Arc lamp

An arc lamp is a lamp that makes light from an electric arc, a sustained discharge through ionized gas or vapor between two electrodes. The first practical form was the carbon arc light, in which an arc burns between carbon rods in open air; Humphry Davy is credited with inventing it in the first decade of the 1800s. The rods burn away in use, so many regulating mechanisms, mostly based on solenoids, were devised to hold the gap steady, and the Yablochkov candle was a simpler design with no regulator but only a few hours of life. Carbon arcs lit streets and large buildings widely from the 1870s until the incandescent lamp displaced them in the early twentieth century, though they stayed in use in searchlights and film projectors until after the Second World War. The name now also covers gas discharge lamps such as the xenon arc lamp, and the common fluorescent lamp is a low-pressure mercury arc lamp.