Edison Screw

The Edison screw fitting is a system of screw mounts used for light bulbs, developed by Thomas Edison and licensed starting in 1909 under the Mazda trademark. The bulb has a threaded metal base which screws into a matching socket. Most have a right-hand threading that tightens when the lamp is turned clockwise. Some lamps and fittings have a left-hand thread to deter theft, since they cannot be used in other light fixtures. The Edison screw base became popular and eventually displaced competing standards for lamp bases for general lighting purposes in several places, especially in North America and continental Europe. In Britain and countries with a historical connection to Britain, the bayonet cap (BC) is the usual mount. In the early days of electrification, the Edison screw-base socket was the only standardized connector, and even non-lighting appliances sometimes connected to power via a light socket fitting. Today Edison screw lamp bases and fittings are made to dimensions prescribed by international standards and are used for general and specialty lamps.