A viscosity measuring system especially useful to pollution and chemical process control applications. A variable gain amplifier drives a magnetic coil which causes a torsion member to oscillate. The amplitude of the oscillations is sensed by a transducer, and converted to a DC voltage by an amplitude monitoring circuit. The DC voltage, which represents the instantaneous amplitude of mechanical oscillation, is compared to a reference DC voltage which represents the desired amplitude of oscillation. The resulting error signal is utilized to control the gain of the amplifier in such a manner that the amplitude of mechanical oscillation is maintained constant and equal to the desired amplitude. The viscosity of the fluid in which the torsion member is immersed is obtained by providing an output signal which is a measure of the power provided by the amplifier to the magnetic coil in order to maintain the desired constant amplitude of oscillation. Preferably, the output signal is obtained from the voltage developed across a resistor in series with the magnetic coil.