The physical theory of the Earth's magnetic and electric phenomena: 5. On the formation of the Earth's magnetic field

Abstract

54. The opinion was expressed in §2 that it was doubtful whether “we should ever be able to solve the problem as to the origin of terrestrial magnetism until we had found out what, causes the Earth's magnetism to vary.” A general study of the time and spasmodic variations of the Earth's magnetic condition led to a relation which was put in the following tentative form (l. c.): “Alterations in the Earth's magnetic condition, whatever their nature or origin may be, appear to be distributed over the globe according to a law profoundly dependent upon that governing the distribution of the Earth's own primary magnetic forces.” The exemplification and deduction of the consequences of this law are reserved for a future number.It seems advantageous at this stage to take up a study of possible disturbing forces which, operating on a simple, uniform magnetization of the Earth about some diameter, such as that of the axis of rotation, could change it into the heterogeneous or complex magnetic system of forces actually observed. The method of the Gaussian analysis results in a cumbersome mathematical expression embracing a large number of terms. Though it has been in use for three‐quarters of a century, and has counted among its devotees most eminent investigators, the precise physical interpretation of the harmonic terms beyond that of the first still awaits solution. Indeed, the general opinion has been that the departure from uniform magnetization is to be ascribed primarily to the heterogeneous structure and formation of the Earth's crust. A favorite theory referred the chief cause of irregularities in the distribution of the Earth's magnetic forces over its surface to the relative distribution of land and water.