While the conductivity gauge is practically an electric lamp of ordinary filament type the ionization gauge of Dushman and Found is practically a thermionic valve (q.v.). The principle of its use has been already described. The glass walls and metal parts must be carefully freed from gas by prolonged heating and pumping before the gauge is used. With suitable conditions the desired proportionality between ionization and pressure is found to hold. The calibration constant depends, of course, on the gas. This type of gauge has been much used in the technical laboratories attached to the great lamp companies. It can probably be used to measure pressures down to microbars.
It is perhaps, of interest to contrast the pressures produced by the pumps and measured by the gauges described with the highest attainable laboratory pressures, produced and measured by Bridg man. The lowest pressures are certainly as low as atmos pheres (a ten-thousand-millionth of an atmosphere). Bridgman's highest pressures are 15,000 atmospheres which far exceed any thing produced in the chamber of a gun during firing. The range
of pressures accessible in the laboratory is therefore from f to 150 million million. Even at the lowest attainable pressure, how ever, there are more than a thousand million molecules present in every cubic centimetre.
BIBLIOGRAPHY.—Historical. The original sources for the most im portant early pumps are quoted in the text. For the general history of early air pumps see E. Gerland, F. Traumiiller, Geschichte der physi kalischen Experimentierkunst, 1899, and E. Gerland, Geschichte der Physik, 1913. A translation into German of Guericke's De Vacuo Spatio has been issued as Number 59 of Ostwald's Klassiker der exacten Wissenschaften. The pumps in use round about the beginning of the present century are described in Winkelmann's Handbuch der Physik, vol. i., part 2, 1908.