Ignitron

ignitron [′ig·nə‚trän]

(electronics)

A single-anode pool tube in which an ignitor electrode is employed to initiate the cathode spot on the surface of the mercury pool before each conducting period.

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Ignitron 

a single-anode ion device with a mercury cathode and controlled arc discharge. It is used as an electric valve in powerful rectifying devices and in electric drives, welding units, and traction substations on railroads.

The emission of electrons, which causes the primary arc discharge between the anode and cathode of the ignitron, takes place when there is a positive voltage at the anode from one or more incandescent sections of the cathode (cathode spots). Cathode spots are created by the pilot arc, which is formed periodically just before the striking of the primary arc by the passage of current pulses up to several tens of amperes (A) in amplitude and several milliseconds in length through a trigger electrode partially immersed in the liquid mercury of the cathode. By changing the instant of striking of the pilot arc, the initiation of striking of the primary arc may be controlled, and thus the average strength of the rectified anode current may be regulated from its maximum value to zero.

Ignitrons are generally constructed in a metal housing and are manufactured for average current values of 20–700 A at voltages of up to 5 kilovolts (kV) and for commutated power of 100-3,600 kVA. Ignitrons with a glass shell are manufactured for average current values of up to 100 A at voltages of up to 5 kV at the anode.