Part 12

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.

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.

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.

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.