Chapter Seven

MAGNETISM AND ELECTRICITY

SECTION 31. Magnets; the compass.

What makes the needle of a compass point north?

What causes the Northern Lights?

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.

EXPERIMENT 61. Lay a magnetic compass flat on the table. Notice which point swings to the north. Now hold a horseshoe magnet, points down, over the compass. Turn the magnet around and watch the compass needle; see which end of the magnet attracts the north point; hold that end of it toward the south point and note the effect. Hold the magnet, ends up, under the table directly below the compass and turn the magnet, watching the compass needle.

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.

EXPERIMENT 62. Take a long shoestring and make a loop in one end of it. Slip the magnet through the loop and suspend it, ends down. Fasten the shoestring to the top of a doorway so that the magnet can swing easily. Steady the magnet and let it turn until it comes to a rest. Mark the end that swings to the north. Turn this end around to the south; let go and watch it. Place the magnet the other way around in the loop so that you can be sure that it is not twisting of the shoestring that makes the magnet turn in this direction.

Now stroke a needle several times along one arm of the magnet, always in the same direction, as shown in Figure 105. Hold the needle over some iron filings or touch any bit of iron or steel with it. What has the needle become? Lay it on a cardboard milk-bottle top of the flat kind, and on that float it in the middle of a glass or earthenware dish of water. Notice which end turns north. Turn this end to the south and see what happens. Hold your magnet, ends up, under the dish, and turn the magnet. What does the needle do?

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.

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.

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 in the same direction--it is almost like combing tangled hair to stroke a needle over a magnet. Then the molecules are arranged more as shown in Figure 108. When all the molecules, each of which is a tiny magnet, pull in the same direction, they make a strong magnet, and they magnetize any iron that comes near them just as they were magnetized.

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.

APPLICATION 47. An explorer lost his compass. In clear weather he could tell the directions by the sun and stars, but in cloudy weather he was badly handicapped. He had with him a gun, plenty of ammunition, a sewing kit, a hunting knife, and some provisions. How could he have made a compass?

INFERENCE EXERCISE

Explain the following:

281. Snow turns to water in the first warm weather.

282. A person's face looks ghastly by the greenish light of a mercury-vapor lamp.

283. If a red-hot coal is touched with a cold poker, the coal turns black at the place touched.

284. Stereopticon slides are put in upside down, yet the picture on the screen is right side up.

285. If the vocal cords of your throat did not vibrate, you could not talk out loud.

286. A watch is sometimes put out of order if it is held near a magnet.

287. The water will be no higher on the inside of a leaky boat than it is on the outside.

288. A bass viol is considerably larger than a violin.

289. Ships that are used by men testing the earth's magnetism carry very sensitive compasses. Explain why such ships are made entirely of wood and brass.

290. Thunder rolls; that is, after the first peal there is a reverberating sound that becomes less and less distinct.