain resistance per foot, according to the nature
of the metal used and the size of the wire--that is to say, the finer
the wire the greater number of ohms resistance it has to the foot.

Water and electricity flow under very similar conditions--that is
to say, each of them must have a channel, or conductor, and each of
them requires pressure to force it onward. Water, however, being a
tangible substance, requires a hollow conductor; while electricity,
being intangible, will flow through a solid conductor. The iron of
the water-pipe and the insulation of the electric wire serve the same
purpose--namely, that of serving to prevent escape by reason of the
pressure exerted.

There is another term which should be mentioned in connection with
resistance, as they are closely related, and that is _opposition_.
There is no general electrical term of this name, but, as it will be
most easily understood from the meaning of the word itself, we have
used it.

Let us give an example of what opposition would mean if applied to
water. Probably every one knows that a water-wheel is a wheel having
large blades, or "paddles," around its circumference.

When the water, in trying to force its passage, rushes against one of
these paddles it meets with its opposition, but overcomes it by pushing
the paddle away. This brings around more opposition in the shape of
another paddle, which the water also pushes away. And so this goes on,
the water overcoming this opposition and turning the wheel around, by
which means we can get water to do useful work for us.

You must remember, however, that it is only by putting opposition in
the path of a pressure and quantity of water that we can get this work.

The same principle holds good in electricity. We make electricity in
different ways, and in order to obtain useful work we put in its path
the instruments, lamps, or machines which offer the proper amount of
resistance, or opposition, to its passage, and thus obtain from this
wonderful agent the work we desire to have done.

You have learned that three important measurements in electricity are
as follows:

The _volt_ is the practical unit of measurement of _pressure_;

The _ampère_ is the practical unit of measurement of the _rate of
flow_; and

The _ohm_ is the practical unit of measurement of _resistance_.




III

MAGNETISM


Now we will try to explain to you something about magnets and
magnetism. There are very few boys who have not seen and played with
the ordinary magnets, shaped like a horseshoe, which are sold in all
toy-stores as well as by those who sell electrical goods.

Well, you know that these magnets will attract and hold fast anything
that is made of iron or steel, but they have no effect on brass,
copper, zinc, gold, or silver, yet there is nothing that you can
see which should cause any such effect. You will notice, then, that
magnetism is like electricity; we cannot see it, but we can tell that
it exists, because it produces certain effects. And here is another
curious thing--magnetism produces electricity, and electricity produces
magnetism. This seems to be a very convenient sort of a family affair,
and it is owing to this close relation that we are able to obtain so
many wonderful things by the use of electricity.

We shall now show you how electricity produces magnetism, and, when we
come to the subject of electric lighting we will explain how magnetism
produces electricity.

[Illustration: Fig. 3]

The easiest way to show how electricity makes magnetism is to find out
how magnets are made. Suppose we wanted to make a horseshoe magnet,
just mentioned above; we would take a piece of _steel_ and wind around
it some fine copper wire, commencing on one leg of the horseshoe and
winding around until we came to the end of the other leg. Then we
should have two ends of wire left, as shown in the sketch. (Fig. 3.)

We connect these two ends with an electric battery, giving, say, two
volts, and then the ampères of current of electricity will travel
through the wire, and in doing so has such an influence on the steel
that it is converted into a magnet, such as you have played with. The
current is "broken"--that is to say, it is shut off several times in
making a magnet of this kind, and then the wire is taken away from the
battery and is unwound from the steel horseshoe, leaving it free from
wire, just as you have seen it. This horseshoe is now a _permanent
magnet_--that is, it will _always_ attract and hold pieces of iron and
steel.

Now, if you were to do the same thing with a horseshoe made of soft
iron instead of steel it would not be a magnet after you stopped the
current of electricity from going through the wires, although the piece
of _iron_ would be a stronger magnet while the electricity was going
through the wire around it.

The steel magnet is called a permanent magnet, and its ends, or
"poles," are named North and South. There is usually a loose piece of
steel or iron, called an "armature," put across the ends, which has the
peculiar property of keeping the magnetism from becoming weaker, and
thereby retaining the strength of the magnet. The strongest part of the
magnet is at the poles, while, at the point marked + (which is called
the neutral point) there is scarcely any magnetism.

It will be well to remember the object of the _armature_ as we shall
meet it again in describing dynamo machines.

The magnets made of iron are called electromagnets because they exhibit
magnetism only when the ampères of current of electricity are flowing
around them. They also have two poles, north and south, as have
permanent magnets. Electromagnets are used in nearly all electrical
instruments, not only because they are stronger than permanent magnets,
but because they can be made to act instantly by passing a current of
electricity through them at the most convenient moment, as you will see
when we explain some of the electrical instruments which are used to
produce certain effects. (Fig. 4.)

[Illustration: Fig. 4]

Of course there are a great many different shapes in which magnets
are made. The simplest is the _bar magnet_, which is simply a flat or
round piece of iron or steel. Suppose you made a magnet of a flat piece
of steel and put on top of it a sheet of paper, and then threw on the
paper some iron filings, you would see them arrange themselves as is
shown in the following sketch. (Fig. 5.)

The filings would always arrange themselves in this shape, no matter
how large or small the magnets were. And, if you were to cut it into
two or half a dozen pieces, each piece would have the same effect. This
shows you that each piece would itself become a magnet and would have
its poles exactly as the large one had.

[Illustration: Fig. 5]

Now, we have another curious thing to tell you about magnets. If you
present the north pole of a magnet to the south pole of another magnet,
they will attract and hold fast to each other, but if you present a
south pole to another south pole, or a north pole to a north pole, they
will repel each other, and there will be no attraction. You can perform
some interesting experiments by reason of this fact. We will give you
one of them.

Take, say, a dozen needles and draw them several times in the same
direction across the ends of a magnet so that they become magnetized.
Now stick each needle half-way through a piece of cork, and put the
corks, with the needles sticking through them, into a bowl of water.
Then take a bar magnet and bring it gradually toward the middle of the
bowl and you will see the corks advance or back away from the magnet.
If the ends of the needles sticking up out of the water are south poles
and the end of the magnet you present is a north pole, the needles will
come to the center; but will go to the side of the bowl if you present
the south pole. You can vary this pretty experiment by turning up the
other ends of part of the needles.

You will remember that when we explained what "resistance" meant, we
told you that electricity would always take the easiest path, and while
part of it will flow in a small wire, the largest portion will take
an easier path if it can get to something larger that is a metallic
substance. Electricity will only flow easily through anything that
is made of metal. You will also remember that you learned that when
electricity took a short cut to get away from its proper path it was
called a _short circuit_.

All this must be taken into consideration when magnets are being made.
In the first place, the wire we wind around steel or iron to make
magnets must always be covered with an insulator of electricity. Magnet
wire is usually covered with cotton or silk. If it were left bare,
each turn of the wire would touch the next turn, and so we should make
such an easy path for the electricity that it would all go back to the
battery by a short circuit, and then we would get no magnetic effect in
the steel or iron. _The only way we can get electricity to do useful
work for us is to put some resistance or opposition in its way._ So
you see that if we make it travel through the wire around the iron or
steel, there is just enough resistance or opposition in its way to give
it work to get through the wire, and this work produces the peculiar
effect of making the iron or steel magnetic.

The covering on the wire, as you will remember, is called "insulation."




IV

THE TELEGRAPH


Every one knows how very convenient the telegraph is, but there are
not many who think how wonderful it is that we can send a message in a
few seconds of time to a distant place, even though it were thousands
of miles away. And yet, though the present system of telegraphing is a
wonderful one, the method of sending a telegram is simple enough. The
apparatus that is used in sending a telegram is as follows:

              The Battery.
              The Wire.
              The Telegraph Key.
              The Sounder.

The different kinds of electric batteries will be mentioned afterward,
so we will not stop now to describe them, but simply state that a
battery is used to produce the necessary electricity. As you all know
what wire is, there is no necessity of describing it further.

The telegraph key is shown in the sketch below. (Fig. 6.)

[Illustration: Fig. 6]

This instrument is usually made of brass, except that upon the handle
there is the little knob which is of hard rubber. The handle, or lever,
moves down when this knob is pressed, and a little spring beneath
pushes it up again when let go. You will see a second smaller knob, the
use of which we will explain later.

The sounder is shown on the following page. (Fig. 7.)

The part consisting of the two black pillars is an electromagnet,
and across the top of these pillars is a piece of iron called the
"armature," which is held up by a spring.

[Illustration: Fig. 7]

Now let us see how the battery and wire are placed in connection with
these instruments. You have seen that we usually have two wires for
the electricity to travel in, one wire for it to leave the battery,
and the other to return on. But you will easily see that if two wires
had to be used in telegraphing it would be a very expensive matter,
especially when they had to be carried thousands of miles. So, instead
of using a second wire, we use the earth to carry back the electricity
to the battery, because the earth is a better conductor even than wire.
Although a quantity of ground equal in size to the wire would offer
thousands of times greater resistance than the wire, yet, owing to the
great body of our earth, its total resistance is even less than any
telegraph wire used.

When two electric wires are run from a battery and connected together
through some instrument, this is called a "circuit," because the
electricity has a path in which it can travel back to the battery. This
would be a "metallic" circuit; _but when one wire only_ is used, and
the other side of the battery is connected with the earth, it is called
a "ground" or "earth" circuit, because the electricity returns through
the earth.

[Illustration: Fig. 8]

If you look at this sketch (Fig. 8) you will see how the telegraph
instruments are connected and will then be able to understand how a
message can be sent.

Here we have two sets of telegraph apparatus, one of which, let us
say, is in New York and the other in Philadelphia.

You will see that one wire from the battery is connected with the
earth, and the other wire with the sounder. Another wire goes from the
sounder to one leg of the key so as to make the brass base of the key
part of the circuit. The other leg of the key is "insulated" from the
brass base by being separated therefrom with some substance which will
not carry electricity, such, for instance, as hard rubber.

We will suppose that there is already a wire strung up on poles between
New York and Philadelphia, and that the key, sounder, and battery in
the latter city are connected in the same way as those in New York.

Now, to enable us to send a message from one city to the other we must
connect the ends of the wires to the instruments in each city; so we
connect one end to the insulated leg of the key in New York, and the
other end to the insulated leg of the key in Philadelphia.

Everything is now completed, and, as soon as we find out what is the
use of that part of the key that has a little round, black handle, we
shall be ready to start. This is called the "switch."

If you will look once more at the picture of the key you will see
under the long handle (or lever) a little point which the lever will
touch when it is pressed down. Now this little point is part of that
insulated leg, and, therefore, this point is also insulated from the
base. If a current of electricity were sent along the wire it could not
get any farther than this point unless we put in some arrangement to
complete the path, or circuit, for it to travel in. We therefore put in
the switch.

One end of the switch (which is made of brass with a rubber handle)
is fastened on the base of the key, so that it may be moved to the
right or left. The other end, when the switch is moved to the left
(or "closed"), touches a piece of brass fastened to the little point
we have mentioned, and so makes a free path for the electricity to go
through the base of the key and through the wire to the sounder, and
from there to the battery, and so back to the earth. This switch must
be opened before the sounder near it will respond to its neighboring
key.

Now we are ready to send a message. Suppose we want to send a telegram
from New York to Philadelphia. The operator in New York opens his
switch and presses down his key several times. The switch on the
Philadelphia key being closed, the electricity goes through to the
sounder, and, this being made an electromagnet by the current passing
through the wire, the iron armature is attracted by the magnetism and
drawn down to the magnet with a snap. It will stay there as long as the
New York operator keeps his lever pressed down, but, when he allows
it to spring up, there is no current passing through the Philadelphia
sounder and there is no magnetism, consequently the armature springs up
again with a click.

As often as the operator presses down his key lever and lets it spring
up again, the same action takes place in the sounder, and it makes that
click, click, which you have heard if you have ever seen telegraph
instruments in operation.

Let us continue, however, to send our message. The New York operator,
having pressed down his key several times to signal the Philadelphia
operator, closes his switch to receive the answer from Philadelphia.
The operator in the latter city then opens his switch and presses down
his key several times, which makes the New York sounder click, in the
same way, to let the operator there know that he is ready to receive
the message. He then closes his switch and receives the telegram which
the New York operator sends after opening _his_ key.

Telegraphic messages are sent and received in this way and are read by
the sound of the clicks.

These sounds may be represented on paper by dots, dashes, and spaces.
For instance, if you press down the key and let it spring back quickly,
that would represent a dot. If you press down the key and hold it a
little longer before letting it spring up again, it would represent a
dash. A space would be represented by waiting a little while before
pressing down the key again.

We show you below the alphabet in these dots, dashes, and spaces, and
these are the ones now used in sending all telegraphic messages.

[Illustration]

Thus, you see, if you were telegraphing the word "and" you would press
down your key and let it return quickly, then press down again and
return after a longer pause, which would give the letter A; then slowly
and quickly, which would be N; then slowly and twice quickly, which
would be D.

Any persevering boy can learn to operate a telegraph instrument by a
little study and regular practice; and, as complete learner's sets
can be purchased very cheaply, this affords a pleasant and useful
recreation for boys.

There are many cases where two boys living near each other have a set
of telegraph instruments in their homes and run a wire from one house
to the other, thus affording many hours of pleasant and profitable
amusement.

In giving the above explanation of telegraphing we have described only
the simple and elementary form. In large telegraph lines, such as
those of the Western Union, there are many more additional instruments
used, which are very complicated and difficult to understand; such,
for instance, as the quadruplex, by which four distinct messages can
be sent over the same wire at the same time. We have, therefore,
described only the simplest form in order to give the general idea
of the working of the telegraph by electromagnetism, which is the
principle of all telegraphing.

When you study electricity more deeply you will find this subject and
the many different instruments very interesting and wonderful.




V

WIRELESS TELEGRAPHY


If it has seemed extraordinary to you that only one wire should be
necessary for sending a message by the electric telegraph, and that our
earth can be used instead of a second wire, how much more wonderful it
is to realize that in these days we can exchange telegraphic messages
with different points without any connecting wires at all between them,
even though the places be many hundred miles apart. Thus, two ships on
the ocean, entirely out of sight of each other, may intercommunicate,
or may telegraph to or receive despatches from a far-distant shore;
indeed, telegraphy without wires has been accomplished across the
Atlantic Ocean. In the language of the day, this is called "wireless
telegraphy," although it is more correct to think of it as aerial, or
space, telegraphy. As you will naturally want to know how this is
effected, we will try to explain the main principles in a simple manner.

If you drop a stone into a quiet pond, you will see the water form into
ring-like waves, or ripples, which travel on and on until they die away
in the far distance. These waves are caused, as we have seen, by a
disturbance of the body of water.

Probably you have already learned in school that all known space is
said to be filled with a medium called "ether," and that this medium is
so exceedingly thin that it penetrates, or permeates, everything, so
that it exists in the densest bodies as well as in free space. For the
sake of obtaining a clear idea of this theory we may imagine that the
ether envelops and permeates every thing in the entire universe. Hence
we can easily realize that, although we cannot see or feel the ether,
any disturbance of it will set it in wavelike motion.

Modern science accounts for light, radiant heat, and electrical
phenomena by reason of wavelike disturbances, vibrations, or pulsations
of this ether. Thus, if you should strike a light, the ether would be
disturbed, causing waves to form, which, like the waves in the water,
would travel in every direction. When these waves reached the eyes of
another person within seeing distance, that person's eyes would be so
acted upon by the waves that he would see the light which you had made,
and would see it instantly, for light waves travel about 186,000 miles
per second.

So, if you create an electrical disturbance, the same kind of an effect
will be produced; that is to say, waves in the ether will be created,
or propagated, and will travel on and on in every direction. Now, if
some form of electrical appliance can be made that will be of the right
kind to respond to them (as the eye responds to light rays), these
electric waves can be made practically useful for transmitting messages
through space. This is just what has been done, and we will now give
you a brief general description of one kind of apparatus used.

For "sending," or "transmitting," as it is usually termed, there
is used an induction-coil, having rather large brass balls on the
secondary terminals; suitable batteries, a condenser, a Morse telegraph
key, and an "aerial," or wire which is carried away up into the air
vertically, and is made fast to a pole or special tower. When these are
connected properly, the closing of the circuit with the key will cause
sparks to jump between the brass balls. This electrical discharge,
or oscillation, is carried by the aerial into the upper air and
causes intense pulsations in the ether, which set up waves as already
mentioned. If the circuit is opened again the disturbance ceases. So,
by alternately closing and opening the circuit, the Morse characters
can be imitated.

But how can these signals be received by the man for whom they are
intended, who may be a hundred miles or more away? He has a "receiving"
set, consisting of a sensitive relay, batteries, resistance-coils, a
Morse register, an aerial, and a special device called a "coherer."
This is the important part of the whole set, because it is sensitive
to the electrical waves. It consists of a little glass tube about as
large around as an ordinary lead-pencil, and perhaps two inches long.
In the tube are two metallic plugs, each having a wire attached so that
one wire projects from each end of the tube. The plugs are separated
inside the tube by a very small space, and in this space are some
metal filings. One wire from the coherer is connected to the aerial
and the other to the ground. When there are no electrical ether waves
to influence them, these filings, being loosely separated, are at
rest and offer high resistance; but when the ether is disturbed by
electrical vibrations and the waves arrive at the coherer (through the
aerial), these filings are drawn together, or cohere. This lowers their
resistance and they become a better conductor. Now, the coherer wires
are also connected through a battery to the relay, which in turn is
connected through another battery to a Morse register. Therefore, when
the filings become a conductor, the current flows through them and the
circuit to the relay is closed. That attracts an armature which closes
the circuit of the Morse register and thus marks the electrical impulse
on a strip of paper tape. In the mean time, a restoring device, called
a "decoherer," operated also by the relay circuit, has tapped upon the
coherer, thus shaking the filings loose again, so that they are ready
to cohere again and register another impulse, or character. Thus, by
pressing the key at the transmitting end for long or short periods, to
represent Morse characters, long and short waves are propagated in the
ether and are received and recorded at the receiving end through the
coherer and other parts of the receiving set. In this way telegraphic
messages are sent and received through space, between points separated
by hundreds or thousands of miles.

We have tried to describe to you the general principles underlying
the art of wireless telegraphy as plainly as possible, using for
illustration the simplest kind of apparatus employed for the practical
sending and receiving of messages. At the present day there are
several systems in actual practice, and with the growth of the art
there have been many elaborations of apparatus that have come into
use. For instance, the coherer is not as much used as formerly. In its
place there are employed several kinds of "wave-detectors" as they
are now termed, and in many of the systems the electrical pulsations
are generated by a dynamo-machine instead of batteries. Then, again,
instead of the messages being recorded by a Morse register at the
receiving end, the operator receives them by means of a telephone
receiver, through which he hears the Morse characters and writes them
down in words as he hears them. Generally the aerial, or "antennæ," as
it is sometimes named, consists of several wires, sometimes a large
number, carried to a considerable height.

There are a great many other details which might be written to explain
all the complicated apparatus which is used in some of the systems,
but it is not intended in this book to offer more than a general
explanation of main principles. We must leave it to you to study the
details elsewhere if you so desire after you have read these pages.




VI

THE TELEPHONE


You probably all know that the telephone is an electrical instrument by
which one person may talk to another who is at a distance. Not only can
we talk to a person who is in a different part of the city, but such
great improvements have been made in these instruments that we can talk
through the telephone to a person in another city, even though it be
hundreds of miles away.

The main principle of the telephone is electromagnetism, as in the
telegraph, but there are other important points in addition to those we
mentioned in describing the latter.

Let us take first the


INDUCTION-COIL

You will remember that an electromagnet is made by winding many turns
of wire around a piece of iron and sending a current of electricity
through this wire.

Now, suppose this current of electricity was being supplied by two
cells of a battery. If you took in your hands the wires coming from
these _two cells_, giving, say, four volts, you could not feel any
shock; but if you were to take hold of the ends of _the wires_ on the
_electromagnet_ and _separate_ them while this same current was going
through, you would get a decided shock.

This separation would "break" the circuit, and the reason you would get
a shock is that, while the electricity is acting on the wire, the iron
itself is magnetized, and on breaking the circuit reacts upon the wire,
producing for a moment more volts of pressure in every turn of it.
Thus, you see, this weak pressure of electricity as it travels through
the wire can yet produce, through its magnetism, strong momentary
effects, but _you cannot feel it unless you break the circuit_.


HOW THE INDUCTION-COIL IS MADE

The object of the induction-coil is to produce high intensity, or
pressure, from a comparatively weak pressure and large current of
electricity; so, if we add still more wire, the magnet has a larger
number of turns to act upon and thus makes a very strong pressure, or
large number of volts, but a lesser number of ampères.

Instead of taking one piece of iron, as we would for an ordinary
electromagnet, we take a bundle of iron wires in making an
induction-coil, as these give a stronger effect. Around this bundle of
wires we wrap many turns of insulated copper wire. This is called the
_primary coil_, and the ends of this wire are to be attached to the
battery.

[Illustration: Fig. 9]

On top of, or over, this primary coil we wrap a great many turns of
very fine wire, of which, as it is so fine, a great length can be used.
This is called the _secondary coil_, and it is in this coil that the
volts, or pressure, of electricity become strongest.

Above we show you a sketch of an induction-coil. (Fig. 9.)

At the left-hand side of the cut is a "circuit-breaker," which is
simply a piece of iron (armature) on a spring placed opposite the iron
core. This armature is made a part of the wire leading to the primary
coil. When the current from the battery is sent through the wires,
the core becomes magnetized and draws this armature away from a fixed
contact point, thus breaking the circuit, but the spring pulls it back,
again completing the circuit, and so it keeps going back and forth very
rapidly with a br-r-r-ing sound.

