De Havilland Propellers Ltd., Hatfield, Herts, have developed a series of torque motors for applications where absolutely reliable operation, together with high torque output and frequency response, low weight and compact size, are of paramount importance. The torque motors provide a mechanical energy output proportional to an electrical input signal; they have been designed specifically to operate control valves in hydraulic and pneumatic servo actuators, but are suitable for recording instruments and many other duties. Provision is made for differential or series excitation; differential excitation is recommended as it eliminates datum shift due to supply variations. A torque, which is proportional to the input current, is developed at the output shaft and this, in conjunction with the stiffness of the system, produces a proportional shaft deflexion.