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.