Maker's Name: 
5 × 5 × 17 cm

This is an inverted thermionic triode, with a hot tungsten filament in the centre, a coaxial grid which is positive wrt the filament and a coaxial ion collector plate which is negative wrt the filament.

Traces of gas between the electrodes are ionised by colliding with accelerated electrons between filament and grid, and are attracted to the collector.

The current that flows in the external collector circuit is a measure of the level of vacuum in the gauge.

This gauge must be calibrated against a directly measurable vacuum using eg a McLeod gauge.

This gauge is suitable for moderate vacuums.