The ratio of the magnetic dipole moment to the mechanical angular momentum of a system.
The electronic spin angular momentum of an electron with spin s is given by
where is h-bar, and in "classical" quantum mechanics, the corresponding magnetic moment is
where is the charge on the electron, is the mass of the electron, and c is the speed of light. This gives a value
where is the Bohr magneton, sometimes known as the Landé factor. The explanation of the factor of 2 appearing in this equation, which should classically be 1 and is caused by orbital electron angular momentum, was one of the successes of Dirac's theory of the electron (Griffiths 1987, p. 153).
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