Ignitron
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An ignitron is a type of controlled rectifier dating from the 1930s. Developed from the Cooper-Hewitt mercury arc rectifier, General Electric was the original manufacturer and owned trademark rights to the name "Ignitron".
It is usually a large steel container with a pool of mercury in the bottom, acting as a cathode. A large graphite block, held above the pool by an insulated electrical connection, serves as the anode. An igniting electrode is pulsed to force conduction through the mercury vapor between the cathode and anode.
Ignitrons were long used as high-current rectifiers in major industrial installations where thousands of amps of AC current must be converted to DC, such as aluminum smelters. Large electric motors were also controlled by ignitrons used in gated fashion, in a manner similar to modern semiconductor devices such as silicon controlled rectifiers and triacs. Many diesel-electric locomotives used them to convert high voltage AC from the alternator to low voltage DC for the motors. Because they are far more resistant to damage due to overcurrent or back-voltage, ignitrons are still manufactured and used in preference to semiconductors in certain installations.