If you were now to take hold of the ends of the secondary coil you
would get a continuous series of quick shocks which would feel like
pins and needles running into you.

Perhaps most of you have taken hold of the handles of a medical battery
and have had shocks therefrom. In so doing, you have simply had the
current from the secondary of an induction-coil. The current may be
made weaker by sliding a metallic cover over part of the iron core and
so shutting off part of the magnetic effect.


SPARKING COILS

While on this subject we may add that these coils will produce sparks
from the two ends of the wire of the secondary coil. These sparks vary
in length according to the amount of wire in the coil. Small ones are
made which give a spark a quarter of an inch in length, while others
are made which will give sparks 10, 12, and 16 inches in length. In the
latter, however, there are many miles of wire in the secondary coil.

The largest induction-coil known is one which was made for an English
scientist. There are 341,850 turns, or 280 miles, of wire in the
secondary coil. With 30 cells of Grove battery this coil will give a
spark 42 inches in length. You may form some idea of the effect of this
induction-coil when we state that if we desired to produce the same
length of spark direct from batteries, without using an induction-coil,
we should require the combined volts of pressure of 60,000 to 100,000
cells of battery.

Having explained to you briefly the induction-coil--how it is made
and its action--we must ask you to bear these principles in mind, and
presently we will tell you how it is used in the telephone.

The next thing we shall try to explain will be


THE VIBRATING DIAPHRAGM

Did you ever take the end of a cane in your hand, raise it up over your
head, and then bring it down suddenly and sharply, so that it nearly
touched the ground, as though you were about to strike something? If
not, try it now with a thin walking-cane or with a pine stick about
three feet long and one-half inch thick, and you will find that there
is a peculiar sound given out. It is not the stick that makes this
sound, but it is owing to the fact that you have caused the air to
vibrate, or tremble, and thus give out a sound.

[Illustration: Fig. 10]

If you strike a tuning-fork sharply you will see the ends vibrate and a
sound will be given. If you put your fingers on top of a silk hat and
speak near it you will feel vibrations of your voice.

Every time you speak you cause vibrations of the air; and the louder
and higher you speak the greater the number of vibrations.

Suppose you take a thin piece of wood in your hands (say, for instance,
the lid of a cigar-box cut in the shape shown in the picture, Fig. 10)
and hold it about two inches from your mouth and then speak. You will
feel the wood tremble in your hand. This is because the vibrations of
the air cause the wood to vibrate in the same manner. These vibrations
are very minute and cannot be seen with the naked eye, but they
actually take place, and could be measured with a delicately balanced
instrument.

[Illustration: Fig. 11]

Now let us try another experiment in further illustration of this
principle. We will take a tube about three inches long and one and
one-half or two inches in diameter. This tube may be made of cardboard.
Now cut out a piece of thin cardboard which will just fit over one
end of the tube. This piece we will call the "diaphragm." Fasten the
diaphragm by pasting it with two strips of thin paper to the tube.
These strips of paper should be fastened only on the ends, and the
middle of the paper allowed to be slack, as shown in the picture, so
that the diaphragm may work backward and forward easily. Take a small
shot about the size seen in the sketch and tie it to a single thread of
fine silk, then let it hang as shown in the sketch (Fig. 11), so that
it will only just touch the diaphragm. Now, if you speak into the open
end of the tube the diaphragm will vibrate and the shot will be seen
to move to and from it according to the strength of the vibrations. If
we could by any means make a diaphragm in another tube reproduce these
same vibrations, we should hear the same words respoken, if the tube
were held to the ear.

[Illustration: Fig. 12]

While the vibrations caused by the human voice are too minute to be
seen, it may seem surprising that they can be made to produce power.
This is done by an ingenious mechanism called a Phonomotor, perfected
by the great inventor Thomas A. Edison, of whom every one has probably
heard. This mechanism, when spoken or sung at (or into) immediately
responds by causing a wheel to revolve. No amount of blowing will start
the wheel, but it can instantly be set in motion by the vibrations
caused by sound.

The Phonomotor (which is shown in the engraving Fig. 12) has a
diaphragm and mouthpiece. A spring, which is secured to the bedpiece,
rests on a piece of rubber tubing placed against the diaphragm. This
spring carries a pawl that acts on a ratchet or roughened wheel on the
fly-wheel shaft. A sound made in the mouthpiece creates vibrations in
the diaphragm; the vibrations of the diaphragm move the spring and pawl
with the same impulses, and as the pawl thus moves back and forth on
the ratchet-wheel it is made to revolve.

The instrument, therefore, is of great value for measuring the
mechanical force of sound waves, or vibrations, produced by the human
voice.


THE TRANSMITTER

That part of the telephone into which we speak is called the
transmitter. This is usually a piece of hard rubber having a round
mouthpiece cut through it. At the other side of this mouthpiece is
placed a diaphragm made of a thin piece of metal, which is held m
place by a light spring. Behind this diaphragm, and very close to
it, is placed a carbon button. Between this carbon button and the
diaphragm is a small piece of platinum, which is placed so as to touch
both the button and diaphragm very lightly. This platinum contact
piece is connected with one of the wires running to the primary of
the induction-coil, and the spring attached to the carbon button is
connected with the battery to which the other wire of the primary is
connected. This is all shown in the sketch of a transmitter. (Fig. 13.)

[Illustration: Fig. 13]

A is the mouthpiece; B, the diaphragm; C, the carbon button; D, the
wire at the end of which is the platinum contact; E, the battery; and
F, the induction-coil; P, P are the wires to the primary, and S, S to
the secondary wires.

We will now say a few words about the receiver, and then describe the
manner in which the telephone works.


THE RECEIVER

This is that part of the telephone which is held to the ear, and
by which we can hear the words spoken into the transmitter of the
telephone at the other end of the line.

[Illustration: Fig. 14]

The receiver is made of hard rubber, and contains a permanent bar
magnet, which is wound with wire so as to make it also an electromagnet
when desired. In front of this magnet is placed loosely a diaphragm
of thin sheet iron. This diaphragm is placed so as to be within the
influence of the magnet, but just so that neither one can touch the
other.

Fig. 14 is a sketch of the receiver. A and B are the wires leading to
the magnet, C, and D is the diaphragm. E and F are where the wires
connect, one from the secondary of the induction-coil in the other
telephone, and the other connected with the earth.


THE CARBON BUTTON

The little carbon button plays an important part in the telephone.
You will see from the sketch of the transmitter that the current of
electricity will flow through the carbon button to the contact point
and through the wire to the primary of the induction-coil.

Now, carbon has a peculiarity, which is this, that if we press this
carbon button, ever so slightly, against the platinum contact, there
would be less resistance to the flow of the electricity through the
wire to the primary, and the more we press it the less the resistance
becomes. The consequence of this would be that more current would go to
the primary, and the secondary would become correspondingly stronger.
If the carbon button were left untouched, and nothing pressed against
it, the flow of current through it would be perfectly even.

Having examined the inside of the transmitter and receiver, and
understanding the effect of pressure on the carbon button, let us now
see


HOW THE TELEPHONE WORKS

When we speak into the mouthpiece of the transmitter, the vibrations of
the air cause the diaphragm to vibrate very rapidly, and, of course,
every movement of the diaphragm presses _more or less_ against the
carbon button, in consequence of which the currents passing through the
primary of the induction-coil are constantly increased or diminished
and thus produce similar effects, but magnified, in the secondary.

The effect of this is that the magnet in the receiver of the other
telephone is receiving a rapidly changing current, which, producing
corresponding magnetic changes, makes the magnet alternately weaker
or stronger. This influences, by magnetism, the iron diaphragm
accordingly, and makes it reproduce the same vibrations that were
caused by the speech at the transmitter of the sending telephone. Thus,
the same vibrations being _reproduced_, the original sounds are given
out, and we can hear what the person at the sending telephone is saying.

The action of the telephone illustrates well the wonderfully quick
action of the electric current by the reproduction of these sound
waves, or air vibrations, for they number many thousands in one
minute's speech.




VII

ELECTRIC LIGHT


We have now arrived at a very interesting part of the study of
electricity, as well as a more difficult part than we have yet told you
of, but one which you can easily understand if you read carefully.

You must all have seen electric lights, either in the streets or in
some large buildings, for so many electric lights are now used that
there are very few people who have not seen them. But perhaps some of
you have only seen the large, dazzling lights that are used in the
streets, and do not know that there is another kind of electric light
which is in a globe about the size and shape of a large pear, and gives
about the same light as a good gas-jet.

These two kinds of electric lights have different names.

The large, dazzling lights which you see in the streets are called
"arc-lights," and the small, pear-shaped lamps, which give a soft,
steady light, are called "incandescent lights." We will tell you later
why these names are given to them.

[Illustration: Fig. 15]

The incandescent lights are generally used in houses, stores, theaters,
factories, steamboats, and other places where a number of small lights
are more pleasant to the eyes. The arc-lights (Fig. 15) are used to
light streets and large spaces where a great quantity of light is
wanted.

It would not be pleasant to have one of these dazzling arc-lamps in
your parlor--although it would give a great deal of light--because
your eyes would soon become tired. But two or three of the small
incandescent lights (Fig. 16) would be very agreeable, because they
would give you a nice, soft light to read or work by, and would not
tire your eyes. So, you see, these two different kinds of lamps are
very useful in their proper places.

Now, if you will read patiently and carefully, we will try and explain
how both these lights are made.

[Illustration: Fig. 16]

You have seen that the telegraph, telephone, electric bells, etc., are
worked by batteries. Electric lights, however, require such a large
amount of current that it is too expensive to produce them in large
quantities by batteries. A small number of lamps could be lighted by
batteries, but if we were to attempt to use them to light 500 or 1,000
lamps together, the expense would be so enormous as to make it entirely
out of the question.

There are many millions of incandescent lamps in use in the United
States, but you will easily see that there could not be that number
used if we had to depend on batteries to light them. You will
understand this more thoroughly when you have finished reading this
little book.

Well, you will ask, if we cannot use batteries, what is used to produce
these electric lights?

Machines called "dynamo-electric machines," or "generators," which
are driven by steam-engines or water-power, are used to produce the
electricity which makes these lamps give us light.

You will remember that in the chapter on Magnetism we explained to you
how electricity makes magnetism, and now we will explain how, in the
dynamo, magnetism makes electricity.

[Illustration: Fig. 17]

It has been found that the influence of a magnet is very strong at
its poles, and that this influence is always in the same lines. This
influence has been described as "lines of force," which you will see
represented in the sketch above by the dotted lines (Fig. 17). Of
course, these lines of force are only imaginary and cannot be seen
in any magnet, but they are always present. The meaning of this term
"lines of force," then, is used to designate the strength of the magnet.

