An instrument is described which measures the torque exerted on the cup of a coni-cylindrical viscometer by balancing it against the action of a coil carrying an electric current in a magnetic field. A servo-system incorporating a novel form of mechanical-electrical transducer automatically adjusts the coil current. The measurement depends only on the electromagnetic constant of the instrument which has been determined with an accuracy better than ±0.6% and should remain unaltered indefinitely. The measured torque is this constant multiplied by the torque coil current.In addition to measuring the torque the instrument can be arranged to control the viscometer speed, and therefore the rate of shear, so as to maintain the shearing stress in the oil under examination at a predetermined value. Design details and the results obtained with castor oil and with an oil having non-Newtonian properties are included.