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 $\frac{1}{S}$. The role played by this minimality condition is made explicit by means of a counterexample.