Note: Principles are mostly the same for hot-cathode ion sources in particle accelerators to create electrons.

A regulated electron current (typically 10 mA) is emitted from a heated filament.

A hot-cathode ionization gauge is composed mainly of three electrodes, all acting as a triode, wherein the cathode is the filament.

Hot-cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot.

Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect.