Magnetic Moment of a Particle with Arbitrary Spin

The magnetic moment of a particle of nonzero spin S is computed by the introduction of minimal coupling into the appropriate Galilean-invariant wave equation. It is shown that the requirement that the differential equations be of first order, together with a minimality assumption on the number of components, uniquely implies a g factor of 1/S. The role played by this minimality condition is made explicit by means of a counterexample.