  We consider a search for a sidereal time variation of the beam trajectory in
the cyclotron motion in a static magnetic field. The combination of two beams
moving in opposite directions could allow a test of the radius stability with
sensitivity approaching $10^{-10}$. Such a level of variation of the Lorentz
force, if it exists, would require a speed of light anisotropy on the level of
$10^{-18}$.
