Mercury-vapor lamp

The mercury-vapor lamp emerged from Peter Cooper Hewitt's converted greenhouse workshop in New York City, but its roots stretched back through half a century of gas discharge experiments. Heinrich Geissler's 1857 tubes had proven that electricity passing through low-pressure gas produces light. His mercury vacuum pump—using falling droplets to create previously impossible vacuums—enabled the precision needed to control gas discharge. J.J. Thomson's 1897 discovery of the electron provided theoretical understanding. What remained was engineering a practical device.

Hewitt filed his patent on September 17, 1901, creating the first efficient alternative to Edison's incandescent bulb. Where incandescent lamps converted only 5% of electrical energy to light, Hewitt's mercury-vapor tubes achieved 35-65 lumens per watt—a revolutionary improvement. The bluish-green light rendered colors poorly, but in an age of black-and-white photography, color didn't matter. Photo studios adopted the lamps immediately, desperate for bright light that enabled shorter exposure times.

George Westinghouse backed the Cooper Hewitt Electric Company in 1902. By 1914, General Electric had assumed control, eventually moving production to Hoboken. The lamps found applications Edison's bulbs couldn't serve: factories, warehouses, the Golden Gate Bridge from 1937 to 1972. More consequentially, low-pressure mercury discharge proved extraordinarily efficient at producing ultraviolet light—82% of output at the germicidal 253.7nm wavelength. Water treatment, air purification, and medical phototherapy all descended from Hewitt's tubes.

The greatest cascade came in the 1930s. Researchers realized that coating mercury tubes with phosphors could convert UV to visible light with far better color rendering. The fluorescent lamp—introduced commercially in 1936—inherited mercury vapor technology while solving its color problem. The oldest high-intensity discharge technology had spawned its successor, but mercury vapor's direct descendants still illuminate industrial spaces more than a century after Hewitt's patent.