  Starting with the results obtained in a previous paper in which classical
local U(1) gauge invariance in terms of the electromagnetic field strenghts
instead of the usual formulation mediated by the four potential was introduced
it is shown that using the gauge freedom associated with the third component of
the magnetic field, previously obtained spinor equations of motion describe the
internal dynamics of a free 1/2 spin particle suggesting a kinematic origin of
its rest mass and helicity. The controversial Zitterbewegung (trembling motion)
appears in a natural way as internal motion with the velocity of light. Such an
interpretation is in contrast with the usual quantum mechanical explanation of
transitions between positive and negative energy states.
