In June 1929 a paper by the 23-year-old Nevill Mott appeared in the Proceedings of the Royal Society of London. As Mott noted in his introduction, theoretical arguments and empirical evidence supported the notion that electrons have an intrinsic magnetic moment, or spin. “The question arises,” he wrote, “has the free electron ‘really’ got a magnetic moment, a magnetic moment that we can by any conceivable experiment observe?”
Mott’s question is subtler than it might first appear. If you turn to the paper’s appendix, you’ll find what Niels Bohr told Mott: The uncertainty principle forestalls any attempt to distinguish an electron’s intrinsic magnetic moment from the magnetic field that arises from its motion. But, as Mott (shown here) demonstrates in his paper, it is possible to quantify the intrinsic magnetic moment because it turns out the probability that an electron scatters off an atomic nucleus in a given direction depends on the orientation of the electron’s spin.
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