"Electric carbon-candles", after Jablotsjkow

Arc lamps require a mechanism to regulate the distance between the carbon rods. To avoid such a roundabout construction, the Russian engineer Paul Jablotsjkow (1847-1894) devised these ‘electric candles’. They consist of two thin, parallel carbon rods, separated by insulating gypsum and placed in a metal stand with electric contacts. When unused, these rods are connected to each other at the top by a piece of graphite, for the electric ignition. As soon as an electric current runs through the graphite, it will burn and an arc is formed. The gypsum melts in the same rate as the burning carbon rods. The carbon rods have to be supplied with alternating voltage, so that both carbon rods burn equally. The distance between the ‘candles’ increases while burning, and this causes the intensity to decrease gradually.