In the Apiezon oil McLeod gauge, the reservoir is in gas communication with the bulb, and the oil is displaced by a plunger externally controlled. The bulb has a volume of 300 cc, and the capillaries have a bore diameter of 1.44 mm. There is no evidence of sticking in the capillaries. For single measurements at pressures of 10−3, 10−4, 10−5, and 10−6mm of mercury the gauge is considered accurate to about 1, 2, 8, and 25 percent, respectively. The equivalent compression ratio for the gauge is 3.5×10−7. To degas the oil it is necessary to raise it about five times, for half‐hour periods, to the freely vaporizing state while the diffusion pumps are in operation. The whole instrument is kept continuously at a low pressure (10−2mm of mercury or less), thus enabling it to be used, at intervals of several months, without further heat treatment, for the measurement of pressure within its range.