Ignitron

A large graphite or refractory metal cylinder, held above the pool by an insulated electrical connection, serves as the anode.

An igniting electrode (called the ignitor), made of a refractory semiconductor material such as silicon carbide,[1] is briefly pulsed with a high current to create a puff of electrically conductive mercury plasma.

The plasma rapidly bridges the space between the mercury pool and the anode, permitting heavy conduction between the main electrodes.

[2] Ignitrons were long used as high-current rectifiers in major industrial and utility installations where thousands of amperes of AC must be converted to DC, such as aluminum smelters.

The anodes in these devices are often fabricated from a refractory metal, usually molybdenum, to handle reverse current during ringing (or oscillatory) discharges without damage.

Although the basic principles of how the arc is formed, along with many aspects of construction, are very similar to other types of mercury-arc valves, ignitrons differ from other mercury-arc valves in that the arc is ignited each time a conduction cycle is started, and then extinguished when the current falls below a critical threshold.