As evidence for a liquid-metal Earth core was beginning to accumulate early in the last century, Joseph Larmor suggested in 1919 that dynamo action of that conducting fluid circulating in Earth’s interior might be what sustains the geomagnetic field. But Larmor’s idea, which geophysicists now take for granted, lay dormant for the next two decades, even as seismological evidence for a liquid core surrounding a solid iron inner core became ever more detailed.
Why such indifference to a plausible answer to one of nature’s great puzzles? “A self-exciting natural dynamo was, at the time, widely thought to violate Lenz’s law,” explains Johns Hopkins geophysicist Peter Olson. Also off-putting was Thomas Cowling’s 1933 annunciation of the first of several “antidynamo theorems.” Starting with the Maxwell equations, Cowling proved that perfectly axisymmetric flow of a conducting fluid cannot generate and sustain an axisymmetric magnetic field. Some physicists concluded, therefore, that planetary and...
