Postscripts on geodynamo’s beginnings

Kolm replies: I think Frank Lowes’s memory is as overpacked as mine. The essence of our disagreement is that he remembers a 1963 model with 5-centimeter-diameter rotors, reversing at a 5-minute period, while I remember seeing a model with 1- to 2-meter-diameter rotors reversing at 20-minute (not 40-minute) periods at a 1967 symposium at the University of Newcastle. ¹

A dynamo of that size is not easily forgotten. The model towered above the heads of the crowd. It was described in the symposium presentation as the “Bullard–Rikitake model,” so I assumed that it was based on Bullard’s collaboration with a thesis student.

I don’t want to engage in a polemic, but I do want to record two facts. First, Earth’s magnetic field in the southern Atlantic Ocean is about one-third as strong as it is in diametrically opposite northern Siberia. I measured it with a proton precession magnetometer aboard the research vessel Pilsbury in 1968, and I doubt that displacement of a dynamo by only 4% of Earth’s diameter will account for so large a difference. Maps of Earth’s magnetic field, published by the US Office of Naval Research, confirm the difference. I was searching for magnetic monopoles in deep-sea sediment at the time.

Second, my invitation to the symposium was prompted by Bullard’s interest in my published supposition that reversals of Earth’s magnetic field might be caused by the planet’s encountering magnetic monopoles. Monopoles would have been attracted to Earth’s opposite poles and trapped in deep-sea sediment or in surface outcrops of magnetite or hematite. ² My supposition was later disproved when I found no monopoles in deep-sea sediments or surface outcrops.