McLeod Gauge

Once a pump has removed most of the air, how do you know how much is left? At a thousandth of an atmosphere a mercury manometer's two columns differ by less than a millimetre, and below that the difference vanishes into the meniscus. Herbert McLeod's answer in 1874 was to stop trying to measure the pressure and instead COMPRESS a known volume of the gas into a tiny capillary, by a known ratio, until its pressure is large enough to read easily. Compress a litre into a cubic millimetre and a pressure a million times too small becomes readable on an ordinary scale. It is a wonderfully direct instrument — it needs no calibration against anything, because Boyle's law and two measured volumes give the answer outright.