Mercury-arc valve

The mercury-arc valve emerged from a twenty-year convergence of observations that no single inventor could have assembled alone. In 1882, Jules Jamin and G. Maneuvrier in France observed that electric current flowing through a mercury arc traveled in only one direction—the rectifying property that would eventually transform power conversion. But they lacked the context to exploit it. Ten years later, Leo Arons in Germany demonstrated that mercury vapor could be controlled commercially in his mercury lamp. The pieces were assembling.

Peter Cooper Hewitt connected them. The American electrical engineer—grandson of industrialist Peter Cooper, son of a New York City mayor—had mastered mercury arc behavior while inventing the mercury-vapor lamp in 1901. The following year, he created the first device to convert alternating current to direct current without moving parts: the mercury-arc rectifier. The glass-bulb device used liquid mercury as a cathode, emitting electrons freely while the graphite anodes emitted almost none, forcing current to flow in a single direction.

The cascade was immediate. Electric railways and streetcars needed massive DC power. The New York subway network ran on mercury-arc rectifiers. Aluminum smelting, electrochemical processing, radio transmitters—any application requiring high-current DC from an AC grid could now bypass expensive, maintenance-heavy rotary converters. By 1908, Hewitt had scaled to steel-tank rectifiers handling thousands of amperes.

The technology's final act was its most consequential. In 1939, Dr. Uno Lamm at ASEA in Sweden invented grading electrodes that enabled high-voltage applications. The 1954 Gotland HVDC link—20 megawatts at 100 kilovolts over 60 miles of submarine cable—proved that mercury-arc valves could transmit power across distances impossible for AC. Every HVDC installation from 1932 to 1972 used mercury-arc devices. The last one operated until August 1, 2012, in New Zealand. Thyristors inherited the application space, but mercury-arc valves had created the entire field of high-voltage direct current transmission.