is a very misleading term. inas much as the Imnino,ity varies with the angle at which the light is emitted: in the vase of are lamps it is greatest at an angle of Pr below the horizon tal. It is spoken of as mean horizontal and mean spherical. according to the diree't ion of the light ray,, whose average is taken, or else the maxi mum value is a,eertained and used. The mean spherieal eandle-power is the most, and the mean horizontal is the least. significant ; hut since the latter is the easiest and the former the hardest to measaire, and as they frequently approximate in value, the horizontal is the one most commonly ,h,erved. ()Mina CII pmeer is an ordinary and generally misleading term. It is assumed that an arc consuming 430 watts yields 2000 candle-power. which may not vary considerably from the truth if maximum instead of spherical candle-power is meant ; but it cannot in ally case he considered as anything more than an approxi mation. It is usually far better that lighting contracts he based on the energy con-Ilined. ex pressed in watts, which may he readily and auto matically measured and recorded, rather than on candlepower, a method which is uncertain at best and require, frequent photonmtrie measure ments to insure a fulfillment of contract. Gn the other hand, if the including the globes of arc lamps, are poorly maintained. much of the energy produced may be lost. Candle-power is generally determined by comparing the intensity of the light in question with a standard. by means of a photometer. The life of an ordinary are-lamp carbon is S to 10 under favor able conditions this, in inclosed arc lamps, may amount to from 100 to 120 hour,. or perhaps more. The life of incandescent lamp,—that the useful life—ranges front 400 to GUI) hours for lamps having an initial efficiency of 3.1 watts per candle-power to 1600 hours for 4 watts.
The NERNST LAMP, invented by Professor Nernst. of Gottingen. employs a -'mall rod or strip of refractory material which is an insulator at ordinary temperatures, but become, a good conductor and Imninant at high temperatures. This is used in place of the carhon filament of the incandescent lamp. and is not inclosed in a in. The medium is a compound of magnesia and rare earths, such as zirconium and thorium oxides, and the lamp may be used with either direct or alternating currents. Some form of heater is required to make the medium con ducting when the ]amp is first lighted. a temper
ature of 700° F. being necessary, \".vCi'I•M Trues; for lighting. have been pro posed by Xikola Tesla, 1'. Cooper Hewitt. and others. hut up to 1902 were not put on a com mercial basis. The light is furnished by a rare lied gas, confined in a tube, through which an electric current would be passed. Long life and great efficieney and candle-power are prom iyed whenever this s•st,n, i, perfected.
The eboice of currents and lamps for electric lighting will depend largely upon local conditions. N\ here the area supplied is within a short radius of the generating stations. the direct current is used: but where the area served is large, or where the generating station is remote. as in the ease of long-distance transmission from a water power plant, a high-potential alternating cur rent is employed. with step-down transformers to reduce the potential. or rotary converters to change it to direct current. the advan tages due to the simple generating machinery (see DYNAMo - ELECTRIc and the smaller used in the alternating system. there is the cost of transformers and extra pre and dangers incident to high-potential current. The accompanying table shows the rela tive number of lamps under each system early in P;99. and also the relative number of arc and incandescent lamp- in service at the same time. Incandescent lamps are invariably used for do mestic and most office lighting, and, in general. where a thoroughly distributed light which is not glaring in its brilliancy at any point is de sired. Arc lamps are more commonly employed than incandescent lamps for street lighting, and are much used in stores and some classes of large assembly halls or rooms. The inclosed arc lamp. the use of arcs of lesser candle power. and the combination of arc and incandescent lamps on the same circuit, permit a wide range of choice to suit local needs or preferences.
BIRLtoRRAFtly. ('rocker, Electric Lighting (New York. 19011. an excellent and complete modern treatise: Houston and Kenellv, Electric Arc Lighting (New York, I S9.5) ; id., Electric Ineandecrent Lighting (New York. 1895) : Salo mon?. Electric Light Installations (3 vols.. Lon don. : Slingo and Brooker. Electric Engi neering for Electric Light Artisans and Students (London, 1898). See DYNAMO-ELECTRIC MA CHINERY for a description of dynamo and motors; STORACE BATTERIES for accumulators and storage systems: TRANSFORMERS: TRANSMISSION OF POWER: ELECTRIC METER.