Mercury-Arc Tube

a general name for ion devices with a self-sustaining arc discharge and a liquid mercury cathode. Mercury-arc tubes are used primarily as electric valves in powerful industrial rectifiers and inverters or as controlled dischargers in pulsed systems. A mercury-arc tube has a hermetic (usually metal) envelope containing a mercury cathode, a main (graphite or steel) anode, and such additional electrodes as a control grid, deionization filter, excitation anode, and ignition electrode.

The pressure of the residual gas inside the envelope amounts to 10^-2–10^-3 newtons per square meter. A small part of the cathode surface, a cathode spot, serves as a source of electrons. During that part of the alternating potential difference period when the mercury arc tube possesses high conductivity, a self-sustaining arc discharge is established between the cathode and the main anode in the mercury vapor formed from the evaporation of cathode mercury. Depending on the method used to control the instant of igniting the arc discharge, mercury-arc tubes are classified as ignitrons or excitrons. The operating voltage at the main anode accounts for a second classification, that between low-voltage tubes (as a rule, up to 5–10 kilovolts) and high-voltage tubes (usually above 50 kilovolts).