Many years ago the great scientist Faraday made the discovery that,
by passing a closed loop of wire through the magnetic lines of force
existing between the poles of a magnet, the magnetism produced the
peculiar effect of creating a current of electricity in the wire. If
the closed loop of wire were passed down, say from U to D, the current
flowed in the wire in one direction, and if it were passed upward,
from D to U, the current flowed in the other direction. Thus, you see,
magnetism produces electricity in the closed loop of wire as it cuts
through the magnetic lines of force. Just why or how, nobody knows;
we only know that electricity is produced in that way, and to-day we
make practical use of this method of producing it by embodying this
principle in dynamo-machines, as we will shortly explain.

In carrying this discovery into practice in making dynamo-machines
we use copper wire. If iron were used, there would be a current of
electricity generated, but it would be much less in quantity, because
iron wire has much greater resistance to the passage of electricity
than the same size of copper wire.

Perhaps you can understand it more thoroughly if we state that when a
closed loop of wire is passed up and down between the poles of a strong
magnet there is a very perceptible opposition felt to the passage of
the wire to and fro.

This is due to the influence of the magnetism upon the current produced
in the wire as it cuts through the lines of force, and, inasmuch as
these lines of force are always present at the poles of a magnet, you
will see that, no matter how many times you pass the loop of wire up
and down, there will be created in it a current of electricity by its
passage through the lines of force.

[Illustration: Fig. 18]

Suppose that, instead of using one single loop of copper wire, you
wound upon a spool a long piece of wire like that in Fig. 18, and that
you turned this spool around rapidly between the poles of the magnet,
you would thus be cutting the lines of force by the same wire a great
many times, and every time one length of the wire cut through the lines
of force some electricity would be generated in it, and this would
continue as long as the spool was revolved. But, as each length would
only be a part of the one piece of wire, you will easily see that there
would be a great deal of electricity generated in the whole piece of
wire.

[Illustration: Fig. 19]

All we have to do, then, is to collect this electricity from the two
ends of the wire, and use it. If we should attach two wires to the
two ends of this wire on the spool, they would be broken off when it
turned around, so we must use some other method. We fix on the end of
the spool (which is called an "armature") two pieces of copper, so that
they will not touch each other (as in Fig. 19), and fasten the ends
of the wire to these pieces of copper. This is called a "commutator,"
and, as you see, is really the ends of the wire on the spool. Now we
get two thin, flat pieces of copper and fix them so that they will
rest upon the copper bars of the commutator, but will not go round with
it. These two flat pieces of copper are called the "brushes," and they
will collect from the commutator the electricity which is gathered in
the wire around the spool. As the brushes stand still, two wires can
be fastened to them, and thus the ampères of current of electricity,
acted upon by the volts pressure, can be carried away to be used in the
lamps, for you must remember that as long as the spool turns around it
gathers more electricity while there is any magnetism for the wire on
the spool to pass through. The constant revolving of the spool creates
so much electricity that it is driven out from the wire on the spool,
through the commutator to the brushes, and there it finds a path to
travel away from the pressure of the new electricity which is all the
time being made.

In this way we get a continuous current of electricity in the two wires
leading from the commutator, and can use it to light electric lamps or
for other useful purposes.

In explaining this to you, so far, we have used as an illustration
of the magnet one of the steel permanent magnets in order to make
the explanation more simple, but now that you understand how the
electricity is made, we must explain to you something about the magnets
that are used in dynamo-machines. We can perhaps make this more clear
by giving another example.

Suppose you had a dynamo which was lighting up 100 of the incandescent
lamps, each of 200 ohms resistance and each requiring 100 volts
pressure. Now each lamp would take just a certain quantity of
electricity, say half an ampère; so, the 100 lamps would require one
hundred times that quantity. But, if you turned off 50 of these lamps
at once, the tendency would be for the pressure to rise above the
100 volts required for the other 50, and they would be apt to burn
out quicker. It is plainly to be seen, then, that we must have some
means of regulating the magnetism so as to regulate the lines of force
for the wire on the armature to cut through. We can do this with an
electromagnet, but not with a permanent magnet, because _we cannot
easily regulate the amount of magnetism which a permanent magnet will
give_.

There is another reason why we cannot use permanent magnets in a
dynamo, and that is because _they cannot be made to give as much
magnetism as an electromagnet will give_.

Thus you will see that there are very good reasons for using
electromagnets in making dynamo-machines. Let us see now how these
electromagnets and dynamos are made, and then examine the methods which
are followed to operate and use them.

You must remember, to begin with, that in referring to wire used
on magnets and armatures and for carrying the electricity away to
the lamps, we always mean wire that is _covered_ or _insulated_. In
electric lighting, insulated wire is _always_ used, except at the
points where it is connected with, the dynamo, the lamps, a switch, or
any point where we make what is called a "connection."

As the shape of the magnets is different in the dynamos of various
inventors, we will take for illustration the one that is nearest
the shape of the horseshoe and the shape that is generally used in
illustrating the principle of the dynamo. This is the form used by Mr.
Edison, whom we have previously mentioned. This form is shown in Fig.
20.

Now, although this magnet appears to be in one piece, it really
consists of five parts screwed together so as to make, practically,
one piece. The names of the parts are as follows: F, F are the "cores";
C the "yoke," which binds them together; and P, P the "pole pieces,"
where the magnetism is the strongest. These pole pieces are rounded out
to receive the _armature_, which, as you will remember, is the part
that turns around.

[Illustration: Fig. 20]

The cores, F, F, are first wound with a certain amount of wire, which
depends upon the use the dynamo is to be made for. Thus, you will see,
there will be on each core two loose ends of the wire that is wound
around it--namely, the beginning of the wire and the end where we leave
off winding, which on the two cores together will make four ends of
wire. We will tell you presently what is done with them.

After the cores are wound, they are screwed firmly to the yoke and to
the pole pieces, so as to make, for all practical purposes, one whole
piece pretty nearly the shape of a horseshoe magnet.

[Illustration: Fig. 21]

Now, to make the dynamo complete, we must put in the armature between
the poles, which are rounded off, as you will see, to accommodate it.
The armature is held up by two "bearings," which you will see in the
sketch of the complete dynamo above. (Fig. 21.)

The armature in a practical dynamo-machine consists of a large spool
made of thin sheets of iron firmly fastened together and having a steel
shaft run through the center, upon which it revolves.

This spool, or armature, is wound with a number of strands of copper
wire. The commutator, instead of consisting of two bars, is made in
many dynamos with as many bars as there are strands of wire, and the
ends of these wires are fastened to the bars of the commutator so as
to make, practically, one long piece of wire, just as we showed you in
explaining how the electricity was produced.

The brushes, resting upon the commutator, carry away the electricity
from it into the wires with which they are connected.

Now we have our dynamo all put together and ready to start as soon as
we properly connect these four loose ends of wire on the cores.

If you will turn back to Fig. 20 you will see that two of the wires are
marked I, and the other two O. The letter I means the inside wire, or
where the winding began, and the letter O means the outside wire, or
where we left off winding.

Now, if we fasten together (or "connect") the two ends of wires, I and
O, near the top of the magnet, we make the two wires round the cores
into one wire, which starts, say, at I near the poles, goes all around
one core, crosses over and around the other core down to the other end
of the wire to O, near the poles.

So far we have called the iron a magnet, although it is not a magnet
until electricity is put into it; so, when the dynamo is started
for the first time, these two ends of wire, I and O, are connected
to a battery or other source of current for the purpose of sending
electricity through the wire on the cores. When the electricity goes
into this wire the iron immediately becomes a magnet, and the lines of
force are present at the poles.

Now, the armature is turned around rapidly by a steam-engine, and, as
the wire on the armature cuts the lines of force with great rapidity
and so frequently, there is quickly generated a large quantity of
electricity, which passes out as fast as it is made through the
commutator and the brushes to the lamp. And so long as the armature is
revolved and the battery attached, the electricity will be made, or, as
it is usually termed, "generated."

As we stated above, a battery is used _the first time the dynamo is
run_, and now we will explain why it is not needed afterward.

Although iron will not become a permanent magnet, like steel, it _does
not lose all its magnetism_ after it has been once thoroughly charged.
When the dynamo is stopped, after the first trial, and the battery is
taken away, you will discover only traces of magnetism about the poles.
They will not readily attract even a needle or iron filings; but there
is, nevertheless, a very small amount of magnetism left in the iron.
Small as this magnetism is, however, it is enough to make very faint
and weak lines of force at the poles of the magnet.

After the battery is taken away, the ends of the wire on the cores,
which were connected to the battery, are connected, instead, to the
wires which carry away the electricity from the brushes to the lamps.
Thus, you will see, if any electricity goes from the dynamo to the
lamps, part of it must also find its way through the wires which are
around the cores.

We will now start up the dynamo without having any battery attached and
see what happens. The armature turns around and the wires upon it cut
through those very faint lines of force which are always at the poles.
This, as you know, makes some electricity; very little, to be sure, but
it comes out through the brushes to the wires leading to the lamps,
and there it finds the wires leading back to the cores. Well, part of
this weak current of electricity goes into these wires and travels back
round the cores and so makes the magnetism stronger. The consequence of
this is that the lines of force become stronger and, as the armature
keeps turning around, the electricity naturally becomes stronger, and
so there is more of it going through the wires back to the cores and
increasing the strength of the magnet all the time, until the dynamo
becomes strong enough to generate all the current it was intended to
give for the lamps.

Of course, you understand that the stronger the magnet becomes,
the greater will be the lines of force and the greater the amount
of electricity made by the turning of the armature. Now, there is
naturally a limit to what can be done with any particular dynamo; so,
while the electricity continues to strengthen the magnetism and the
magnetism increases the electricity, this cannot go beyond what is
called the "saturation" point of the magnet.

Saturation means that the iron is full of magnetism, and will hold
that much but no more. You will learn more as to the saturation of
magnets when you study electricity more deeply, and we therefore do not
intend to enter into that subject in this book. We will only state,
however, that the magnets of dynamos are not always charged up to their
saturation point.


THE LAMPS

So far you have learned how the current of electricity is produced, and
now we will follow along the wires to find out how it makes the lamps
give out both strong lights and the smaller, pleasant ones.

Suppose we take first the large, dazzling lights we see in the streets,
which, as you know, are called


ARC-LIGHTS

Those who have seen the arc-lamps will readily recognize them from the
picture in Fig. 22.

You will see that there are two sticks, or "pencils," of carbon. Now
you will remember that in the chapter on Magnetism we told you that _in
order to have electricity do work for us we must put some resistance or
opposition in its way_. When we get light from an electric lamp it is
because we make the electricity do some work in the lamp, and this work
is in pushing its way through a resistance or opposition which is in
the lamp.

[Illustration: Fig. 22]

When we generate electricity in the dynamo and put two wires for it to
travel in, the current goes away from the dynamo through one of the
wires and will go back to the dynamo through the other one if it can
possibly get a chance to get to this other one. Now, the electricity
which is constantly being made fills the wires and acts as a pressure
to force the current through the wires back to the dynamo, and, if we
put no resistance or opposition in the way, it would have a very easy
path to travel in and would do no work at all. The wires leading to an
electric lamp should have very little resistance, not sufficient to
require any work from the current in passing through.

