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electronelectron tube functioning as a rectifier to convert alternating current (AC) to direct current (DC). Each conduction cycle is started by an external voltage applied to the igniter, a small electrode touching the tube's cathode, which is a pool of mercury. Electrons released by the igniter from the surface of the mercury initiate a conducting arc through the tube. The arc lasts until voltage on the ignitron's plate has been reduced to the point that the arc can no longer be sustained