A speed control has been designed for the Beckman model E analytical centrifuge which employs as the velocity transducer a photoelectric pickoff and a digital counter. The controller regularly gives a short term stability of one part in 20 000 and a long term stability (24 h) of one part in 10 000 at rotor speeds greater than 15 000 rpm. The speed controller can be used for any motor which can be controlled by changing the current in the field winding. Even further, the method of control can be used to control any quantity that can be measured by digital means if the relatively low sampling can be tolerated.