mercury arc rectifier
A rectifier which converts alternating current to direct current, using a cathode tube with a pool of liquid mercury. Now superseded by solid-state rectifiers.
mercury arc rectifier: high-power AC-to-DC conversion via ionized mercury vapor
A mercury arc rectifier is a large glass or steel tube containing a pool of liquid mercury at the bottom and a cathode immersed in it, with an anode mounted above. When alternating current is applied, the mercury vaporizes and ionizes, allowing electrons to flow from cathode to anode during the positive half-cycle of AC input. The mercury vapor conducts in one direction only, converting alternating current into direct current with relatively little voltage drop across the tube itself, typically 10 to 15 volts.
The device operates at temperatures around 700 to 900 degrees Celsius at the arc column. Multiple anodes could be added to the same mercury pool to create multiphase rectifiers: a three-anode tube could convert three-phase AC input simultaneously. The tube required a starter electrode or ignitor to initiate the arc, since mercury vapor does not conduct until ionization begins. Once running, the arc was self-sustaining as long as current flowed through the device.
Applications and limitations
Mercury arc rectifiers dominated heavy industrial and utility applications from the 1920s through the 1960s. They were used to supply DC power to streetcars and railway systems, to convert AC power for electroplating and electrolysis, and in early telephone exchanges. They could handle much higher currents than copper oxide or selenium rectifiers available at the time, with some units rated for thousands of amperes.
The technology had serious drawbacks. The tube had to be mounted vertically to keep the mercury pool in place. Mercury vapor was toxic and leakage posed an occupational hazard. The mercury had to be periodically cleaned of oxides. The tube had a limited lifespan; electrodes eroded, and the glass envelope could crack from thermal stress or mechanical shock. Starting the device required care, and a tube that failed could introduce DC offset into the circuit during the restart sequence.
Solid-state rectifiers, first silicon diodes in the 1950s and later thyristors and other semiconductor devices, made mercury arc tubes obsolete. They required no warm-up time, generated less heat, were compact, handled transients better, and posed no toxic hazard. By the 1970s mercury arc rectifiers had almost entirely disappeared from new installations, though some remained in service for decades afterward.