The Pirani Gauge for the Measurement of Small Changes of Pressure

The application of the Pirani gauge to the measurement of small pressure changes is discussed. Both nickel and tungsten wires are used as filaments in the gauge. Nickel wire not only has the greater sensitivity but possesses several other advantages. The theory of the gauge is developed so that it is possible to predict the effect of change in length or diameter of the wire upon the sensitivity of the gauge. The theory also predicts that there is an optimum temperature to which the wire should be heated for maximum sensitivity of the gauge. The observed and computed values of the optimum temperature are compared. In some cases the agreement is as good as can be expected and in the others the discrepancy is easily explained. The maximum sensitivity attained is a galvanometer deflection of 1 mm for a pressure change of air equivalent to 5×${10}^{-9\mathrm{}}$ mm of mercury.