The relativistic Lagrangian for a spinning particle in an electromagnetic field is derived from the known Lagrangian in the particles rest frame. The resulting relativistic Stern-Gerlach and Thomas precession forces on the particle are then derived f
rom the Lagrangian in the laboratory frame. In particular, the longitudinal component of this combined Stern-Gerlach-Thomas force does not contain a term proportional to gamma-squared as was claimed in a previous derivation [1].
Tests were performed to pass a 100 MeV, 430 kWatt c.w. electron beam from the energy-recovery linac at the Jefferson Laboratorys FEL facility through a set of small apertures in a 127 mm long aluminum block. Beam transmission losses of 3 p.p.m. throu
gh a 2 mm diameter aperture were maintained during a 7 hour continuous run.
