Spin in a variable magnetic field: the adiabatic approximation

The problem of spin precession in a time-dependent magnetic field is considered in the adiabatic approximation where the field direction or the angular velocity of its rotation is changing slowly. The precession angles are given by integrals in a way similar to the semi-classical approximation for the Schr\"{o}dinger equation.Comment: LaTeX, 6 pges, no figures, submitted to Comptes rendus Acad. Sci.Pari