EQUATIONS OF MOTION FOR SPINNING PARTICLES IN EXTERNAL ELECTROMAGNETIC AND GRAVITATIONAL-FIELDS

The equations of motion for the position and spin of a classical parti cle coupled to an external electromagnetic and gravitational potential are derived from an action principle. The constraints ensuring a corr ect number of independent spin components are automatically satisfied. In general the spin is not Fermi-Walker transported nor does the posi tion follow a geodesic, although the deviations are small for most sit uations.