So, if we bring the two carbons in an arc-lamp together they really
form part of the wire, and do not interrupt the current in its
travels, but, if we _separate the carbons_, we make a gap which the
current must jump across if it wants to go on. As the volts, or
pressure, is so great, the current must jump, and this _against the
resistance or opposition_ in an arc-lamp is that which gives the
current so much work to do. Indeed, so hard is it for the current to
jump across this gap that it breaks off from one carbon a shower of
tiny particles as fine as the finest dust, and makes them white hot in
passing to the other. This shower of fine carbon dust, together with
the ends of the carbons, being white hot, of course makes a light, and
this is the dazzling light which you see in the arc-lamp.

Of course, when the electricity has jumped over from one carbon to the
other, it goes through it to the wire, and so passes on to the next
lamp, where it has to jump again, and so on until it has gone through
the last lamp, then it has an easy path to get back to the dynamo.

Now, we want you to understand more thoroughly how that much resistance
or opposition will cause heat, so we will try to give you a simple
example.

Most of you know that if you were holding a rope tightly in your hands
and some one pulled it through them quickly and suddenly, it would get
very hot and your hands would feel as though they were being burned.
This is heat caused by your hands resisting or opposing the passage of
the rope through them, and if you could hold on tightly enough and the
rope was drawn through quickly enough, it would take fire. This fire
would, therefore, cause heat and light.

It is just this principle of resistance to the passage of the current
which causes the light in an arc-lamp, as we have shown you.


INCANDESCENT LAMPS

You have just learned that the light in an arc-lamp is caused by the
current forcing off from the carbon sticks tiny particles and heating
them up until they give a brilliant light. So, you see, in an arc-light
there is a wearing away of carbon by electricity, and therefore these
sticks, or pencils, of carbon in time are all burned away. In practice
the carbon pencils last about eight or ten hours, and then new ones
must be put in.

Now, in the incandescent lamp there is also carbon used, but the light
is not produced by the combustion or wasting away of the carbon, as we
will show you.

The picture below will show you the appearance of an incandescent lamp.
(Fig. 23.)

[Illustration: Fig. 23]

You will see that this lamp consists of a pear-shaped globe, and inside
is a long U-shaped strip of carbon no thicker than an ordinary thread.
This is a strip of bamboo cane[1] which has been carbonized to a thread
of charcoal. It is joined to two wires which come through the glass.
These two wires come down through the bottom of the globe, and one is
fastened to a brass screw-ring, while the other wire is fastened to a
brass button at the bottom of the lamp. These two (the ring and button)
must, as you know, be separated from each other by something which
will not carry electricity, or they would make a short circuit when the
electricity was applied. We separate the ring and the button in various
ways.

Now, if we took the ends of two wires which were charged with the
proper amount of electricity and put one wire on the screw-ring and the
other on the button, the lamp would light up, because there would be a
complete path for the current to travel in.

[Illustration: Fig. 24]

It will, however, be plain to you that it would be awkward to light the
lamps in this way, so we use a "socket" into which the lamp is screwed.
(Fig. 24.)

The wires from the dynamo carrying the electricity are connected in the
socket, one wire with the screw thread into which the screw-ring fits,
and the other with a button which the button on the lamp touches when
the lamp is screwed into the socket. Thus we have a connected path for
the current to travel in, or, as it is termed, a _complete circuit_.

You will notice that in the incandescent lamp the electricity does
not need to jump, as it does in the arc-light, because we give it one
continuous line to travel in.

In order, however, to get the current to do work for us, we put some
resistance in its path, which it must overcome in order to travel back
to the dynamo. The resistance in an incandescent lamp is the U-shaped
carbon strip (or, as it is called, "filament"). This charcoal filament
has so much greater resistance than the wires that it opposes, or
resists, the passage of the electricity through it; but the electricity
_must_ go through, and, as it is strong enough to force its way, it
overcomes this resistance and passes on through the carbon to the wire
at the other end. You see it is a struggle between the carbon and the
electricity, the current being determined to go on and the carbon
trying to keep it back; and, in the end, the electricity, being the
stronger, gets the best of it; but the struggle has been so hard that
the carbon has been raised to a white heat, or incandescence, and so
gives out a beautiful light, which continues as long as the current of
electricity flows.

You will remember that in the arc-light the carbons are slowly consumed
and new ones must be put in. If the carbon in the incandescent light
were consumed, it would not last many minutes, because it is only about
the size of a horsehair. Now, you will naturally inquire why this fine
strip is not burned up when it is raised to so high a heat. Well, we
will tell you.

You know that if you light a match and let it burn the wood will all be
consumed. But did you ever light a match, put it into a small bottle,
and put the cork in? If you never did, do so now as an experiment, and
you will see that the match will keep lighted for an instant and then
go out without consuming the wood.

The reasons for this are very simple. In order to burn anything up
entirely it is absolutely necessary to have the gas called oxygen
present, and, as the air you live in contains a very large amount of
oxygen, there is more than sufficient in your room to cause the wood
of the match to be entirely consumed after it is lighted. But there is
such a small quantity of oxygen in the bottle that it is not enough to
keep the fire going in the match, and, consequently, it will not burn
up the wood.

The reason the filament in an incandescent lamp is not burned up is
because there is _no oxygen_ inside the globe. After the carbon is
put in its place all the oxygen is drawn out through a tube, and the
glass is sealed up so that no more oxygen can get in. This is called
obtaining a "vacuum," and vacuum means a space without air.

There being no oxygen in the globe, it is impossible for the carbon to
burn up; so the incandescent lamp will continue to give its light for a
very long time, some of them lasting for thousands of hours. Some day,
however, from a great variety of obscure causes, the filament becomes
weak in some particular spot and breaks, and the light ceases. When
this happens, we unscrew the lamp and put another one in, and the light
goes on as usual.

Now you have learned how the incandescent lamp is made to give light.
We will add that it is a beautiful, soft, white light, almost without
heat, it will not explode, throws off no poisonous fumes like gas or
oil lamps, and has many other points of comfort and convenience which
make it very desirable.


ELECTRIC-LIGHT WIRES

Before closing the subject of electric light you would perhaps like to
know something about the way in which we place the wires leading to
the lamps.

[Illustration: Fig. 25]

If you remember what we told you about measurements in the beginning of
this book, it will be easy to understand what follows:

You know that if you have a very great pressure you can force a
quantity through a small conductor. This is the principle upon which
the arc-lamps are run. Every arc-lamp takes about 40 to 50 volts and
from 5 to 10 ampères to produce the light, and they are connected with
the wires as shown in Fig. 25.

This is called running lamps in "series," and, as you will see from
the sketch, the wire starts out from the dynamo and connects with one
carbon of the first arc-lamp, and to the other carbon is connected
another wire which goes on to the next lamp, and so on until the last
lamp is reached, and then the wire goes back to the dynamo. This forms,
practically, one continuous loop from one brush to the other of the
dynamo.

The current starts out, makes its way through the first lamp, goes on
to the next, makes its way through that, and so on till it has jumped
the last one; then it goes back to the dynamo.

Now, as each of these jumps requires a pressure of 40 or 50 volts, you
will easily see that the total pressure, in volts, of the electricity
must be as many times 40 or 50 volts as there are lamps to be lighted;
so, if there were 60 lamps in circuit, there would be 2,400 to 3,000
volts pressure, which, while it gives very fine lights, might cause
instant death to any one touching the wires.

Suppose anything happened to the first lamp, which stopped the current
from jumping through it. There would be no path for the current to
travel farther, and, consequently, all the lights would go out. To get
over this difficulty there is sometimes used what is called a "shunt,"
which only acts when the lamp will not light. This shunt carries the
current round the lamp to the other wire, so that it may travel on and
light up the other lamps.


WIRES FOR INCANDESCENT LAMPS

The wiring for incandescent lamps is carried out in an entirely
different way, which you can see by comparing Fig. 25 A with Fig. 25
which shows the wiring for arc-lamps.

[Illustration: Fig. 25 A]

This is called connecting in "multiple arc."

You will notice that the two wires running out from the dynamo (which
are called the main wires) do not form one continuous loop as in the
arc-light system, but that a smaller wire is attached to one of the
main wires and then connected with the screw-ring in the lamp-socket;
then another wire is connected with the button in the socket and
afterward to the other main wire. Every lamp forms an independent path
through which the current can travel back to the dynamo.

Now, if we turn one of these incandescent lamps out, we simply shut off
one of these paths and the electricity travels through the other lamps,
and, if we wish, we can turn out all the lamps but one and there will
still be a way for the electricity to go back to the dynamo.

In the arc-lamps we must have a very high number of volts pressure,
because the electricity has only one path, and it all has to pass
through the first and other lamps till it comes to the last one. In
the incandescent light the electricity has as many paths as there are
lamps, so we only need to keep _one_ certain _pressure_ in volts in the
main wires all the time. This pressure is _even_ all the way through
the main wires, and, therefore, it is ready to light a lamp the instant
it is turned on, because, as you have seen, electricity will always get
back to the dynamo if there is a possible chance, and the lamp opens a
path.

The volts pressure used to operate any number of incandescent lamps is
altogether very much less than for a number of arc-lights. For example,
in the Edison system the pressure (sometimes called "electromotive
force") is only about 110 volts, which is very mild and not at all
dangerous. This electromotive force would be _the same_ if there were
_one lamp or ten thousand_ lighted.

While this Edison current would not hurt any one, you should remember
that it is much the better plan not to touch _any_ electric-light wires
until you have learned a great deal more on this subject.

We may add that each of the standard incandescent lamps requires only
about one-quarter of an ampère of current to make them give a light of
16 candle-power, which is about the light given by a very good gas-jet,
and while the electromotive force, or pressure, would only be about
110 volts, whether there were one lamp or ten thousand lighted, there
must be sufficient ampères in the wires to give each lamp its proper
quantity.


SWITCHES

We have made mention several times of turning on or off one or more
lights, and now, perhaps, you would like to know how this is done.

Suppose the electricity was traveling through wires to one or several
lamps, it would light up those lamps as long as the wires provided
a path to travel in, but if you were to cut out one of them, which
is called "breaking the circuit," there would be no road for the
electricity to follow, and, consequently, its course would be stopped
short and the lamps would go out. You will remember that _electricity
must have a complete circuit_ or it can do no work, and in electric
lighting it is always a _metallic circuit_ that is used.

Now, the switch is simply a device which is used to break the circuit
so that the current cannot pass on. The simplest form of switch is seen
in the sketch. (Fig. 26.)

[Illustration: Fig. 26]

You will see that there is a wire cut in two, and to one piece is
attached a metallic piece, A, which turns one way or the other, and
when it is turned so as to touch the other part of the wire the circuit
is closed and the electricity goes from the lower part of the wire
through the metallic piece A to the other part of the wire, thus making
a complete circuit or path for the electricity to travel in.

If we turn the piece A away from the upper wire this breaks the circuit
and cuts off the path, and, of course, the lamps would go out.

