This paper describes a simple feedback circuit for the hot wire Pirani gauge. This gauge is used for measurement at low pressures where the heat conducted away from a thin hot wire is a function of the gas pressure. The wire is included in a Wheatstone bridge circuit which serves both to heat the wire and to measure its temperature. In the circuit described here the heat input 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 magnetic amplifier which is controlled by the wire temperature, a fall in temperature increasing and a rise in temperature decreasing the power supply.In this way pressure measurements can be made over the range zero to 3 mm Hg with wire temperatures of the order of 50 to 100°C by using either the feedback signal or the power output from the amplifier to give a measure of the pressure.