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.