  We derive the relativistic velocity addition law, the transformations of
electromagnetic fields and space-time intervals by examining the drift
velocities in a crossed electromagnetic field configuration. The postulate of
the light velocity invariance is not taken as a priori, but is derived as the
universal upper limit of physical drift velocities. The key is that a physical
drift of either an electric charge or a magnetic charge remains a drift motion
by inertial reference frame transformations. Such a simple fact is incompatible
with the Galilean velocity addition. This derivation provides a way to
introduce relativity via elementary electromagnetism.
