A vacuum pressure gauge in which a relatively large volume of a low-pressure gas is compressed to a small volume in a glass apparatus (see illustration). The volume is reduced to an extent that causes the pressure to rise sufficiently to support a column of fluid high enough to read. This simple device, which relies on Boyle's law, is suitable for measuring pressures in the range 103 to 10-3 pascal. It is named after H. McLeod (1841–1923).
McLeod gauge