This is the principle of the switch, and, although they are made in
thousands of ways, switches all have the same object--namely, the
closing and breaking of the circuit, whether it is for one or a hundred
lamps.


WIRE ON DYNAMOS

In explaining to you the construction and working of dynamo-machines,
we did not state anything about the amounts of wire used in winding the
machine.

It is not our intention to say exactly how much is used on any one
dynamo, because that is among the things you will have to learn when
you come to study the subject of electricity more deeply.

We simply want to have you understand that upon the number of turns of
wire on any one machine depends the effect that that amount of wire,
carrying electricity, will have upon a certain weight of iron when the
armature is revolved a certain number of turns per minute.

A certain number of strands of wire on an armature will only do a
certain amount of work at the most, so you will see that a small dynamo
will not produce as much electricity as a larger one containing more
iron and wire. For high pressure there must be more strands of wire
cutting the lines of force more frequently than would be required for
low pressure; and, to produce a great many ampères, the armature must
be larger and the wire upon it thicker than it would need to be if only
a small number of ampères were wanted.

This of itself is a very deep and complicated subject, and many books
have been written upon it alone. We shall, therefore, not attempt to go
more deeply into it in this little book, but simply content ourselves
with giving you the general idea, which will be sufficient until you
make a thorough study of the subject.




VIII

ELECTRIC POWER


One of the most convenient uses to which electricity is put is in
producing motive power for driving all kinds of machines, from a
sewing-machine to a railway train, and we will now try to explain how
we can get this kind of work from electricity.

To begin with, you all know that a piece of machinery is usually made
to work by revolving a wheel which is part of the machine, either by
means of a steam-engine or by water-power, or, as a sewing-machine, by
foot-power. Now, when we work a piece of machinery by electricity we do
just the same thing by using, instead of the steam-engine or water or
foot power, an electric-engine called an "electromotor," which operates
in the same way--namely, by turning the wheel of the machine it is
applied to.

Foot-power is hard work for the person who is applying the power, and,
as you can easily see, one person can make only a very little power by
use of the feet. Steam and water power can be used for any large amount
of work, but the work must be within a few hundred feet of the engine
or the power cannot be used.

If there were a factory using steam-power a block or two away from
where you lived, and you had a lathe in your house which you would like
to have run by the steam-power in the factory, it would be practically
impossible to do this. Now, if the factory were still farther away from
your house, it would be still more impossible, and if it were a mile
away it would be foolish to dream of taking steam-power from a place so
far away.

Suppose, however, that this factory was lighted by electric lights,
it would be a very easy matter to take some of the power over to your
house. This could be done, even if the factory were miles away, by
taking two wires from their electric-light wires and running them
into your house to an electromotor connected with your lathe. This
electromotor would then run your lathe just as well as if it were
belted to a steam-engine.

So, you see, power can be carried in the form of electricity through
two wires over very great distances and made to do work at a long way
from the engine which is turning the dynamo to make the electricity.
Thus, you may have brought into your house wires which will give lights
and, at the same time, power to run a sewing-machine, a lathe, or any
other piece of machinery.

Having learned so far that a dynamo will make a continuous current of
electricity, and that two wires will carry this current to any place
where it is wanted, let us now see what takes place in the electromotor
to transform the electricity into power.

An electromotor (which we will now call by its short name, motor) is
simply a machine made like a dynamo. Curious as it may seem to you, it
is a fact that if you take two dynamo-machines exactly alike, and run
one with the steam-engine so as to produce electricity, and then take
the two main wires and attach them to the brushes of the other dynamo,
the electricity will drive this other dynamo so as to produce a great
deal of power which could be used for driving other machines. Thus, the
second dynamo would become a motor.

In the chapter on dynamos we explained something about the way they
were made and how the electricity was produced.


THE MOTOR

You will remember that the armature consists of a spool wound with
wire. This spool is made of iron plates fastened together so as to form
one solid piece. The armature of a motor may be made in the same way;
in fact, the whole motor is practically a dynamo-machine.

There is something more about magnetism which we will tell you of
here, because you will more easily understand it in its relation to an
electromotor.

If we take an ordinary piece of iron and bring one end of it near to
(but not touching) one pole of a magnet, this piece of iron will itself
become a weaker magnet as long as it remains in this position. This
is said to be magnetism by "induction." The end of the piece of iron
nearest to the magnet will be of the opposite polarity. For instance,
if the pole of the magnet were north, the end of the iron which was
nearest to this north pole would be south, and, of course, the other
end would be north. To make this more plain we show it in the following
sketch. (Fig. 27.)

This would be the same whether the magnet were a permanent or an
electromagnet.

You will remember also that the north pole of one magnet will _attract
the south pole_ of another magnet, but will _repel a north pole_.

These are the principles made use of in an electromotor, and we will
now try to show you how this is carried into practice.

[Illustration: STEEL PERMANENT MAGNET

IRON

Fig. 27]

Although a motor is made like a dynamo, we will show a different form
of machine from the dynamo already illustrated, because it will help
you to understand more easily. (Fig. 28.)

Here we have an electromagnet with its poles, and an iron armature
wound with wire, just as in the dynamo we have described, except that
its form is different.

[Illustration: Fig. 28]

A commutator and brushes are also used, but the electricity, instead
of being taken away from the brushes, is taken _to_ them by the wires
connected with them. Two wires are also connected which take part of
the electricity around the magnet, just as in the dynamo.

Now, when the volts pressure and ampères of electricity coming from a
dynamo or battery are turned into the wires leading to the brushes of
the motor, they go through the commutator into the armature and round
the magnet, and so create the lines of force at the poles and magnetize
the iron of the armature.

Let us see what the effect of this is.

The poles of the magnet become north and south, and the four ends on
the armature also become north and south, two of each.

By referring to Fig. 28 again we shall see what takes place.

The north pole of the magnet is doing two things: it is repelling, or
forcing away, the upper north pole of the armature and at the same time
drawing toward itself the lower south pole of the armature.

In the mean time the south pole of the magnet is repelling the south
pole of the armature and at the same time drawing toward itself the
north pole of the armature.

This, of course, makes the armature turn around, and the same poles
are again presented to the magnet, when they are acted upon in the
same manner, which makes the armature revolve again, and this action
continues as long as electricity is brought through the wires to the
brushes. Thus, the armature turns around with great speed and strength,
and will then drive a machine to which it is attached.

The speed and strength of the motor are regulated by the amount of iron
and wire upon it, and by the volts pressure and ampères of electricity
supplied to the brushes. Motors are made from a small size that will
run a sewing-machine up to a size large enough to run a railway train,
and are often operated through wires at a great distance from the place
where the electricity is being made, sometimes miles away.

They are also made in a great many different forms, but the principle
is practically the same as we have just described to you.




IX

BATTERIES


So far we have only described one way of producing electricity--namely,
by means of a dynamo-machine driven by steam or water power. The supply
of electricity so obtained is regular and constant as long as the steam
or water power is applied to the dynamo.

There is another and very different way of producing electricity, and
this is by means of a chemical process in what is called a battery.

To obtain electricity from the dynamo we must spend money for the coal
to make the steam which operates the steam-engine, or for the water
which turns the water-wheel, as well as for an engineer in both cases.
When we obtain electricity from a battery we must spend money for the
chemicals and metals which are constantly consumed in the battery.


PRIMARY BATTERIES

An electrical battery is a device in which one or more chemical
substances act upon a metal and a carbon, or upon two different metals,
producing thereby a current of electricity, which will continue as long
as there is any action of the chemicals upon the metal and carbon, or
upon the two metals.

Batteries for _producing_ electricity may be divided into two classes,
called "open circuit" batteries and "closed circuit" batteries.

Open-circuit batteries are those which are used where the electricity
is _not_ required constantly without intermission--for instance, in
telephones, electric bells, burglar alarms, gas-lighting, annunciators,
etc.

Closed-circuit batteries are those which are used where the effect
produced must be continuous every moment, as, for instance, in electric
lights and motors.

The open-circuit battery is made in many different ways, so we only
describe two of the principal ones.

As we told you in an early part of this book, we do not know just what
electricity is, nor why it is produced under the conditions existing
in a battery. But we do know that by following certain processes and
making certain chemical combinations we can make as much electricity
and in such proportions as we want.

The two metals, or the metal and carbon, in a battery are called the
"elements," and to these are connected the wires which lead from the
battery to the instruments to be worked by it.

_The Leclanché Battery._--This form of open-circuit battery consists of
a glass jar in which is placed the elements. One element consists of a
rod of zinc, and the other element is carbon and powdered black oxide
of manganese. These two (the carbon and black oxide of manganese) are
placed in an earthenware vessel called a "porous cup." This is simply a
small jar made of clay which is not glazed. Thus, the liquid which is
in the glass jar penetrates through the porous cup to the carbon and
manganese which it contains, and so the chemicals affect both these
and the zinc at once, for, in order to obtain electricity, you will
remember that the chemical action must take place at the same time upon
both the elements in the same vessel. (Fig. 29.)

The chemical substance used in this battery is sal-ammoniac, or salts
of ammonia. A certain quantity of this salt is dissolved in water,
and this solution is poured into the glass jar. When this is done the
battery will generate electricity at once.

[Illustration: Fig. 29]

It should be remembered that the proper term for the chemical mixture
which acts upon the elements in any battery is "electrolyte."

_The Dry Battery._--The cleanliness, convenience, high efficiency, and
comparatively low internal resistance of the dry cell has brought it
into great favor in the last few years. It is now extensively used in
preference to the Leclanché and other open-circuit batteries having
liquid electrolyte for light work, such as bells, gas-lighting, burglar
alarms, ignition on motor-boats, automobiles, etc.

The dry cell is also used in great numbers for pocket flash-lamps,
and in other ways where it would be impossible to employ batteries
containing liquids.

A dry cell consists of zinc, carbon, and the electrolyte, which is a
mixture so made that it is in the form of a gelatinous or semi-solid
mass, so that it will not run or slop over.

A piece of sheet zinc is formed into a long tube, and a round, flat
piece of zinc is soldered at one end, thus making a cup open at one
end. This forms the cell itself, and at the same time becomes one of
the elements. The other element is a piece of battery carbon which is
long enough to project out of the top of the cell about half an inch or
more. While the cell is being filled with the electrolyte the carbon is
held up by a support so that it does not touch the zinc at the bottom
of the cup. Of course, the zinc cup and the carbon are provided with
proper binding-posts or other attachments, so that conducting wires can
be connected.

The electrolyte is packed into the cup and around the carbon in such
a way that the cup is entirely filled within about half an inch from
the top, and then some melted tar or pitch is poured over the top of
the electrolyte. This seals the cell and binds the contents solidly
together. Just before the sealing compound hardens, one or two holes
are made in it so that the gases may escape.

The composition of the electrolyte itself is not exactly alike in all
dry cells, as the various manufacturers follow their own particular
formulas. However, as you may be curious to know something about it, we
would state that one formula embraces flour, water, plaster of Paris,
granulated carbon, zinc chloride, ammonium chloride, and manganese
binoxide.

