  Section 31. Magnets; the compass.
For many hundreds of years sailors have used the compass to determine directions. During all this time men have known that one point of the needle always swings toward the north if there is no iron near to pull it some other way, but until within the past century they did not know why. Now we have found the explanation in the fact that the earth is a great big magnet. The experiment which follows will help you to understand why the earth's being a magnet should make the compass needle point north and south.
The earth is a magnet, and it acts just as your magnet does: one end attracts one point of the compass, and the other end attracts the other point. That ought to make it clear why the compass points north. But how is the compass made? The next experiment will show this plainly.
Now it should be easy to understand why the compass points north. One end of any magnet pulls on one end of another magnet and drives the other end away. The earth is a big magnet. So if you make a magnet and balance it in such a way that it is free to swing, the north end of the big earth magnet pulls one end of the little magnet toward it and pushes the other end away. Therefore one end of your compass always points north. Other effects of the earth's magnetism. Another interesting effect of the earth's being a big magnet is to be seen if you lay a piece of steel so that it points north and south, and then pound it on one end. It becomes magnetized just as your needle became magnetized when it was rubbed on the small magnet. Fig. 105. Fig. 105. Magnetizing a needle.Fig. 106. Fig. 106. A compass made of a needle and a piece of cardboard.And still another effect of the earth's magnetism is this: Tiny particles of electricity, called electrons, are probably shooting through space from the sun. It is believed that as they come near the earth, the magnetism of the north and south polar regions attracts them toward the poles, and that as they rush through the thin, dry upper air, they make it glow. And this is probably what causes the Northern Lights or Aurora Borealis. What happens when a needle is magnetized. The reason that a needle becomes magnetic if it is rubbed Fig. 107. Fig. 107. Diagram of molecules in unmagnetized iron. The north and south poles of the molecules are supposed to be pointing in all directions.Fig. 108. Fig. 108. Diagram of magnetized iron. The north and south poles of the molecules are all supposed to point in the same direction.But when a piece of steel or iron that is already magnetized is brought near the unmagnetized needle, all the north poles of the molecules of the needle are pulled Steel will stay magnetized a long time; but ordinary soft iron loses magnetism almost as soon as another magnet is taken away from it,—the molecules become all disarranged again. In a later section you will find that whenever electricity flows through a wire that is coiled around a piece of iron, the iron becomes magnetized just as when it is rubbed with a magnet.
Inference Exercise
Section 32. Static electricity.
The Greeks, 2000 years ago, knew that there was such a thing as electricity, and they used to get it by rubbing amber with silk. In the past century men have learned how to make electricity do all sorts of useful work: making boats and cars and automobiles go, ringing bells, furnishing light, and, in the telephone and telegraph, carrying messages. But no one knew what electricity really was until, within the last 25 years, scientists found out. Atoms and electrons. When we talked about molecules, we said that they were as much smaller than a germ as a germ is smaller than a mountain. Well, a molecule is made up, probably, of some things that are much smaller still,—so small that people thought that nothing could be smaller. Those unthinkably tiny things are called atoms; you will hear more about them when you come to the parts of this book that tell about chemistry. But if you took the smallest atom in the world and divided it into 1700 pieces, each one of these would be about the size of a piece of electricity. Electricity is made up of the tiniest things known to man—things so small that nobody really can think of their smallness. These little pieces of electricity are called electrons, and for all their smallness, scientists have been able to find out a good deal about them. They have managed to get one electron all by itself on a droplet of oil and they have seen how it made the oil behave. Of course they could not see the electron, but they could tell from various experiments that they had just one. Scientists know how many trillions of electrons flow through an incandescent electric lamp in a second and how many quadrillions of them it would take to weigh as much as a feather. They know what the electrons do when they move, how fast they can move, and what substances let electrons move through them easily and what substances hold them back; and they know perfectly well how to set them in motion. How the scientists came to know all these things you will learn in the study of physics; it is a long story. But you can find out some things about electrons yourself. The first experiment is a simple one such as the Greeks used to do with amber. Fig. 109. Fig. 109. When the comb is rubbed on the coat, it becomes charged with electricity.
Objects negatively and positively charged with electricity. There are probably electrons in everything. But when there is just the usual number of electrons in an object, it acts in an ordinary way and we say that it is not charged with electricity. If there are more than the usual number of electrons on an object, however, we say that it is negatively charged, or that it has a negative charge of electricity on it. But if there are fewer electrons than usual in an object, we say that it has a positive charge of electricity on it, or that it is positively charged. Fig. 110. Fig. 110. The charged comb picks up pieces of paper.You might expect a "negative charge" to indicate fewer electrons than usual, not more. But people called the charge "negative" long before they knew anything about electrons; and it is easier to keep the old name than to change all the books that have been written about electricity. So we still call a charge "negative" when there are unusually many electrons, and we call it "positive" when there are unusually few. A negative charge means that more electrons are present than usual. A positive charge means that fewer electrons are present than usual. Before you rubbed your comb on wool, neither the comb nor the wool was charged; both had just the usual number of electrons. But when you rubbed them together, you rubbed some of the electrons off the wool on to the comb. Then the comb had a negative charge; that is, it had too many electrons—too many little particles of electricity. When you brought the comb near the hair, the hair had fewer electrons than the comb. Whenever one object has more electrons on it than another, the two When you brought the charged comb near your ear, some of the electrons on the comb pushed the others off to your ear, and you heard them snap as they rushed through the air, making it vibrate. How lightning and thunder are caused. In thunderstorms the strong currents of rising air blow some of the forming raindrops in the clouds into bits of spray. The tinier droplets get more than their share of electrons when this happens and are carried on up to higher clouds. In this way clouds become charged with electricity. One cloud has on it many more electrons than another cloud that is made, perhaps, of lower, larger droplets. The electricity leaps from the cloud that has the greater number of electrons to the cloud that has the less number, or it leaps from the heavily charged cloud down to a tree or house or the ground. You see the electricity The electricity you have been reading about and experimenting with in this section is called static electricity. "Static" means standing still. The electricity you rubbed up to the surface of the comb or glass stayed still until it jumped to the bit of paper or hair; then it stayed still on that. This was the only kind of electricity most people knew anything about until the nineteenth century; and it is not of any great use. Electricity must be flowing through things to do work. That is why people could not invent electric cars and electric lights and telephones before they knew how to make electricity flow steadily rather than just to stand still on one thing until it jumped across to another and stood there. In the next chapter we shall take up the ways in which electrons are made to flow and to do work.
Inference Exercise
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