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figure do not show this. So we see that the compass cannot tell us true north at any place, unless we know how far it points east or west of true north at that place. This difference is called the declination of the compass.
If you were in Greenland, in what direction would the compass point? If you were in Alaska ? At the north pole?
Fig. 132. — The lines show the direction in which a compass needle points in the Western Hemisphere and in the oceans that wash its shores. Thus, at any point of the line cutting off the eastern corner of Brazil and marked 10, the compass will point 10° west of true north.
220. How Can We Find the Earth's Magnetic Poles ? — How do you suppose men found where the magnetic pole north of us is located? If we balance a knitting
needle nicely and support it (Fig. 133) so that it can swing in an up and down, or vertical, circle instead of in a horizontal circle, and if we then magnetize the needle, we shall find that its north pole dips more and more as we take it farther north, until a place is reached at which it stands vertical; the needle is then over the earth's north magnetic pole. In the Southern Hemisphere there is also a place where the needle becomes vertical, but here the
FIG. 133. south pole of the needle
The dipping needle
points downward toward the earth's points downward.
magnetic pole. The nearer it is to needle is called a dipping position is.
the pole, the more nearly vertical its
How does it behave needle.
when brought over a large magnet?
Try it. Does it seem strange that our earth is a great magnet ? Do you suppose it is the magnetic force of the earth that has magnetized the natural “lodestones”?
221. Exercises. 1. Pass a magnet through some dry sand; do you get any evidence that the sand contains iron particles ?
2. In what ways could a ship which was out of sight of land be steered in the right direction on a bright day? On a bright night? On a cloudy night?
3. How does a compass needle behave if brought near a large body of iron, such as a stove? How would a dipping needle behave if brought over such a body of iron ?
4. If you hold the north-seeking pole of a bar magnet near the north-seeking pole of a compass, is that end of the compass needle
attracted, or repelled? Tell, then, how you can find out, without suspending the bar magnet, which of its ends is north-seeking. Try it.
5. Can a compass be depended upon absolutely to give the correct directions ?
6. How would you describe the magnetic field about a magnet? Is it flat (a plane) or does it extend in all directions ?
Summary. Iron and steel are magnetic : they are attracted by magnets. Soft iron forms a temporary magnet, while it is near another magnet. Steel can be magnetized permanently.
Lodestones are natural magnets. All magnets, whether natural or artificial, take a general north-and-south position when free to move.
A magnet has a north-seeking and a south-seeking pole. When a magnet is broken in two or more pieces, each piece is a magnet.
A magnetic field is the region surrounding a magnet, in which a magnetic body becomes magnetized.
The earth is a great magnet, with a north-magnetic and a southmagnetic pole. Compass needles point toward these poles, except as they are interfered with by other magnetic fields.
A dipping needle is a magnet that can move in a vertical direction. CHAPTER XXIV
FIRST THINGS ABOUT ELECTRICITY
222. Electric Magic. — Have you ever held a sheet of writing paper up against a smooth wall, on some cold winter's night, and rubbed the paper with a woolen cloth or with a silk handkerchief? Try it. You will find that the paper is attracted by the wall, and does not fall when you let go of it. If you pull the paper away, it will be drawn back. Why is the paper attracted after being rubbed?
Comb your hair rapidly with a rubber comb, or rub the comb briskly with a woolen cloth or with fur; then bring the comb near bits of paper, cloth, or cork. What happens ? Can you tell why?
Have you ever seen amber beads ? Amber was once a gum that was produced by trees, but ages ago it hardened and so was preserved for us. It is a fossil gum (see § 147). Sometimes the amber contains the remains of insects that were caught in the gum while it was yet soft and sticky (Fig. 134). The ancient Greeks were observing enough to know that after amber had been rubbed with a cloth it would attract small bodies, such as bits of wood, paper, and cloth. They called amber “ electron ” (ē-lěk'tron);
() so this power of rubbed amber to attract bodies came to be called electricity. We say the amber becomes “charged with,” that is, loaded with, electricity.
(Courtesy of the Field Museum of Natural History.) Fig. 134. — Amber is a fossil gum that is easily charged with electricity,
when rubbed. For what is it used ?
If a glass rod is rubbed with silk cloth, both the rod and the silk will attract light bodies, such as paper, cork, and cloth (Fig. 135). This experiment succeeds best in dry, cold weather, as during a "cold
Fig. 135. — A glass rod or slender bottle will be charged with electricity
when rubbed, and will pick up objects of paper, cloth, and the like.