<P> The most common ion gauge is the hot - cathode Bayard--Alpert gauge, with a small ion collector inside the grid . A glass envelope with an opening to the vacuum can surround the electrodes, but usually the nude gauge is inserted in the vacuum chamber directly, the pins being fed through a ceramic plate in the wall of the chamber . Hot - cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot . The measurements of a hot - cathode ionization gauge are always logarithmic . </P> <P> Electrons emitted from the filament move several times in back - and - forth movements around the grid before finally entering the grid . During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron (electron ionization). The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current . Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current . </P> <P> The low - pressure sensitivity of hot - cathode gauges is limited by the photoelectric effect . Electrons hitting the grid produce x-rays that produce photoelectric noise in the ion collector . This limits the range of older hot - cathode gauges to 10 Torr and the Bayard--Alpert to about 10 Torr . Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect . In the extraction type the ions are not attracted by a wire, but by an open cone . As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam . This ion beam can be passed on to a: </P> <Ul> <Li> Faraday cup </Li> <Li> Microchannel plate detector with Faraday cup </Li> <Li> Quadrupole mass analyzer with Faraday cup </Li> <Li> Quadrupole mass analyzer with microchannel plate detector and Faraday cup </Li> <Li> Ion lens and acceleration voltage and directed at a target to form a sputter gun . In this case a valve lets gas into the grid - cage . </Li> </Ul>

Discuss in brief the scales of measuring pressure