XV. The diurnal variation of terrestrial magnetism

Abstract

In the year 1839 Gauss published his celebrated Memoir on Terrestrial Magnetism, in which the potential on the Earth’s surface was calculated to 26 terms of a series of surface harmonics. It was proved in this Memoir that, if the horizontal components of magnetic force were known all over the Earth, the surface potential could be derived without the help of the vertical forces, and it is well known now how these latter can be used to separate the terms of the potential which depend on internal from those which depend on external sources. Nevertheless Gauss made use of the vertical forces in his calculations of the surface potential in order to ensure a greater degree of accuracy. He assumed for this purpose that magnetic matter was distributed through the interior of the Earth, and mentions the fair agreement between calculated and observed facts as a justification of his assumption. In the latter part of the Memoir it was suggested that the same method should be employed in the investiga­tion of the regular and secular variations. The use of harmonic analysis to separate internal from external causes has never been put to a practical test, but it seems to me to be especially well adapted to enquiries on the causes of the periodic oscillations of the magnetic needle.