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 substances; author of the epoch-making book "De Magnete, etc.," published in London in 1600

THE SPERRY GYROCOMPASS

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. Today, 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

graphic pole was found in 1831 by Sir James Ross in Boothia, Canada, latitude 70° 30' N., longitude 95° W. It was located again in 1905 by Captain Amundsen (the discoverer of the geographic south pole in 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 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 which the north pole of a compass needle at any given place on the earth points away from the true north is called its declination at that place. Each of the lines in the map opposite page 238 is so drawn that at each point on it the declination is the same. Lines drawn over the earth through points of equal declination are called isogonic lines. The heavy lines pass through all the points where the needle points exactly to the north. These lines correspond, therefore, to places where the declination is zero. Lines of zero declination are called agonic lines.

FIG. 218. Arrangement for showing dip

276. The dipping needle. Let an unmagnetized knitting needle a (Fig. 218) be thrust through a cork, and let a second needle b be inserted as shown. 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 the needle is allowed to swing in a north-and-south vertical plane it will dip at an angle of from 60° to 70° with the horizontal, N pole downward.

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° (see map opposite page 238). The dipping needle is shown in Fig. 219.

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 be found to have become a magnet with its upper end an S pole, like the north pole of the earth, and its lower end an N pole. If the rod is reversed and tapped again with the hammer, its magnetism will be reversed. If it is held in an east-and-west position and tapped, it will become demagnetized, as will be shown by the fact that either end of it will attract either end of a compass needle. In some respects a softiron rod is more satisfactory for this experiment than a steel rod, on account of the smaller retentivity.

SUMMARY. Like poles repel; unlike poles attract.

A unit pole is one which at a distance of 1 centimeter from an equal and similar pole repels it with a force of 1 dyne.

A magnetic line of force is the path along which an independent N pole tends to move.

A magnetic field of unit strength is one in which a unit pole experiences 1 dyne of force.

The declination at a given locality is the number of degrees by which the N pole points away from the true north.

The dip, or inclination, in a given locality is the angle between a horizontal plane and the direction of the dipping needle.

The magnetic equator is an irregular line drawn over the earth through points where the inclination is zero.

QUESTIONS AND PROBLEMS *

1. Devise an experiment which will show that a piece of iron attracts a magnet just as truly as the magnet attracts the iron.

2. In testing a needle with a magnet to see if the needle is magnetized why must you get repulsion before you can be sure it is magnetized?

3. Given two bar magnets, the poles of which are not marked, how would you (1) locate the north-seeking pole of each? (2) determine which is the more strongly magnetized?

4. Make a diagram to show the general shape of the lines of force between unlike poles of two bar magnets; between like poles. 5. When a piece of soft iron is made a temporary magnet by bringing it near the N pole of a bar magnet, will the end of the iron nearest the magnet be an N pole or an S pole? Explain.

7. Two bar magnets of equal strength are combined as in Fig. 220 (1) and then as in Fig. 220 (2). Diagram their magnetic lines of force as you imagine them to be in the two cases. Test by using iron filings and cardboard; also by lifting a bunch of small nails.

8. Examine the map opposite page 238 and tell where a compass would point north; south; east; west.

9. Examine the map opposite page 238 to locate the line of no dip (the magnetic equator). Does it ever coincide with the geographical equator? If so, where?

* Supplementary questions and problems for Chapter XII are given in the Appendix.