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.
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.