ELECTRIC LIGHTING. 764 ELECTRIC LIGHTING. sation. He placed liis two carbons side by side, vertically, with baked kaolin between them, and did away willi the carbon feed inechanisui. It was only an ingenious arrangement which did not come into use. In the same year, however. Charles F. Brush, of Cleveland, Ohio, devised a complete system oi arc electric lighting, including a special form of dynamo and lamps arranged in series. About the same time the Thomson- Houston system was developed, as well as a number of others. The Edison incandescent lamp was first exhib- ited in 1S79 in Edison's laboratory, at ilenlo Park, X. J., and three years later, at the Pearl Street Edison lighting-station in New York, the first incandescent plant was put in operation. Of the previous attemjjts at incandescent lamps the most promising was one descrilicd in 1845 in a British patent to an American named Starr, who died when only 2.5 years old. In ISS.i or 1886 occurred the first use of a transformer (q.v.. also see below). Xotable develoi)ments of more re- cent years have been snp])lying both arc and in- candescent lights from the same dynamo and wire system, using the inclosed arc lamp, and the employment of rotary converters to change alternating to direct currents, or the reverse. These various improvements give great flexibility to the generation, distribution, and use of electric current for liglit or power, so that almost any local condition may bo met with ease. Central Stxtion is a term used to designate a plant at which electricity is generated to supply many customers, while wliat are known as iso- Jated stations supply the wants of one building or possil)ly a group of Ijuildings. The central station may be located many miles from the point or points where the current is utilized. Perhaps the best known example of the latter is the Niagara Falls plant, from which current is transmitted to Buffalo for electric-lighting and street-railway service, a distance of 27 miles. This transmission line is short compared with many others. ( See Power Traxsmissiox.) The central stations in large American cities contain steam generating plants of enormous capacity, from some of which the current is distributed to sub-stations. Thus one of the central stations of the Edison Electric Illuminating Company in the borough of Brooklyn. Xew York Cit.v. serves an area of 120 square miles. Its dynamos furnish alternating current which is transmitted by un- derground conduits to the sub-stations, where it is transformed to suit the needs of the several sub-districts supplied. The Xew York Gas, Elec- tric Light. Heat, and Power Cnmpanv controls six light and power coni|)anies and has an authorized capital of .'?72.flnn.nno. One of the companies controlled had, in l!tOO, a total of 5060 arc and 466.533 incandescent lamps, besides over 25.000 motors in use. The latest available summary of electric-lighting stations, given herewith, will serve to show something of the extent and char- acter of the industry in the United States as it was early in 1800. Separate figures are given for plants under public and private ownership. DiSTRiBiTiXG SYSTEMS are almost invariably of copper wire, but aliuninium is beginning to be employed extensively, though its use depends largely on the relative |>rices of the two metals. Under certain conditions it may be economical, and it is usually preferable wlierc liglitness is essential. Up to the year 1001 aluminium has been employed chielly for transmission linet,, rather than distribution proper. Copjier is used on account of its high conductivity. The con- ductivity of aluminium is only about one-half that of copper, but owing to its lesser density an aluminium conductor weighs only 01 per cent, as much as copper wire of equal length and re- sistance; on the other hand, the greater cross-sec- tion of aliuninium wire calls for more insulating material and also for more space, an important consideration in tinderground work. Wires of either material are most commonly sup])orted on poles, erected along the higliways. or less fre- quently on i)rivate land. There is an increasing tendency, however, to jilace street wires for all classes of electrical service undcrgi'ound. (For a discussion of underground wires, sec Cable, Electric, and Sibways.) Distributing systems are divided broadly into series, constant-current systems, and parallel constant-])otential systems. These may be complicated by various combina- tions, but only the simplest forms will be de- scribed here. In the .series system the current passes through each lamp on the circuit in succession and then returns (as is always the case) to the dynamo. The current is generally constant, and of high initial potential, the latter dropping at each lamp as many volts as is lost there by the re- sistance. Parallel systems employ constant po tential, or voltage, but the current varies accord- ing to the number of lamps in circuit. The lamps are placed between the two wires of the circuit, the current to each lamp returning di rectly to the station. Xearly all incandescent and many arc lamps and most electric motors are operated on this system. Three- and fire- rrire systems are designed to save copper. Taking up the three-wire system as the simplest, it may be considered as the combination of two parallel systems into one. by uniting the positive main of one circuit with the negative main of the other. If one of the lamps in a series is extingnislied the current for the other returns by the common conductor. The five-wire system is merely an extension of the same principle as the three- wire. Recent improvements in incandescent lamps permit the use of current at 220 volts or more, instead of 110 volts, thus partly doing away with the necessity for the nuiltiple-wire system. Tb.>;sfokmer.s (q.v.) are extensively employed to cliange alternating currents from low to high pressure for transmission through small wires, thus saving great expense for copper. A reversed process turns the current back to any desired low pressure for actual tise. To raise or lower the pressure of alternatinsr currents. siin])le induction coils, or static transformers, are employed. For varying the voltage of a direct current, a motor- generator must he used, while to change an al- ternating to a direct current, or vice versa, a rotary converter or transformer is employed. Arc Lamps consist of the carbons, regulating and feeding devices for the carbons and the globes, besides such accessories as automatic cut-outs, so an accident to one lamp will not lead to the extinguishment of others in the series, and light- ning arresters, the object of which i* evident. Tn .merica arc lamps are cenerallv on varying potential circuits, but in Europe they are quite as ircnerally on constant potential circuits. Carbons are made from petroleum coke, gas
