A thyratron includes an envelope containing a gas filling, an anode, an intermediate grid, control grids and a cylindrical structure which acts as a cathode heat shield in a conventional thyratron. In operation the intermediate grid is maintained at cathode potential and, when triggering pulses are applied to the control grids, the gas filling ionizes and the thyratron becomes conducting. The main part of the current comes from the intermediate grid which acts as a "hollow" cathode, and enables the thermionic cathode conventionally used to be dispensed with if desired.