Spin and Magnetic Field

Classically, a point charge that circulates on a ring of radius r produces a current \(i = \frac{ev}{2 \pi r}\), which causes a magnetic dipole moment \(\overrightarrow{\mu }= \frac{i}{c} S \overrightarrow{n}\), \(S= \pi r^{2}\), in obvious notation; \(\overrightarrow{\mu }= \frac{e v}{2c} r \overrightarrow{n}= \frac{e}{2c} \overrightarrow{r} \wedge \overrightarrow{v}= \frac{e}{2 m c} \overrightarrow{L}.\).