The carbon button lamp is a single-electrode incandescent lamp invented by Nikola Tesla. A carbon button lamp contains a small carbon sphere positioned in the center of an evacuated glass bulb. This type of lamp must be driven by high-frequency alternating current, and depends on an electric arc or perhaps a vacuum arc to produce high current around the carbon electrode. The carbon electrode is then heated to incandescence by collisions by ions which constitute the electric current. Tesla found that these lamps could be used as powerful sources of ionizing radiation.
In February, 1892, Tesla gave a lecture to the Institution of Electrical Engineers, in which he described the carbon button lamp in detail. He also described several variants of the lamp, one of which uses a ruby drop in place of the carbon button. Some enthusiasts have argued that this is an early ruby laser.