Bayard-Alpert ion gauge

The gauge is a triode with the heated filament serving as a cathode. The electrons emitted from the heated cathode are attracted to a helical grid by a DC potential. Most of the electrons pass through the grid and collide with gas molecules. The positive ions formed in this way are attracted to the central ion collector wire by a negative voltage applied to it. When the temperature is constant, the gauge reading is proportional to pressure. The gauge is sometimes called a hot-filament gauge or Bayard–Alpert gauge and it is the widely used for the region from 10−3 to 10−10 Torr.

Pictured is a glass tubulated Bayard–Alpert gauge with a single thorium oxide-coated iridium (ThO2/Ir) filament. These filaments are prepared by depositing a layer of thoria on the base metal by cataphoresis and conditioning in a vacuum at about 1800 °C. They are very resistant to poisoning and do not burn out if exposed for a short time during operation to a sudden inrush of air. The gauge also contains Kovar seal and a 3/4" stainless steel connecting arm. The exhibit served in a quadrupole mass spectrometer HP SERIES 5972 MSD (Hewlett Packard) coupled to a capillary gas chromatograph (manufactured in 1993).

Wikipedia: Hot filament ionization gauge
Wikipedia: Pressure measurement