A thyratron consists of a hot cathode, an anode, and one or more control grids between the anode and cathode in an airtight glass or ceramic envelope that is filled with gas. The gas is typically hydrogen or deuterium at a pressure of 300 to 500 mTorr (40 to 70 Pa). Commercial thyratrons also contain a titanium hydride reservoir and a reservoir heater that together maintain gas pressure over long periods regardless of gas loss.