again it will attract both ends of the needle, showing that it is not polarized. Place the bar again in the line of dip and give it one or two sharp blows with a hammer. Now place it in the first horizontal position; and on testing it with the needle it is found to be polarized. Hold it horizontally in an east and west line and give it a few sharp blows; and it will be found to be not polarized.

Test the steel or iron rods about the building for polarity. Most of them will be found polarized, especially if they have been in a vertical position. Test a vertical rod with the magnetized sewing needle. If the upper end of the rod repels the point, the lower end will repel the eye and vice versa. What is the polarity of the lower end of the rod? Why?

337. Molecular Magnets. If we consider the molecules of iron to be polarized, we can explain magnetism and magnetic induction as follows. A magnet is a piece of iron composed of molecular magnets which lie partly or wholly in a uniform direction. When a piece of iron is in a neutral condition, not polarized, the molecular magnets of which it is composed have no uniform direction, their positions being determined by the mutual action they have upon one another. When, however, a magnet is brought near, their mutual action is overpowered by the greater force of the magnet, they assume parallel directions, and the iron becomes a magnet.

Demonstration

FIG. 290

them, and with a strong pointed punch make an indentation in the middle. Make a support for each by cutting out a disk of sheet lead and driving a pin through it to the head. Mount the needles upon these supports, as in Fig. 290. Set these needles upon a board as near as they can be placed without touching one another, and they will take various positions, as shown in the figure. Now bring one end of a strong bar magnet near one end of the board, and at once

the magnets begin to turn in parallel directions. Figure 290 illustrates an unmagnetized iron bar, and Fig. 291 the inductive action of a magnet upon such a bar.

338. The Effect of Breaking a Magnet.-Demonstration.Magnetize a knitting needle. Determine its poles. File a notch at the middle and break it. Examine for polarity again, and compare with the polarity the needle had before breaking. Break one of these pieces in the middle and examine for polarity. ing of the broken needle, and mark the polarity of each end of each piece.

Make a draw

339. The Effect of Heating a Magnet. If a sewing needle is magnetized and then heated to redness by being held in the flame of a Bunsen burner, it will be found, on testing it after cooling, that it has lost practically all its magnetism. This is probably due to the fact that, in heating, the vibrations of the molecules have increased in velocity until they no longer retain the positions which determine the polarity

of the magnet. If the needle is heated red-hot from end to end, it cannot be picked up by a magnet, which shows that at that temperature steel is not a magnetic substance.

Questions

1. How do natural magnets become polarized?

2. Suppose a magnetic needle is attracted by a certain body; does this prove that the body is a magnetic substance? Does it prove that the body is polarized? What is the only action that proves polarity?

3. In what direction would the north end of a magnetic needle point at the northern end of Greenland?

A

N

4. What is meant by a uniform magnetic field?

B

FIG. 292

C

example.

Give an

5. The square piece of land shown in Fig. 292 was surveyed, starting from the point A. The line AB was found to have the direction N. 20° E. An old survey gave the direction of this line as N. 16° E. Show by a drawing the effect of running this out on the old directions, without allowing for a change in the declination. 6. Why are scissors, knives, and other steel tools often found to be magnets?

7. What position would a magnetic needle take if suspended, at the earth's surface, over the magnetic north pole? Over the magnetic south pole? At the magnetic equator?

8. Suppose you were to place a magnetic needle upon a thin cork disk on the surface of water in a wooden pail. Would it go toward the north? Explain. Make the experiment.

9. What is the effect of near-by iron ore, iron posts, steel wire fences, and the like, upon the surveyor's compass?

10. Why are the iron posts of a fence generally found to be magnetized? What is the polarity of their upper ends?

11. Show by a drawing the polarity of a magnet broken in two. 12. A test tube filled with iron filings can be magnetized by stroking with a bar magnet. Explain.

Problems

500.500 10000

1. A bar magnet sends out 500 lines of force. How many dynes of attraction will there be between the + pole of this magnet and the pole of a magnet of equal strength if they are 5 cm.

apart?

2. It requires a pull of 12,000 dynes to keep the + pole of a magnet, having a strength of 200 lines of force, from going toward the pole of another magnet placed 2 cm. from the first. What was the polarity of the second magnet and what was its strength?

3. If two bar magnets placed end to end are repelled with a force of 1200 dynes when the poles are 20 cm. apart, what will be the repulsion when they are 30 cm. apart, if we consider only the effect of the adjacent poles?

4. A cylindrical bar magnet is 2 cm. in diameter, and the strength of its field adjacent to the end of the magnet is 800 gausses. How many maxwells has it in this field?

5. A bar magnet is 2 cm. wide and 8 mm. thick. Its field adjacent to the end has 1200 maxwells. What is the strength of the field in gausses?

CHAPTER IX

ELECTRICITY

I. STATIC ELECTRICITY

340. Electrification.

Demonstrations. - Hold a warm, dry

glass rod over a handful of cork filings, pith balls, bits of paper, etc., and the rod will not affect them. Rub the rod briskly a few times

FIG. 293

with a piece of silk or flannel, and the light bodies will begin at once to fly to the rod, will remain there for an instant, and will then fly back to the table.

Place a warm, dry sheet of glass over the bits of paper, supporting it by a book at each end. No effect will be noticed until the glass is rubbed with the silk, when the bits of paper will at once begin to jump to the glass and back again.

Using a flannel pad for a rubber, repeat the first demonstration with (1) a stick of sealing wax, (2) a rod of ebonite, and (3) a hard rubber comb.

The above demonstrations show that when glass is rubbed with silk, or sealing wax with flannel, there is imparted the property of attracting light bodies. The first record of such a phenomenon was made by the Greeks about 600 B.C. Because they noticed it in amber, which they called elektron, the name electricity has been given to the cause of these