Rensselaer Union, Volume 7, Number 48, Rensselaer, Jasper County, 19 August 1875 — Terrestrial Magnetism. [ARTICLE]

Terrestrial Magnetism.

If a magnetized steel needle is suspended by its center of gravity, or placed upon a point, it will take a determinate direction toward a point of the horizon w hich is very nearly north and south. The force which produces this direction is called terrestrial magnetism. It is one of the modes of manifestation of the natural sources of electricity, since magnetism itself is only a particular form of electricity. The magnetic force of our globe is manifested at rts surface by three classes of phenomena, namely: the declination of the magnetized needle, its inclination, ■and the intensity with which the force acts. The declination is the angle that is formed with the direction of the meridian of the. place by the direction of the magnetized needle placed upon a vertical pivot. The inclination is the angle that is formed with the horizon in the magnetic meridian by the direction of a magnetized needle sustained by its center of gravity, around which it is able to turn freely in a vertical plane. These three elements—declination, inclination and intensity—not only vary from one place to another, but in the same place with time. They also manifest irregular and accidental variations, designated under the name of disturbances, the existence of which is connected with the presence of some natural phenomena-, such in» particular as that of the aurora borealis. It is well established that the forces) which act upon the magnetized needle emanate directly from the terrestrial globe, and we are naturally led to regard the earth as a great magnet, and as having one pole situated to the north of us, attracting the north pole of a needle in that direction. If we suspend a magnetic needle by its center of gravity, so "that it may move freely either in a vertical or horizontal plane, the extremity which turns toward the north will incline below 7 the horizon, making at New York an angle with the horizon of about 72 deg. Hence we conclude that, if the earth be a great magnet, giving direction to the needle, its pole must be situated, not on the north horizon, but almost vertically Itetteath us. If the earth is really a magnet, the magnetism of soft iron ought to be decomposed by it, in the same manner as is done by a bar magnet, and such is the fact. If a bar of soft iron is held in the direction which a magnetic needle assumes when freely suspended, its lower end immediately becomes a north pole and its upper end a south pole, as is shown by bringing a small magnetic needle near each end of the bar. On inverting the bar it will be found that its poles "have immediately changed, the low’er end being again a north pole, and the upper one a south pole. If the bar is held horizontally, pointing east and west, no such effect takes place. A similar but slightly diminished effect is produced on a bar of iron suspended in a vertical position; and iron rods which have remained long in a vertical position frequently acquire permanent magnetism. When a bar of iron is rendered magnetic by the influence of terrestrial magnetism, a stroke of a hammer will sometimes fix the magnetism, and the poles will not be reversed When the bar is inverted. But if several blows with the hammer be struck when in the inverted position its magnetism may be destroyed or its poles be reversed. The action exerted by the earth upon a magnetic needle is simply to give direction to the needle, for the weight of a needle is not increased by its magnetism. Hence it is concluded that the attraction of the earth for one pole of the needle is exactly equal to its repulsion for the other. If a magnetic needle be placed upon a cork floating on water, it will soon adjust itself to the magnetic meridian; but it has no tendency to travel either toward the north or south.

Although a magnetic needle, when fully suspendedf-generally points nearly north and south, it is found in almost all parts of the world that the north pole of the needle deviates a few degrees from the astronomical meridian. This deviation is called the magnetic declination. The declination is said to be east or west, according as the north pole of the needle deviates to the east or west of the true meridian. The declination of the needle is very different at different places on the earth’s surface. There are places w’here the declination is 10 deg., 20 deg., 30 deg. and even 90 deg. west; and there are places where the declination is as much to the east. At most places on the earth's surface, the dipping needle will not rest in a horizontal line, one pole pointing downward and the other upward. This dip varies at different places from 0 deg. to 90 deg., and observations to determine its amount have been made in almost every part of the world. In order to represent all these observations conveniently upon a chart, a line is drawn connecting all those places where the dip is the same. A line connecting all those places where the needle rests horizontally is called the magnetic equator. This line exhibits numerous sinuosities in its course around the globe, but does not depart much from a great circle. It crosses the terrestrial equator near the western coast of Africa, attains its greatest southern latitude in South America, where it is 15 deg. south of the geographical equator, crosses the equator again near the meridian of New Zealand, and attains a north latitude of 12 deg. near the southern part of Hindostan. As we travel northward from the magnetic equator, the north end of the needle inclines downward and the dip continually increases, at the rate of about 1 deg. for 1 deg. of latitude, until we reach the north magnetic pole, where the needle stands vertically, in latitude 70 deg. 5 min. N., longitude 96 deg. 45 min. W. As we travel southward from theipagnetic equator, the south end of the medle inclines dow rikard, and this dip continually increases until we reach the south magnetic pole. That terrestrial magnetism is not produced, in any important degree, by magnetic forces external to the earth is probable, because, if there were an external cause for magnetism, it is scarcely conceivable that some large part of it would not act in planes parallel to the geographical equator, and, if so, its effects at any one place would undergo very great changes in the earth’s diurnal revolution, evety part of the earth being presented, in the course of the day, in different aspects toward forces so acting. Now the fact is that the diurnal changes are very small, only about 1-500 part of the whole horizontal force. It would seem certain, therefore, that external bodies or spaces do not produce any sensible part of the magnetism in the "planes to which the earth’s axis is normal. That terrestrial magnetism does not reside, in any important degree, in the earth’s surface, is probable, because of the non magnetic property of the materials of which the earth’s surface is composed, and upon the general absence of any perceptible change in magnetism depending on the change of soil. 1 Humboldt adopted the idea that the

principal phenomena of terrestrial magnetism could be explained by the action of a powerful magnet, of limited dimensions, near the center of the earth; but it was found that the theory upon which this idea depended, though well .representing the broad facts of terrestrial magnetism, failed in accuracy when applied in many special cases. Hansteen suggested the theory of two large magnets within the earth, but this failed to meet the facts of observation. Gauss attempted to explain the cause of terrestrial magnetism by supposing that magnets are distributed irregularly through the earth, and the results of observations generally accord with the necessities of his theory. Regarding the earth as a heterogeneous compound of different substances, which may possess in some degree the properties of different metals, and conceiving (as is the opinion of many physicists) that there is in the interior a great store of caloric which may heat the points of contact, some of them steadily and some of them by occasional bursts of flame, it seems within the range of possibility that such a combination of heat with dissimilar substances may be the cause of terrestrial magnetism. But there is no evidence of this beyond mere conjecture. It is worthy of reifiark that the isothermal lines on the earth’s surface bear a striking resemblance to the lines of equal magnetic intensity. On the whole we must express the opinion that the general cause of the earth’s magnetism still remains one of the mysteries of cosmical physics.— Scientific American.