molecules of the steel are magnets even when the bar as a whole is not magnetized, and that magnetization may consist in causing them to arrange themselves in rows, end to end, just as the magnetization of the tube of iron filings mentioned above was due to a special arrangement of the filings.

272. Theory of magnetism. In an unmagnetized bar of iron or steel it is probable, then, that the molecules themselves are tiny magnets which are

arranged either haphazard or in little closed groups or chains, as in Fig. 217, so that, on the whole, opposite poles

FIG. 217. Arrangement of molecules in an unmagnetized iron bar

neutralize each other throughout the bar. But when the bar is brought near a magnet, the molecules are swung around by the outside magnetic force into an arrangement somewhat like the one shown in

Fig. 218, where the opposite poles completely neutralize each other only in the middle of the bar. According to this view, heating and

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FIG. 218. Arrangement of molecules in a magnetized iron bar

jarring weaken the magnet because they tend to shake the molecules out of alignment. On the other hand, heating and jarring facilitate magnetization when the bar is between the poles of a magnet because they assist the magnetizing force in breaking up the molecular groups and chains and getting the molecules into alignment. Soft iron has higher permeability than hard steel, because the molecules of the former substance are much easier to swing into alignment than those of the latter substance. Steel has a very much greater retentivity than soft iron, because its molecules are not so easily moved out of position when once they have been aligned.

273. Saturation. Strong evidence for the correctness of the above view is found in the fact that a piece of iron or

steel cannot be magnet

ized beyond a certain limit, no matter how strong the magnetizing force is. This limit probably corresponds to the condition in which

FIG. 219. Arrangement of molecules in a saturated magnet

the axes of all the molecules are brought into parallelism, as in Fig. 219. The magnet is then said to be saturated, since it is as strong as it is possible to make it.

TERRESTRIAL MAGNETISM

274. The earth's magnetism. The fact that a compass needle always points north and south, or approximately so, indicates that the earth itself is a great magnet having an S pole near the geographic north pole and an N pole near the geographic south pole; for the magnetic pole of the earth which is near the geographic north pole must of course be unlike the pole of a suspended magnet which points toward it, and the pole of the suspended magnet which points toward the north is the one which, by convention, it has been decided to call the N pole. The magnetic pole of the earth which is near the north geographic pole was found in 1831 by Sir James Ross in Boothia Felix, Canada, latitude 70° 30' N., longitude 95° W. It was located again in 1905 by Captain Amundsen (the discoverer of the geographic south pole, 1912) at a point a little farther west. Its approximate location is 70° 5′ N. and 96° 46′ W. It is probable that it shifts its position slowly.

275. Declination. The earliest users of the compass were aware that it did not point exactly north; but it was Columbus who, on his first voyage to America, made the discovery, much to the alarm of his sailors, that the direction of the compass

WILLIAM GILBERT (1540-1603)

English physician and physicist; first Englishman to appreciate fully the value of experimental observations; first to discover through careful experimentation that the compass points to the north not because of some influence of the stars, but because the earth is itself a great magnet; first to use the word "electricity "; first to discover that electrification can be produced by rubbing a great many different kinds of substances; author of the epoch-making book entitled "De Magnete, etc.," published in London in 1600

Although the action of the mariner's compass was first correctly explained by Gilbert, the magnetic compass itself was invented by the Chinese and came to Europe about A.D. 1200. Until very recently it has been the sole reliance of the mariner. To-day, however, it has found a competitor in the gyrocompass, which is now used to a considerable extent on battleships, and exclusively on submarines, within whose encircling shell of iron the magnetic compass will not function at all. It consists of a heavy wheel driven 8600 revolutions per minute about a horizontal axis by an induction motor. Because of its inertia this wheel tends to maintain the plane of its rotation. The revolution of the earth, however, tends to make it leave this plane unless the axis of rotation of the gyro and the earth's axis are already in the same plane. This calls into play a couple which swings the axis of the gyro into the same plane with the axis of the earth

needle changes as one moves about over the earth's surface. The chief reason for this variation is found in the fact that the magnetic poles do not coincide with the geographic poles; but there are also other causes, such as the existence of large deposits of iron ore, which produce local effects upon the needle. The number of degrees by which at a given point on the earth the needle varies from a true north-and-south line is called its declination at that point. Lines drawn over the earth through points of equal declination are called isogonic lines.

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276. The dipping needle. Let an unmagnetized knitting needle a (Fig. 220) be thrust through a cork, and let a second needle b be passed through the cork at right angles to a and as close to it as possible. Let a pin c be adjusted until the system is in neutral equilibrium about b as an axis, when a is pointing east and west. Then let a be carefully magnetized by stroking one end of it, from the middle out, with the N pole of a strong magnet, and the other end, from the middle out, with the S pole of the same magnet. If now the needle is replaced on its supports and turned into a north-and-south position, its N pole will be found to dip so as to cause the needle to make an angle of from 60° to 70° with the horizontal.

FIG. 220. Arrangement for showing dip

The experiment shows that in this latitude the earth's magnetic lines make a large angle with the horizontal. This angle between the earth's surface and the direction of the magnetic lines is called the dip, or inclination, of the needle. At Washington it is 71° 5' and at Chicago 72° 50'. At the magnetic pole it is of course 90°, and at the so-called magnetic equator, which is an irregular curved line near the geographic equator, the dip is 0°.

277. The earth's inductive action. That the earth acts like a great magnet may be very strikingly shown in the following way:

Hold a steel rod (for example, a tripod rod) parallel to the earth's magnetic lines (the north end slanting down at an angle of about 70° or 75°) and strike it a few sharp blows with a hammer. The rod will