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.