In his article “Rutherford’s Geophysicists” (PHYSICS TODAY, July 2010, page 42), Greg Good writes that Patrick Blackett “had noted that the magnetic fields attributed to the Sun and Earth seemed proportional to their angular momentum. Could it be that every large, rotating gravitational mass generates a magnetic field according to a simple, elegant relation”? Good also says that Blackett “first presented a sketch of his theory in a seminar in November 1946.” The impression is that Blackett originated the idea that the magnetic field of a planetary or stellar body is related to the body’s angular momentum or more generally to its rotation. While he certainly adopted the idea and spent considerable effort on testing it, the hypothesis seems to go back to Blackett’s predecessor at the University of Manchester, Arthur Schuster.
Revisiting an idea he had already aired in 1891 before the Royal Institution, Schuster in 1912 discussed different mechanisms by which the rotation of any body could give rise to a magnetic field.1 He was opposed to the idea that electric currents deep underground should be the cause of Earth’s magnetism, because no suitable electromotive force was known and because for the currents to be decaying remnants of some ancient initial state, unreasonably strong magnetic fields had to be assumed for the remote past.
Schuster held that the close alignment between the axes of the dipole and of rotation could not be a coincidence; as an alternative to the currents, he suggested that any spinning body would have a magnetic field. The dependence would not be a straightforward relation between angular velocity and field intensity, because the effect of such a mechanism would already have been observed. On the other hand, the field would be too difficult to detect at the laboratory scale if it were proportional to the product of angular velocity and the radius squared. Schuster proposed that the rotation would instead give rise to a magnetic force that may or may not induce magnetization, depending on a body’s chemical composition. He also pointed out that magnetic molecules should align with the axis of rotation like gyrostatic compasses and in so doing precess about that axis—a possible explanation for the secular variation of the field. At the time, he was carrying out experiments at Manchester to measure such effects in rapidly rotating bodies and to measure the effect of pressure on magnetization, which he speculated to be opposite to that of temperature.