As one who knew both Edward “Teddy” Bullard and one of Patrick Blackett’s doctoral students in paleomagnetism, I read with great interest “Rutherford’s Geophysicists” by Greg Good. I am concerned, though, that the article suggests, even if inadvertently, that Bullard was the first to propose electrical currents in Earth’s liquid-metal core as being capable of sustaining the geomagnetic field by dynamo action and that his March 1948 paper 1 contained a dynamo model.
That paper makes no mention of the first suggestion, by Joseph Larmor in 1919, that Earth’s magnetic field might be maintained by something like a dynamo in its interior. 2 In a footnote in his 1948 paper (page 249), Bullard acknowledged the prior and “similar” arguments by Walter Elsasser in his two 1946 papers, but strangely does not mention Elsasser’s important 1947 paper on the toroidal field modes. 3 Elsasser’s papers discuss at length the mathematical representation of the poloidal and toroidal parts of the geomagnetic field and the dynamics and energetics of the feedback mechanisms necessary to sustain it. But Bullard’s March 1948 paper concentrates on the secular variation rather than on dynamo action and offers only the briefest summary of possible causes of motions in the core. In fact, the first of Bullard’s many papers on geomagnetic dynamo models 4 was not submitted for publication until November 1948. That paper concludes with his generous acknowledgment of the influence of Elsasser’s work on his own ideas.
My comments are not intended to detract from Bullard’s extensive and original contributions to geomagnetic dynamo theory, only to remind readers that Elsasser did foundational work at a time when the origin of Earth’s magnetism still appeared to be a nearly impenetrable mystery.
