The thyratron is a modification of a gas-filled triode. Mercury vapor or one of the noble gases neon or xenon are used.

Instead of a grid the thyratron contains an ignition electrode (Z_E).

When voltage is applied between the cathode _K and the anode _A , nothing will happen. If one now puts a voltage pulse, which must be high as the so-called ionization voltage of the gas, between the ignition electrode and the cathode, then the entire gas ionizes in the tube. (the tube starts up like a neon sign) The internal resistance of the tube sinks to a very low value and between the cathode and anode a strong current flows. In this regard a thyratron behaves like a relay, but without the wearing of the contacts.
However the difference (between the thyratron and) between the mechanical relay is that the thyratron can be only switched off by removing the voltage between the cathode and anode.
However if one uses an alternating current instead of direct current, then with each zero crossover (The anode becoming more negative than the cathode.) of the alternating current then the tube independently switches off. However if voltage is at the ignition electrode, then the thyratron restarts immediately with the negative half of the wave Translation needs checking (The anode becoming more positive than the cathode.).

This occurs practically without delay so the thyratron can be used as relay for alternating current. At the same time it works as an electric rectifier. It was therefore used much in electric spot welders.

The modern counterpart in semiconductor technology is the SCR and the triac.