The FX227 is a (triode) hydrogen thyratron for pulse modulator duty and this exhibit is marked as CV372 as well as BT79. This thyratron would work with a peak anode voltage of 3 kV and a pulse power of 60 kW.
It was originally used in some WWII small airborne radars but remained in the lists until the late 1970s. WWII originals are now rare and this is much later.
In-service life was rather short so the valves had to be replaced quite frequently. This is a more modern specimen that includes a small hydrogen reservoir cylinder absent in the originals.
The electrodes are firmly held, and the stout wire to the top cap is insulated by glass. The filament is vertical within the anode and helical in the hydrogen cylinder..
The BT79 lettering to the left of the hydrogen reservoir with an oxide coated helical heater inside.
Above the base is the cylinder with the heater within.
The heater, both parts, is a single coated wire.
The central heater wire passing through insulation and the cathode. The outer cylinder is the control element.
The wide glass tube envelope is 38 mm in diameter and, excluding the UX4 base pins, is 108 mm tall.
Reference: Data-sheet. Type FX227 was first introduced in 1952.