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.