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