[ig-nahy-tron, ig-ni-tron] 
| a cathode-arc vacuum tube with an auxiliary electrode projecting into a pool of mercury: it conducts current when the anode is positive. |
| ig·ni·tron (ĭg-nī'trŏn', ĭg'nĭ-) n. A single-anode, mercury-vapor rectifier in which current passes as an arc between the anode and a mercury-pool cathode, used in power rectification. [igni(te) + -tron.] |
ignitron
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
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