INCANDESCENT LAMP

any of various devices that produce light by heating a suitable material to a high temperature. When any solid or gas is heated, commonly by combustion or resistance to an electric current, it gives off light of a colour (spectral balance) characteristic of the material. In 1801 Sir Humphrey Davy demonstrated the incandescence of platinum strips heated in the open air by electricity; but the strips did not last long. Frederick de Moleyns of England was granted the first patent for an incandescent lamp in 1841; he used powdered charcoal heated between two platinum wires. The first practical incandescent lamps became possible after the invention of good vacuum pumps. The Englishman Sir Joseph Wilson Swan in 1878 and the American inventor Thomas Alva Edison in the following year independently produced lamps with carbon filaments in evacuated glass bulbs. Edison has received the major credit because of his development of the power lines and other equipment needed to establish the incandescent lamp in a practical lighting system. In 1911 the drawn tungsten filament was introduced; in 1913 filaments were coiled, and bulbs were filled with inert gas. Nitrogen alone was used first, and, later, nitrogen and argon in proportions varied to suit the wattage. These steps increased efficiency. Beginning in 1925, bulbs were frosted on the inside with hydrofluoric acid to provide a diffused light instead of the glaring brightness of the unconcealed filament. The carbon-arc lamp, a very bright electric lamp used for spotlights and in motion-picture projection, consists of two carbon electrodes with a high-current arc passing between them. The light comes from heating of the electrodes and from the ionized gases in the arc. Nonelectric incandescent lamps include the gas-mantle lamp. The mantle is a mesh bag of fabric impregnated with a solution of nitrates of cerium and one or more of the following metals: thorium, beryllium, aluminum, or magnesium. The mantle is fixed over an orifice carrying a flammable gas such as natural gas, coal gas, propane, or vaporized benzene or other fuel. When the gas is ignited, the mantle fabric burns away, leaving a brittle residual lattice of metal oxides. Light is produced when this lattice is heated to glowing by the gas combustion, although the mantle itself does not burn. Gas lamps may operate without mantles. The limelight is a very bright gas lamp, invented in 1825 and widely used for theatrical lighting until about 1900. It consists of a block of lime (calcium oxide) heated in an oxyhydrogen flame.