A gas-filled tube containing three electrodes, the anode, cathode, and gate. The tube exhibits bistable characteristics: a high impedance between anode and cathode – the ‘off’ state – and a low-impedance ‘on’ state. A voltage placed on the gate electrode initiates a discharge in the tube, switching from off to on. The current is limited by the external circuitry. The extinction of the discharge is effected by reducing the anode potential to a low value. The thyratron is now superseded by its solid-state analogue, the thyristor.