You will remember that the Leclanché and the dry batteries are purely
open-circuit cells, and that they can be used to advantage for electric
bells, annunciators, burglar alarms, gas ignition, etc., where _the
current of electricity is not doing_ continuous work, but only for a
few seconds at a time. Consequently, the batteries have a little rest
in between, if only for a few seconds.

Now, if we were to attempt to use open-circuit batteries for electric
lights or motors, where the electricity must work constantly every
second, the batteries would "polarize"--that is to say, they would only
work a few minutes and then stop, because the chemicals used in them
are of that kind that they will only allow the battery to do a little
work at a time.

The batteries we have been describing will do the ordinary work for
which they are intended for sometimes a year without requiring any
attention, but if we try to make them do work for which they were not
intended, they would only last a few days.

If we should want to operate electric lights or motors continuously
from a battery we must, therefore, use


CLOSED-CIRCUIT BATTERIES

There is a great variety of ways in which closed-circuit batteries
are made, but, as the main principles are very much alike, we will
only describe two general kinds, those with and those without a porous
cup.[2]

In the first place, we must state that closed-circuit batteries proper
usually consist of a glass jar and two elements--carbon and zinc.
Sometimes a porous cup is used; for what reason you will soon learn.

The chemicals that are used are usually different from those used in
the open-circuit batteries and are much stronger. These chemicals are
usually sulphuric acid and bichromate of potash (or chromic acid),
which are mixed with water.

We will now examine two of the types of closed-circuit batteries,
taking first the one without the porous cup, of which the Grenet is a
good example.

[Illustration: Fig. 30]

This battery, as you see, consists of a glass jar, in which are placed
two plates of carbon and one of zinc. (Fig. 30.) The latter is between
the two carbon plates and is movable up and down, so that it may be
drawn up out of the solution when it is not desired to use the battery.
When the zinc is in the solution there is a steady and continuous
current of electricity developed, which can be taken away by wires from
the connections on top of the battery.

If the zinc were left in the solution when the battery was not in use,
the acid would act upon it almost as much as though the electricity
were not being used, and thus the zinc would be eaten away and the acid
would be neutralized, so that no more action could be had when we
wanted more electricity.

Now, in the Grenet battery we can light a lamp or run a motor for
several hours continuously, but at the end of that time the solution
would become black and it would do no more work. Then we must throw out
that solution and put in fresh, and the battery will do the same work
again, and so on.

If you should only want to light your lamp or run your motor for a few
minutes, you could pull the zinc up from the solution and put it down
again when you wanted the electricity once more. The carbon element in
the battery is not consumed by the acid, although the zinc is.

[Illustration: Fig. 31]

Now you will see the use of the porous cup. We will take as an
illustration of this type an ordinary battery in which a porous cup is
used. (Fig. 31.)

Here, you will see, the carbon is placed in the porous cup, while the
zinc is outside in the glass jar. In the glass cell with the zinc
is usually used water made slightly acid, and the strong solution of
sulphuric acid and bichromate of potash (or chromic acid) is poured in
the porous cup, where the carbon is placed.

The strong solution penetrates the porous cup very slowly and gets to
the zinc, when it immediately produces a current of electricity. But
the acid does not get at the zinc so freely as it does in the battery
without a porous cup, and, consequently, neither the acid nor the zinc
is so rapidly used up.

Where porous cups are used, the batteries will give a continuous
current for a very much longer time than without them, and will,
sometimes, give many hours' work every day for several months without
requiring any change of solution.

_Polarization._--There is one other reason why a longer working time
can be had from a battery with a porous cup, and that is, in a battery
without a porous cup the action of the acid upon the zinc is so rapid
that the carbon plates become covered with gas, and, therefore, the
proper action by the acid cannot take place upon them. Thus, the
battery ceases to work, and is said to be "polarized." When a porous
cup is used, the action of the acid upon the zinc is slow enough to
give off only a small amount of gas, and thus the acid has a chance to
act upon the carbon plates and develop a steady current of electricity.


THE WORK DONE BY BATTERIES

The pressure and quantity of electricity given off continuously by open
and closed circuit batteries is very different.

The pressure (or "electromotive force") of one cell of an ordinary
open-circuit battery is only about one volt, and the current is usually
very much less than one ampère, except in a dry cell, which may give
more.

In the closed-circuit batteries described, the electromotive force
of each cell is about two volts, while the current varies from 1 to
perhaps 50 ampères, according to the size of the zinc and carbon plates.

It would not matter if you made one cell as big as a barrel, nor if you
put in a _dozen carbons and zincs_, the _electromotive force would not
exceed the volts mentioned for each type of battery_, but the _ampère
capacity would be greater_ than in a smaller cell on account of the
larger size of the carbon and zinc plates.

_Internal Resistance._--There is one other point which affects the
number of ampères which can be obtained from a closed-circuit battery,
and that is whether there is a large or small internal resistance in
the battery itself.

This depends upon the solution which is used and the arrangement of the
plates.

If there is a high resistance in the battery itself (called "internal
resistance"), the electricity must do work to overcome this resistance
before it can get out of the battery to do useful work through the
wires, and, consequently, the capacity in ampères is limited.

If, on the other hand, there is very little resistance in the battery,
the current has very little work to flow to the wires leading from the
battery, and we can get a larger quantity, or greater number of ampères.

Thus, you will see that while the closed-circuit battery is the
stronger, and will do all that the open-circuit battery will do, and
even more, in a short time the latter, though weaker, will do about as
much work for the same amount of zinc and carbon as the former, but
takes a much longer time.


BATTERIES FOR ELECTRIC LIGHT

As we have explained to you, closed-circuit batteries are used for
producing incandescent electric lights in small numbers, as well as
for running motors.

To operate incandescent lights, a number of batteries connected
together are used. The number used depends upon the pressure which the
lamps require to make them give the required light. We will now explain
how the batteries are connected together for this purpose.

[Illustration: Fig. 32]

Suppose you wished to light an incandescent lamp of, say, three
candle-power, which required six volts. We would take three
closed-circuit batteries which would each give two volts, and connect
by a piece of wire the zinc of the first to the carbon of the second,
and the zinc of the second to the carbon of the third, as shown in the
sketch. (Fig. 32.)

We would then attach a wire to the carbon of the first and one to the
zinc of the third, and there would be six volts in these two wires,
which would light up one six-volt lamp nicely.

This is called connecting in series, or for intensity.

Now if each of these cells gave ten ampères alone, the three will only
give ten ampères together when they are connected in series.

If our lamp only required one ampère, you would naturally think that
ten similar lamps put on the wires would give as good light as the one,
but that is not so.

Although you might light up two lamps, the pressure would drop and the
lights would become less brilliant if you put on the whole number. So,
if we wished to put on the whole ten lights we would connect another
battery and thus increase the pressure, which would probably make these
ten lamps burn brightly.

These rules hold good for connecting any number of batteries for lamps
of any number of volts--that is to say, there should be calculated
about two volts for each cell and an allowance made for drop in
pressure.


CONNECTING IN MULTIPLE

There is another way of connecting batteries, and that is to obtain a
larger number of ampères. This is called connecting in multiple arc,
or for quantity.

[Illustration: Fig. 33]

Let us take again for an illustration the three cells giving each 2
volts and 10 ampères. This time we connect the carbon of the first to
the carbon of the second, and the carbon of the second to that of the
third; then we connect the zinc of the first to that of the second, and
the zinc of the second to that of the third, as shown in the sketch.
(Fig. 33.)

We then attach a wire to the zinc and one to the carbon in the third
cell, and we then can obtain from these two wires _only 2 volts_, but
30 ampères.

There are, again, many ways of connecting several of these sets
together, but it is not intended in this book to go into these
at length, for the reason that we only set out to give a simple
explanation of the first principles of this subject.

We shall therefore only give an illustration of one more method of
connecting batteries which will be easy to understand. This is called


MULTIPLE SERIES

The sketch we have last given shows three batteries connected in
multiple. These we will call set No. 1.

Now, suppose we take three more batteries exactly similar and connect
them together just in the same manner. Let us call this set No. 2. Now
take the wire leading from the carbon of set No. 2 and connect it with
the wire leading from the zinc of set No. 1. Then take a wire leading
from the zinc of set No. 2, and a wire leading from the carbon of set
No. 1, and connect them with the lamps or motors. These two sets being
connected in multiple series, we shall get 4 volts and 30 ampères.

This is called connecting in multiple series, and may be extended
indefinitely with any number of batteries.

We should add that one of the elements in a battery is called
"positive," and the other "negative."


THE EDISON PRIMARY BATTERY

As this type of battery will work efficiently on _either_ open or
closed circuit, we have thought best to describe it separately at this
place, in order not to confuse your ideas while reading about batteries
generally.

The type of cell we will now describe was originated by an inventor
named Lalande, and was known by that name; but it has been greatly
improved and rendered more efficient by Edison, and is now manufactured
and sold by him under the name of the Edison Primary Battery.

Before describing the cell itself, let us consider the action that
takes place in a battery of this kind.

If certain metals are placed in a suitable solution, and are connected
together, outside of the solution, by wires, vigorous chemical action
will take place at the surfaces of the metals, and electrical energy
will be produced. The plates must be of different metals, and the
solution should be one that will dissolve neither of them except when
an electric current is allowed to flow.

One of the metals is usually zinc, which is gradually eaten away or
dissolved by the solution while the battery is delivering electrical
energy. It is the chemical combination of the zinc and the solution
that produces this energy, which leaves the zinc in the form of an
electric current, and passes through the solution to the other metal,
out of the cell to the wire, and thence back by another wire to the
zinc, where it is once more started on its circuit.

At the surface of the other metal, which may be, and frequently is,
copper, small bubbles of the gas called hydrogen are produced. This gas
rises to the surface of the liquid and gradually passes off into the
air. But its presence offers resistance to the passage of the current;
so that generally there is associated with the copper a supply of the
gas oxygen. Oxygen and hydrogen are always very eager to mix with
each other, and, therefore, when the hydrogen bubbles appear they are
quickly taken up by the oxygen near by. The mixture of these two gases
forms water, which becomes part of the solution. All of this happens so
quickly that the hydrogen cannot be perceived so long as there is any
oxygen left in the copper-oxide plate.

[Illustration: Fig. 34]

In the Edison Primary Battery (Fig. 34) the plates are zinc, known as
the negative, and copper oxide (copper and oxygen), or the positive.
These are suspended in a solution of caustic soda and water, the plates
and solution being contained in jars of glass or porcelain. The plates
are provided with suitable wires for connecting the cells with one
another and with the lamps, motors, or other devices which they are to
operate. There are usually two zinc plates and one copper-oxide plate,
or multiples thereof. The quantity of current that may be withdrawn
depends on the size and number of the plates, as well as upon their
construction and arrangement.

The voltage of these cells is low, being about 0.65 volt each; but this
is more than compensated for by the fact that the internal resistance
of the battery is so low that the voltage is not perceptibly affected
even at continuous high-discharge rates, and that the voltage remains
practically constant throughout the life of the cell.

