  We derive rigorously the relativistic angular momentum conservation equation
by means of quantum electrodynamics. The novel nonrelativistic spin current and
torque in the spin-orbit coupling system, up to the order of $1/c^{4}$, are
exactly investigated by using Foldy-Wouthuysen transformation. We find a
perfect spin Hall coefficient including the contribution of spin torque dipole.
A novel spin motive force, analogue to the Lorentz force, is also obtained for
understanding of the spin Hall effect.
