# Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/156

124 TERRESTRIAL MAGNETISM. [469.

downwards, but where the compass-needle, when carried in a circle round the point, instead of revolving so that its north end points constantly to the centre, revolves in the opposite direction, so as to turn sometimes its north end and sometimes its south end towards the point.

If we call the points where the potential is a minimum true north poles, then these other points may be called fals&.north poles, because the compass-needle is not true to them. If there are p true north poles, there must be p I false north poles, and in like manner, if there are y true south poles, there must be y I false south poles. The number of poles of the same name must be odd, so that the opinion at one time prevalent, that there are two north poles and two south poles, is erroneous. According to Gauss there is in fact only one true north pole and one true south pole on the earth s surface, and therefore there are no false poles. The line joining these poles is not a diameter of the earth, and it is not parallel to the earth s magnetic axis.

469.] Most of the early investigators into the nature of the earth s magnetism endeavoured to express it as the result of the action of one or more bar magnets, the position of the poles of which were to be determined. Gauss was the first to express the distribution of the earth s magnetism in a perfectly general way by expanding its potential in a series of solid harmonics, the coefficients of which he determined for the first four degrees. These coeffi cients are 24 in number, 3 for the first degree, 5 for the second, 7 for the third, and 9 for the fourth. All these terms are found necessary in order to give a tolerably accurate representation of the actual state of the earth s magnetism.

To find what Part of the Observed Magnetic Force is due to External and what to Internal Causes.

470.] Let us now suppose that we have obtained an expansion of the magnetic potential of the earth in spherical harmonics, consistent with the actual direction and magnitude of the hori zontal force at every point on the earth s surface, then Gauss has shewn how to determine, from the observed vertical force, "whether the magnetic forces are due to causes, such as magnetization or electric currents, within the earth s surface, or whether any part is directly due to causes exterior to the earth s surface.

Let V be the actual potential expanded in a double series of spherical harmonics,

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