A method based on some work of Thomas Davis is described for controlling the rotor speed of an air‐driven ultracentrifuge. A permanent magnet mounted on the shaft of the centrifuge induces an e.m.f. in a series inductance, capacity resistance circuit which has the same frequency as the rotor. When the rotor speed approaches the resonance frequency of the circuit, energy is absorbed, and the driving air pressureversusrotor speed relation increases abruptly. By regulating the driving air pressure at a few lbs./in.2above that necessary to maintain the speed of the centrifuge where resonance begins, the rotor speed is held constant to roughly 0.1 percent over the range of speeds employed.