The electric and magnetic state of the interior of the earth, as inferred from terrestrial magnetic variations

Abstract

1.1. By spherical harmonic analysis it is possible, as was shown by GAUSS, to distinguish between the parts of the earth’s magnetic field, at any time, which originate respectively within and above the earth’s surface. In this way GAUSS confirmed and extended GILBERT’s conclusion that the field is almost entirely of internal origin. 1.2. Sir ARTHUR SCHUSTER applied the method to the field of the daily magnetic variation at the earth’s surface, and found that the major part is of external origin, but that there is also a part produced within the earth. He attributed the latter to electric currents induced in the earth by the outer varying field, which he regarded as primary. In co-operation with Prof. H. LAMB he showed that the relation between the two parts of the field is consistent with this hypothesis ; the calculations referred to currents induced in a uniformly conducting sphere. They showed that the conducting sphere must be distinctly smaller than the earth, that is, it is an inner core, and not the whole earth, which is effective.