A new kind of experiment is described which measures the ‘effective mass’ of mobile electrons in semiconductors and metals. Samples are placed in a magnetic field which causes the charge carriers to move in circular paths, and a microwave oscillating electric field accelerates them in the same way as charged particles in a vacuum are accelerated in a cyclotron. The effective mass thus determined measures the momentum transfer between the crystal lattice and the electron waves and thus gives detailed information about the nature of the conduction electron states.