The spin-orbit interaction in the one-body problem of the point structuralparticle

In the present paper the classical equations of motion of a Point Structura l Particle (PSP) in an electric external field are obtained. Taking into ac count:the Lorentz transformations of the fields, the spin-orbit interaction is obtained for nu much less than c, as in the Dirac equation, but with th e advantage that it appears with the proper gyromagnetic factor for each pa rticle. The Darwin contact interaction is also obtained, with a clear physi cal meaning: it is the quadrupolar interaction of the particle with an exte rnal electric field. The dipolar interaction term, which allows to classica lly obtain the 1/nu law for the capture of a neutron in a Coulomb field als o appears, and predicts a value for the electric polarizability of the neut ron in good agreement with the experimental data.