IGNITRON

electron tube functioning as a rectifier to convert alternating current to direct current. Each conduction cycle is started by an external voltage applied to the igniter, a small element 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 is reduced to the point that the arc can no longer be sustained. Large ignitron devices may be built inside vacuum tanks instead of tube envelopes. Electrical welding equipment incorporates an ignitron as a heavy-duty relay.