Mini 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 the Series 342 Mini ion vacuum gauge capable of measuring vacuum pressures from less than 1 x 10^-6 Torr to 5 x 10^-2 Torr. Its filament is made of thoria-coated iridium, and the lime glass bulb is sealed to 0.75 inch diameter tube. In this gauge, the filament (cathode), grid (anode), and ion collector are maintained at +30 V, +180 V, and 0 V, relative to ground, respectively. This gauge was removed from the ion trap mass spectrometer LCQ Fleet produced by Thermo Fisher Scientific in 2008.

Wikipedia: Pressure measurement
Wikipedia: Hot filament ionization gauge