Two forces that
cancel at one speed
Cross an electric field with a magnetic one and a charged beam splits by speed alone. The electric push is constant; the magnetic push grows with velocity. Exactly one speed, v = E/B, feels nothing at all and flies straight through the slit.
whatA channel with an electric field across it and a magnetic field through it. Three particles enter at different speeds; their paths are integrated, not drawn.
lawThe electric force qE is constant; the magnetic force qvB grows with speed. They balance at exactly one velocity, v = E/B.
tryChange |B| and watch which particle makes it through the slit.
noticeThe selected speed depends only on the field ratio — charge and mass drop out entirely, which is why this works as a filter for any beam.
in lifeMass spectrometers put one of these in front of the analyser so only one speed enters.
found byWien used crossed fields in 1898; J. J. Thomson used the same trick to measure the electron's charge-to-mass ratio.