In field emission experiments with spin polarized electrons a magnetic field is superposed on the electric emission field to define the preferred spin direction. The motion of the polarization vector in these fields was calculated for rays emanating from individual points of the emitter by integrating the equation of motion and taking into account relativistic terms. There is a slight shift of the polarization vector from its initial direction. If the initial polarization is aligned with the magnetic field and the emission tip is sufficiently well centred in the magnetic field, the tilting of the polarization vector for a beam of electrons starting not too far from the tip apex is less than 10 0
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