Modification of the McLeod Gauge.—In order to extend the range of a McLeod gauge, the pressure of the gas which is forced into the capillary is measured by means of a hot-wire gauge. Since the ratio of the compression is known, it is possible to determine the true pressure. The hot-wire gauge consists of a loop of tungsten wire, sealed into the closed end of the capillary and connected so as to form part of a Wheatstone bridge. The hot wire gauge is calibrated directly against the McLeod gauge. Because of the fact that the cooling effect varies with the character of the gas, a separate calibration for each gas is necessary.