Good replies: David Stevenson’s letter raises an interesting issue that has more to do with the nature of history than with Earth’s internal heat. As Stevenson notes, from our current vantage point we know that ignorance of radioactivity was not the main problem with Lord Kelvin’s calculation of Earth’s age. And the belief of Kelvin’s peers that this invalidated his calculations “undoubtedly influenced the development of geophysics,” as Stevenson says. What counts in history is what people thought at the time. Although “could haves” interest me, too, we historians usually struggle sufficiently just establishing what did happen.
I hope I did not suggest that Teddy Bullard was the first to think Earth’s magnetic field might be due to electrical currents deep within. In fact, wonderful letters in which Bullard, Patrick Blackett, and Walter Elsasser debate details of such currents still exist in the Bullard archive. And even before Joseph Larmor’s 1919 paper on stellar interiors, Arthur Schuster had also considered those currents in several papers. Michael Rochester’s comments are much appreciated, since he knew and worked with Bullard.
As Nick Rogers points out, Bullard came from a wealthy family and experienced opportunities less available to his working-class peers. Social background is always relevant to biography. I thank Cinna Lomnitz for the welcome correction of my error regarding Harold Jeffreys. I did know that Jeffreys was a fellow of St. John’s College; I spent several enjoyable weeks in the school’s archives reading his manuscripts.
Bullard and Blackett actually had more in common than their interest in geophysics. Both were government advisers and department directors, both involved themselves in operational research and the governance of science. Their personalities, of course, could not have been more divergent.
