A negative experiment relating to Magnetism and the Earth's Rotation

Abstract

The discovery by Babcock of the magnetism of certain rapidly rotating stars led me to study the hypothesis, first clearly discussed by Schuster and by Wilson, that the magnetism of rotating astronomical bodies might be due to some new and general property of matter. The well-known theoretical difficulties attending such a view were matched by the difficulty of finding a quantitative explanation of even the earth’s magnetic field in terms of the known laws of physics. A detailed study of the possibility of making a direct test of the Schuster-Wilson hypothesis, by measuring the very small magnetic field of the order of 10 -9 G which would be produced by a rotating body of reasonable size in the laboratory, led me to conclude that the experiment would perhaps be possible but would certainly be exceedingly difficult. However, a much easier but still worth-while subsidiary experiment presented itself. This was to test whether a massive body, in fact a 10 x 10 cm gold cylinder, at rest in the laboratory and so rotating with the earth, would appear to an observer, also rotating with the earth, to produce a weak magnetic field with a magnitude of the order of 10 -8 G. That such a field might exist is a plausible deduction from a particular form of the Schuster-Wilson hypothesis considered in some detail by Runcorn and by Chapman. This paper describes the design, construction and use of a magnetometer with which this ‘static-body experiment’ was carried out. Since few detailed studies of the design of sensitive magnetometers to measure steady fields appear to have been made since the days of the classical experiments of Rowland and of Eichenwald, I found it necessary to investigate the theory and use of such an instrument in considerable detail. The bulk of this paper, that is, §§ 2 to 5, is concerned with this instrumental study. The actual static-body experiment is described in § 6, and it is there shown that no such field as is predicted by the modified Schuster-Wilson hypothesis is found. This result is in satisfactory agreement with the independent refutation of the hypothesis by the measurements by Runcorn and colleagues of the magnetic field of the earth underground. When the magnetometer was completed it was found to be very suitable for the measurement of the remanent magnetism of weakly magnetized specimens, in particular certain sedimentary rocks.