In the late spring of 1947, the experimental physicist P. M. S. Blackett succumbed to the temptations of theory. At this time, Blackett (1897–1974) was fifty years old. He was a veteran of the Cavendish tradition in particle physics and he was on his way to an unshared award of the 1948 Nobel Prize for his experimental researches in nuclear physics and cosmic-ray physics. His photographs of cloud-chamber tracks of alpha particles, protons, electrons and positrons were well known to practitioners of particle physics, even as they now grace the pages of physics textbooks.

Blackett's turn toward theory in 1947 involved some risk for a well-established experimental physicist. The 3 May 1947 issue of Nature carried an announcement of his forthcoming lecture at the Royal Society:

Professor P. M. S. Blackett, Langworthy Professor of Physics in the University of Manchester, will deliver a lecture on ‘The Magnetic Field of Massive Rotating Bodies’ at a meeting of the Royal Society on May 15, at 4:30 p.m.

Blackett circulated a preliminary draft of his paper among colleagues in several different fields, including the geophysicist Sydney Chapman and the astrophysicist Harry Plaskett.