The Weiss magneton was based on a model of the atom
that preceded the work of Ernest Rutherford and Niels Bohr.
Weiss knew that something in some kinds of atoms
was responsible for creating magnetic fields,
but he didn’t know what it was.
Theoretical physicists complained that it ought to be related
to the Planck constant, which is the quantum unit of action,
but the Weiss magneton could not be related to the Planck constant.
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