ELECTRIC LIGHT, a brilliant light emitted by the white-hot points of two pieces of carbon when used as the elec trodes of a powerful voltaic battery, or other generator of electric currents; also the light emitted by the incandes cence of a metallic wire, or carbon fila ment, when subjected to the passage of an electric current.
In 1809 Sir Humphry Davy, while experimenting with a powerful battery, discovered the phenomenon of the vol taic arc. He used as electrodes points of charcoal. Foucault and later experi menters replaced these by pencils of gas retort carbon, and this material is yet used in some forms of regulators. A better result, however, is obtained from manufactured carbon pencils, and this manufacture already represents a dis tinct trade both here and in Europe. lampblack, cane sugar, etc., are the ingredients used for these pencils, which are subsequently placed in molds and submitted to a red heat.
The discovery by Faraday (1830) that an electric current could be induced in a coil of wire by the approach to it or recession from it of a magnet may be said to have given electricians the first hope of giving the electric light a com mercial importance. The magneto-elec tric machines which followed upon Fara day's discovery were soon many in num ber, each one exhibiting some improve ment upon its predecessor. Of these pioneer machines may be mentioned that of Siemens, who in 1854 introduced a new form of armature or coil, which superseded the bobbins formerly used; Wilde, of Manchester, who produced a powerful machine in which the electro magnet was first employed in this con nection, it being excited by a permanent or ordinary horseshoe magnet.
In England the Gramme machine was first used in 1874, to provide a light for the summit of the Westminster clock tower. Since that date it has been greatly improved. The Gramme ma chine gives a continuous current like that afforded by a voltaic battery. An electric current sent through a bridge of vapor or arc between electrode con ductors produces the arc light. A varia tion is the incandescent lamp in which a wire of carbon or metal of more or less resistance is mounted in an exhausted glass bulb. The reaction of the filament to the current generates a heat which glows into vivid incandescence. Dynamos moved by water power, gas or steam, generate the electric current at a central station or plant of more or less dimen sions according to the volume of current in requisition. The organization of high voltage power has made it possible tc distribute electric current over an ever widening radius. In cities in the United States the stations of the electric light ing systems have been shown to be capa ble of development to the extent of transmitting current over whatever area required, but the clearest demonstration is made by the power stations at Ni agara, from which center current is transmitted to cities in some cases al most 200 miles distant. There is per haps no larger electric-lighting system in anv country than that owned in Chi cago by the Commonwealth Edison Co., in which current is distributed by four stations with a total capacity of 320,000 kilowatts, one single generating unit being of 35,000 kilowatts. Both alumin ium and copper filaments are largely used in these systems of distribution, but copper being superior in conduc tivity is the wire most in use. Alumin ium, owing to its lightness, is found especially valuable in long-distance work, and the two metals, differing in density and consequently in bulk, have to be employed variously, having regard to the requirements. When electric
lighting first became general, overhead installation was the method most in vogue as being the easier and less expen sive, and the more easily manageable for repairing purposes. It was recognized, however, that the aesthetic results were not quite desirable and as the system developed the underground cable was in troduced in the larger cities. Despite the larger expense it was considered that underground cables being protected by conduits were less liable to accident, while the unsightly overhead wires were removed. Of the fundamental divisions of the distributing systems the parallel or constant-potential type is in general vogue for general power and lighting. Direct-current generators are used in isolated plants and alternating-current generators in other systems. Where the system is large, as in the central dis trict of cities, substations receive the alternating current at high potential and convert it by dynamo machines to direct current, which is distributed by a three-wire system. The alternative to the parallel system is the series, con stant current, system in which the cur rent is sent from a dynamo through each lamp successively returning to the dy namo without subdivision. Regulators keep the current constant, the voltage being differentiated according to the number of lamps. Carbon, flame car bon, metallic oxide and mercury arc lamps may be used or incandescent lamps using filaments of carbon, metal lized carbon, tantalum or tungsten. In the arc lamps are the two electrodes with regulators establishing the arc and feeding the electrodes as they burn, the connection being maintained with an ex ternal circuit. There are various divi sions among arc lamps, such as the parallel and series type, and open and inclosed arcs. In carbon arcs cylindrical electrodes are used prepared from gas coke or petroleum coke. The metallic oxide arc is provided with a positive electrode of copper, while the mercury arc can be produced only in exhausted tubes of glass or quartz. In incandes cent lamps the tungsten metal filament has been found very efficient. Its dense ness having been overcome, its high melting point rendered it possible to heat it without too speedy evaporation. It has a conductivity much superior to carbon, which is apt to evaporate much below its boiling point. The nitrogen filled tungsten lamp has been highly de veloped for the lighting of thorough fares. The average incandescent lamp is exhausted by methods of air pumping supplemented by the consumption in each chamber of phosporus compound. The lamp invented by Dr. Nernst of Gottingen University had considerable vogue before the advent of the tungsten lamp. Rods of earth oxide, such as yttria and zirconia, were used as illu minating elements, but as these ma terials were lacking in conductivity when cold they had to be supplemented with an external heating apparatus. As success was attained in the use of luminous elements, methods had to be found to temper the glare. As a result opportunities for the use of variously formed and colored lamps and glass cov erings has led to considerable artistic development. The concealment of the lamps by various devices and the pro jection of the direct lighting onto white ceilings are methods that have come largely into vogue with a resultant dif fusion of light resembling daylight.