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. Magnetizing a needle.
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 over a magnet is probably this: Every molecule of iron may be an extremely tiny magnet; if it is, each molecule has a north and south pole like the needle of a compass. In an ordinary needle (or in any unmagnetized piece of iron or steel) these molecules would be facing every way, as shown in Figure 107.