This paper describes a simple feedback circuit for the Pirani gauge in which the power supply to the Wheatstone bridge is adjusted automatically so as to keep the wire temperature approximately constant over a wide range of pressure changes. This is achieved by supplying the power from a d.c. amplifier which is itself controlled by changes in wire temperature, a small fall in temperature serving to increase the power output from the amplifier. A transistor is employed as the input stage of the amplifier. The comparatively large zero drifts in the transistor d.c. amplifier due to temperature changes are no particular disadvantage in this application as the Pirani gauge signal is also dependent upon ambient temperature.