The physical theory of the Earth's magnetic and electric phenomena, no. VI: On the origin of the Earth's magnetic field

Abstract

62. The results obtained in the previous number, if not considered as definitely proving that a part at least of the Earth's magnetism is in some manner dependent upon the Earth's rotation, gave strong indications that such, indeed, is the case. Perhaps the result of greatest importance was that the forces which cause the Earth's magnetization to depart from that of a uniformly magnetized sphere follow, as a first approximation, certain definite and comparatively simple laws. When, for example, the Earth's magnetic moment was obtained separately for each parallel of latitude from 60 N. to 60 S., on the assumption that the measured magnetic forces are those resulting from a uniformly magnetized sphere, a systematic increase in the value was found in both hemispheres with approach towards the equator. This fact, as will be seen, furnishes a crucial test of the sufficiency of the theories already propounded to account for the origin of the Earth's magnetism.