Fused quartz membrane manometers, used as null instruments, have been developed for measuring pressures of Hg, or other vapors (or gases) from a few centimeters of Hg to 50 atmospheres, with an accuracy of the order of 0.1 percent. The membranes are clear fused quartz disks, plane polished, 0.1 to 0.5 mm in thickness, and 0.5 cm to 2.5 cm in diameter. These are sealed onto fused quartz heads, either plane or slightly concave. In the first case, the motions of the diaphragms are magnified mechanically by a three‐legged rocker, the two outer legs of which rest on fixed bearings and the middle one of which rests on the diaphragm. The three legs being only slightly out of line, a high magnification (up to 6000) is obtained when a mirror and a long light path are used, or a pointer and a microscope are employed. In the second case, interference fringes set up between the diaphragm and the near‐by slightly concave quartz surface indicate the motions of the former. Volumes as small as 4 mm3have been attained, and all‐quartz manometers, remarkably constant in calibration, are operable at temperatures of 800°C, or higher. Certain forms of these manometers are extremely rugged and free from explosion hazard at high pressures. These manometers are being used to measure the operating pressures in high pressure (H‐type) Hg lamps.