A simple form of Knudsen gauge has been developed in which a thin strip of aluminum leaf is suspended from its upper end in the middle of a glass tube whose axis is vertical. Alongside the aluminum leaf is mounted a platinum strip which can be heated electrically. In the presence of gas at low pressure, the aluminum leaf is pushed away from the heated strip because the molecules hitting it on the side facing the heated strip impart to it more momentum than those hitting it on the other side. The deflection of the aluminum leaf, which can be measured by a microscope with a scale in the eyepiece, is a function of the pressure of the gas, and consequently by suitable calibration, the device may be made to measure pressure. By varying the heating current through the platinum strip a wide variety of sensitivities can be readily obtained. Sensitivities ranging from 1 eyepiece scale division=3.2×10−7 mm to 1 eyepiece scale division=304×10−7 mm have been used. The upper limit at which the gauge will work satisfactorily is approximately 0.01 mm.