Furthermore, when the battery is not in use there is practically no
local action. Consequently, the cells may remain on open circuit (that
is, doing no work) for years and there will be no loss of energy. The
cell will then operate with the same practical efficiency as if it were
new. In some classes of work this battery remains in service from four
to six years without attention.

Another peculiar advantage of this battery lies in the fact that the
plates and the electrolyte are so well proportioned that they are all
exhausted at the same time, and then new plates and solution can be put
in the jar, restoring it to its original condition. These batteries are
used in great numbers for railway signal work and for other purposes,
such as fire and burglar alarm systems, various telephone functions,
operation of electric self-winding and programme clock systems, small
electric-motor work, for low candle-power electric lamps, gas-engine
ignition, electro-plating, telegraph systems, chemical analysis, and
other experimental work where batteries are required that will remain
in use for long periods of time without requiring any attention or
renewal.

The remarks that have been made on previous pages about connecting up
batteries in series, multiple, and multiple series apply also to these
Edison Primary Cells. Fig. 35 shows a battery of four of these cells
connected in series.


SECONDARY, OR STORAGE, BATTERIES

The open and closed circuit batteries we have so far described are used
to produce electricity by the action of the chemicals upon the elements
contained in them. They are called primary batteries.

[Illustration: Fig. 35]

The batteries which we will now tell you of are called secondary, or
storage, batteries, and do not of themselves make any primary current,
but simply act as reservoirs, so to speak, to hold the energy of the
electric current which is led into them from a dynamo or primary
battery. At the proper time and under proper conditions these secondary
batteries will give back a large percentage of the energy of the
electric current which has been stored in them.

This class of battery has been called by these three names: "secondary
battery," "accumulator," and "storage battery"; but as the latter name
is used almost exclusively in this country, we shall use it in the
following description.


TWO TYPES

There are two distinct types of storage battery. One is called the
"lead" or "acid" storage battery, and the other the "alkaline" or
"nickel-iron" storage battery. Each of them simply acts as a reservoir
to hold the energy of the electric current which is led into it, and
each of them, under proper conditions, will give back that energy.
As the lead storage battery is the oldest in point of discovery and
invention, we will describe it first.


THE LEAD STORAGE BATTERY

A lead storage battery usually consists of a glass or hard-rubber jar
containing lead plates and a solution consisting of water and sulphuric
acid. A single unit is usually called a "cell." (Fig. 36.)

There are always at least two lead plates in a storage-battery cell of
this kind, although there may be any number above that. For the sake of
making a clearer explanation to you, we will take as an illustration a
cell containing only two plates.[3]

[Illustration: Fig. 36]

We have, then, a glass or hard-rubber jar containing two lead plates
and a solution consisting of water and sulphuric acid. These plates are
called the "elements," and one is called the positive and the other the
negative element. The solution is called the "electrolyte."

The positive element is a sheet of lead upon which is spread a paste
made of red-lead. The negative element is a similar sheet of lead upon
which is spread a paste made of litharge.

Now, when these plates are thus prepared, they are put into the acid
solution in the jar, and a wire attached to each plate is connected
with the two wires from a dynamo or other source of electric current,
just as a lamp would be connected.

The electric current then goes into the storage-battery cell, entering
by the positive plate and coming out by the negative. These plates
and the paste upon them offer some resistance, or opposition, to the
passage of the current, so the electricity must do some work to get
from one to the other. The work it does in this case is to so act upon
the paste that its chemical nature is changed.

So, after the primary current has been passed from one plate to the
other for some time, and after several "discharges," the storage
battery may be disconnected, being now "formed."

The paste on the lead plates is now found to have changed its chemical
nature, the paste on the positive plate having been transformed into
peroxide of lead, and that on the negative plate into spongy lead. On
arriving at this condition, the paste on the plates is called "active
material."

This process of "formation" is absolutely essential before the lead
storage battery is ready to be used for actual work. So, when the
plates have been fully "formed," the storage battery may be again
connected with a source of electric current which again enters by the
positive plate and leaves by the negative. This current so acts on the
active material that it combines with the acid solution and, through
the energy of the charging current, forms other chemical compounds
which may for convenience be called "sulphates." When the charging
current has flowed through the battery long enough to produce these
changes in the active material the battery is said to be "charged," and
is ready for useful work.

If the two wires attached to the plates are now connected with electric
lamps, or a motor, or other device, the active material will develop
energy in the effort to again change its nature. This energy takes
the form of an electric current, which leaves the battery and passes
through the conductors and operates the lamps, motors, or other devices
in its passage.

In this way the battery is said to be "discharged," and at the end
of its discharge it can again be charged and discharged in a similar
manner for a long time, until the active material is either used up or
drops off the plates.

So far as the actual details of construction are concerned, lead
storage batteries are made in a great many different ways, but the
materials are, in general, of the same nature as those we have
mentioned above.


THE ALKALINE STORAGE BATTERY

We shall now describe an entirely different type of storage battery,
which contains neither lead nor acid. It is one of the many inventions
of Thomas A. Edison.

In the alkaline storage battery the gas called oxygen plays a very
important part, and we will try to make it clear to you what this part
is.

You are well aware of the fact that if you leave your pocket-knife
out in the air it will get rusty. The reason for this is that iron or
steel quickly tends to combine with the oxygen of the air, and this
combination of oxygen and iron is rust, otherwise called oxide of iron,
or iron oxide.

This iron oxide, or rust, is therefore the result of a chemical action
between the iron and the oxygen.

Now as all chemical actions require the expenditure of energy, there
has been developed either heat or electricity in the process. The
oxygen may be taken away from the iron oxide, chemically; but here
again would be another chemical action which would require energy to be
once more expended.

Iron oxide may be made chemically in many different ways. It is
frequently made in the form of a powder. Therefore, we do not have to
depend upon iron rust for a supply of this material.

Before going further we must consider another oxide--namely, nickel
oxide. It is characteristic of nickel that when it is combined with
oxygen to a certain degree so as to form the compound known as nickel
oxide, it will receive still more oxygen.

Now, if under proper conditions we compel iron oxide to give up its
oxygen to some other kind of chemical compound, such as nickel oxide,
we must expend energy. But, on the other hand, if this nickel oxide
gives back the oxygen to the iron--which it will do if opportunity is
given--there is energy produced again in receiving the oxygen. In other
words, the energy previously expended, or part of it, is now returned.

This action and reaction are practically those that take place in the
Edison alkaline storage battery. For simplicity of illustration we
will consider a cell containing only two plates, one positive and one
negative.

The negative plate is made up of a number of small, flat, perforated
pockets containing iron oxide in the form of a fine powder. The
positive plate is made up of small, perforated tubes containing
nickel oxide mixed with very thin flakes of metallic nickel. (Fig. 37
illustrates these plates, the positive being in front.)

[Illustration: Fig. 37]

These two elements, positive and negative, having wires or conductors
attached, are placed in a nickeled-steel can containing the
electrolyte, which consists of a potash solution. You will see that
this differs from a lead storage battery, in which the electrolyte is
sulphuric acid and water. If we were to put this acid solution into a
metallic can (except one made of lead) the can would not last long, as
the acid would quickly eat holes through it.

Now let us see what takes place in the Edison alkaline storage
battery. If an electric current from a dynamo or other source of
electricity is caused to pass through the positive to the negative
plate the oxygen present in the iron oxide passes to and remains with
the nickel oxide. During all the time this is going on the battery is
said to be "charging," and when all the oxygen has been removed from
the iron oxide and is taken up by the nickel oxide, then the battery
is said to be "charged," and the flow of current into the battery is
stopped.

A change has now taken place. The powder in the negative plate is no
longer iron oxide, but has been reduced to metallic iron, because the
oxygen has been removed. The powder in the positive plate is now raised
to a higher or super oxide of nickel, because it has taken the oxygen
that was in the iron.

But the nickel oxide will readily give up its excess of oxygen, and the
iron will receive it back freely if permitted. If the proper conditions
are established, this transfer of oxygen will take place, but the iron
cannot receive it without delivering energy.

[Illustration: Fig. 38]

The proper conditions are established by providing a conducting
circuit between the two elements, in which lamps, motors, or other
electrical devices are placed. As soon as this circuit is provided, the
opportunity is given to the iron to receive the oxygen. This it does,
and in so doing develops electrical energy.

This energy is in the form of electric current which is then delivered
by the battery on what is called the "discharge," and this current may
be used for lighting lamps or for operating motors or other electrical
devices.

The battery is said to be discharging as long as the iron is receiving
oxygen from the nickel oxide. As soon as it becomes iron oxide once
more, the giving out of energy ceases and the battery is said to be
"discharged," and must again be charged to obtain further work from it.
Such a battery can be charged and discharged an indefinite number of
times.

This type of battery is very rugged, and its combinations are not
self-destructive. It is very simple, as it provides chiefly for the
movement of the oxygen back and forth; besides, it gives much more
current for its weight than the lead type of storage battery. (Fig. 38
shows the plates of a standard Edison cell removed from container.)


CONNECTING STORAGE BATTERIES

On the discharge, one cell of a lead storage battery gives an average
of about 2 volts, and a cell of alkaline storage battery about 1.2
volts, no matter what its size or the number of plates may be. When
there are more than two plates in one cell, all the positives in that
cell are connected together by metallic strips or bands, and all
negatives in the cell are connected together in a similar way.

Although we cannot obtain more than the above-named electromotive
force from one cell of either type of storage battery, we can obtain a
greater ampère capacity by using large plates instead of small ones, or
by using a larger number of small size.

The same effects are produced by connecting the cells in series, or
multiple, or multiple series, as we showed you in regard to primary
batteries; and the storage batteries may be charged as well as
discharged when connected in any one of these ways.


CHARGING CURRENT

The current which is used for charging must always be greater in
pressure than that of the storage batteries which are being charged. If
it is not, the storage batteries will be the stronger of the two and
will overpower the charging current and so discharge themselves.




X

CONCLUSION


We will now bring this little volume to a close, having given you a
brief outline of the simplest rudiments of that wonderful power of
nature, Electricity.

We may compare this subject to a beautiful house the inside of which
you would like to examine from top to bottom. We have opened the door
for you; now walk in and examine everything. There may be a great many
stairs to climb, but what you see and learn will repay for all the
trouble.


THE END




FOOTNOTES:

[1] The filaments in modern "Mazda" lamps, as made at the Edison Lamp
Works, are strips of metallic tungsten.

[2] The batteries we will now describe are for closed-circuit work
_only_, and they are never used for open-circuit work. But there is
a type of battery made that is available for either open or closed
circuit operation. This is the Edison Primary Battery, which will be
described later on.

[3] Practically, there is always one more negative plate than positive
plates in a _regular_ storage-battery cell. Consequently, a standard
cell always contains an odd number of plates.




                          TRANSCRIBER'S NOTE

-Plain print and punctuation errors fixed.






End of Project Gutenberg's ABC of Electricity, by William Henry Meadowcroft