Abstract: Concentric cylinder viscometer comprising means for rotating an outer sleeve which exerts a viscous drag on an inner cylinder, the torque of which is registered by spring means, and in which the outer sleeve is rotated at any of several pre-selected speeds by a direct current motor, the speed of which is controlled by a phase-locked feedback circuit in which the instantaneous speed of the motor is detected by an incremental encoder, the signal from which is fed to a digital display showing the rotational speed and is also fed back to the phase-locked comparator circuit which causes the application to the motor of the voltage required to drive it at the desired speed. The circuit includes an oscillator and frequency dividers which provide a plurality of precisely controlled frequencies, and also includes a variable oscillator which provides any desired hand-settable frequency over a wide range. The device is particularly adapted to the testing of drilling fluids.