Abstract :
Use of metal-tank rectifiers for converting alternating current to direct current has increased rapidly in the United States and Canada since 1925. Initial applications were in electric railway service and the rectifiers were of the multianode design. Direct potentials were at levels of 600, 1,500 and 3,000 volts, and capacities were of the order 1,000 to 3,000 kw. Development of single-anode mercury-arc rectifiers received a decided impetus in 1933 with the introduction of the ignitron. In the ignitron a new cathode-spot is established for each conducting cycle by passage of a current pulse through the ignitor — a small tapered rod of highly refractory material immersed in the mercury pool. The excitron differs from the ignitron in that a continuously excited cathode-spot is maintained on the mercury pool and a grid controls the starting of anode conduction electrostatically. Single anode tubes are made in both the sealed and pumped construction. Improved efficiency of the single-anode units extended the field of application for mercury-arc rectifiers to the industrial 250–600-volt levels.
