A modified form of the Pirani gauge has been constructed in which a potential difference between two extra electrodes attached directly to the heated filament is measured rather than the change of resistivity of the whole filament assembly. In this way changes in the long term stability, which are a common feature of most Pirani gauges, can be avoided. The modified instrument may be relied on to maintain its stability after calibration for very long periods, and hence it can be used as a precision gauge for the accurate measurements of pressures in the range 1 to 10−3 mm Hg.