Electric Light
a brilliant light emitted by the white-hot points of two pieces of carbon when used as the electrodes of a powerful voltaic battery, or other generator of electric currents; also the light emitted by the incandescence of a metallic wire, or carbon filament, 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 voltaic arc. He used as electrodes points of charcoal. Foucault and later experimenters replaced these by pencils of gasretort 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 distinct trade both here and in Europe. Coke, 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 commercial importance. The magneto-elec- ELECTRIC LIGHT tric machines which followed upon Faraday's discovery were soon many in number, each one exhibiting some improvement 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these pioneer machines may be mentioned that of Siemens, who in 1854 introduced a new form of armature or coil, which superseded the bobbins formerly used; Wilde, of Manchester, who produced a powerful machine in which the electromagnet was first employed in this connection, it being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 machine gives a continuous current like that afforded by a voltaic battery. An electric current sent through a bridge of vapor or arc between electrode conductors produces the arc light. A varia- 1. 2. 3.
ELECTRIC LIGHT-TYPES OF ELECTRIC BULBS
1. Carbon Lamp 2. Mazda Lamp 3. New Mazda 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 centra station or plant of more or less dimen sions according to the volume of curren in requisition. The organization of high voltage power has made it possible t distribute electric current over an even widening radius. In cities in the Unite States the stations of the electric light ing systems have been shown to be capa ble of development to the extent transmitting current over whatever are required, but the clearest demonstratio is made by the power stations at N agara, from which center current transmitted to cities in some cases a most 200 miles distant. There is pe haps no larger electric-lighting syste in any country than that owned in CH ELECTRIC LIGHT 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 aluminium and copper filaments are largely used in these systems of distribution, but copper being superior in conductivity is the wire most in use. Aluminium, 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 expensive, and the more easily manageable for repairing purposes. It was recognized, however, that the æsthetic results were not quite desirable and as the system developed the underground cable was introduced 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 district 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, constant current, system in which the current is sent from a dynamo through each lamp successively returning to the dynamo without subdivision. Regulators keep the current constant, the voltage being differentiated according to the number of lamps. Carbon, flame carbon, metallic oxide and mercury are lamps may be used or incandescent lamps using filaments of carbon, metallized 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 external circuit. There are various divisions 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- ELECTRIC RAILWAYS cent lamps the tungsten metal filament has been found very efficient. Its denseness 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 nitrogenfilled tungsten lamp has been highly developed for the lighting of thoroughfares. 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 Göttingen University had considerable vogue before the advent of the tungsten lamp. Rods of earth oxide, such as yttria and zirconia, were used as illuminating elements, but as these materials were lacking in 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 coverings has led to considerable artistic development. The concealment of the lamps by various devices and the projection of the direct lighting onto white ceilings are methods that have come largely into vogue with a resultant diffusion of light resembling daylight.