Page:Collier's New Encyclopedia v. 03.djvu/569

ELECTRIC LIGHT 495 ELECTRIC RAILWAYS 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 tung^sten 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. ELECTRIC RAILWAYS, cars driven along tracks by electric power, supplied either from a central power station, or storage batteries, the latter method being no longer in practical use. The first ex- periment made in an electrically driven vehicle, interesting from a historical point of view rather than from any practical results it attained, was under- taken by Thomas Davenport, of Bran- don, Vermont, a blacksmith with a self- developed education in electricity and mechanics. In 1835 he attempted to propel a wagon by means of a revolving electro-magnet, without any degree of success. A more significant attempt was made three years later, in 1838, by Robert Davidson, in Aberdeen, Scotland, who built a small locomotive which was able to move along a track for a con- siderable distance. In 1850 the first practical electrical locomotive was built in this country, by Prof. C. I. Page, of Washington, D. C. This electrically driven vehicle, of sixteen horse power, was tested on the tracks of the Balti- more and Ohio railroad, an3 attained a speed on a level stretch of track of nine- teen miles an hour. In both these me- chanically successful cases, however, the commercial yalUQ of the experiment was