Common Science Part 26

SECTION 31. _Magnets; the compa.s.s._

What makes the needle of a compa.s.s point north?

What causes the Northern Lights?

For many hundreds of years sailors have used the compa.s.s 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 compa.s.s needle point north and south.

EXPERIMENT 61. Lay a magnetic compa.s.s flat on the table.

Notice which point swings to the north. Now hold a horseshoe magnet, points down, over the compa.s.s. Turn the magnet around and watch the compa.s.s 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 compa.s.s and turn the magnet, watching the compa.s.s needle.

The earth is a magnet, and it acts just as your magnet does: one end attracts one point of the compa.s.s, and the other end attracts the other point. That ought to make it clear why the compa.s.s points north. But how is the compa.s.s 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 gla.s.s 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?

[Ill.u.s.tration: FIG. 104. The compa.s.s needle follows the magnet.]

Now it should be easy to understand why the compa.s.s 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 compa.s.s 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.

[Ill.u.s.tration: FIG. 105. Magnetizing a needle.]

[Ill.u.s.tration: FIG. 106. A compa.s.s 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 s.p.a.ce 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 compa.s.s. 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.

[Ill.u.s.tration: FIG. 107. Diagram of molecules in unmagnetized iron.

The north and south poles of the molecules are supposed to be pointing in all directions.]

[Ill.u.s.tration: 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 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 compa.s.s. 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 compa.s.s?

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 ba.s.s viol is considerably larger than a violin.

289. s.h.i.+ps that are used by men testing the earth's magnetism carry very sensitive compa.s.ses. Explain why such s.h.i.+ps are made entirely of wood and bra.s.s.

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

SECTION 32. _Static electricity._

What is electricity?

What makes thunder and lightning?

Why does the barrel or cap of a fountain pen pick up small bits of paper after it has been rubbed on your coat sleeve?

Why do sparks fly from the fur of a cat when you stroke it in the dark?

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, furnis.h.i.+ng 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 n.o.body 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.

EXPERIMENT 63. Rub a hard rubber comb on a piece of woolen cloth. The sleeve of a woolen coat or sweater will do. Rub the comb quickly in the same direction several times. Now hold it over some small bits of paper or sawdust. What does it do to them? Hold it over some one's hair. The rest of this experiment will work well only on cool, clear days. Rub the comb again, a dozen or more times in quick succession. Now touch it gently to the lobe of your ear. Do you hear the snap as the small spark jumps from the comb to your ear?

Pull a dry hair out of your head and hold it by one end.

Charge your comb by rubbing it again, and bring it near the loose end of the hair. If the end of the hair clings to the comb at first, leave it clinging until it flies off. Now try to touch the hair with the comb. Next, pinch the end of the hair between your thumb and finger and again bring the charged comb near it. Is the hair attracted or repelled? After touching the comb what does it do?

You can get the same effects by rubbing gla.s.s or amber on silk.