ELECTRIC LIGHTING. 767 ELECTRIC LIGHTING. any vinevenness makes either briglit or dull spots wlien lighU'd. To corrcet this, the tilaineiit is burneJ lor a few inoinents in a livdrooailioii vapor. The spots whieli burn bri^'htest on the iilament are those of smallest diameter, anil they have a higher temperature than the rest of the Iilament. Xow carbon is deposited from the hydrocarbon vapor on to the lilment when a certain temjierature is reaehed. and the higher the temjierature the faster the deposit. The spots of higher temperature will take on the deposit of carbon faster than the rest of the Iilament. and in this way the diameter throughout becomes per- fect, even in a few moments. Incandescent lamps are generally connected in multiple — that is, each lamp is comiected across the two conductors leading from the poles of the generator. The energj" of power eonsinned in electric lamps, as in the case of other electrical apparatus, is meas- ured in watts. 740 of which are equivalent to a horsepower. The number of watts in a circuit is obtained by nuiltiplying the strength of cur- rent in amperes by the dilTerence in potential in volts, as one watt represents the energj" devel- oped by a current of one ampere, with a difference of potential of one volt. In electric lighting the chief consideration is to obtain the maximum amount of light with the least expenditure of energy-. An ordinary arc lamp may require 450 watts, or a current of 10 amperes of current at 4.5 volts, while a lO-eandle power incandescent lamp might utilize '-^ ampere at 110 volts, or 5.5 watts. The actual energy' required is increased by losses in the conductors leading to the lamps. Incandescent Iami)s now range from 1 candle-power or less, if desired, to, say. 100 candle-power or more, and arc lamps from some 350 candle-power, in the case of the closed lamps, to 2000 candle-power. A far high- er power is obtained in lamps designed for spe- cial purpose, such as search-lights (q.v.). In- candescent lamps are generally made to operate at voltages of 50 to GO. 100 to 120. and 200 to 250, but special types are constructed with which much lower voltages are employed, such as bi- cycle lamps, taking 'i ampere of current at 5 volts, and giving •'■4 candle-power ; and surgical lamps using about 1 am|)ere of current at 3 V(dts and giving '-j candle-power. Incandescent lamps may be run in series — as in the case of advertis- ing signs — on a 110-volt circuit, where each lamp consumes 0.33 amperes at V2 to 15 volts, and gives 1 candle-])ower-. or series incandescent lamps may be placed on a lO-ampere arc circuit, where they utilize about ly volt per candle-power. As already stated, the ordinary open-are direct- current lamp operates at 45 volts, using from 61^ to 10 amperes. The inclosed arcs may run some 50 per cent, higher in voltage. Meters of various kinds are used to indicate or record the amperage and voltage of electric currents and the energv- or power consumed in watts. See Elec- tric SIeter. C.KNDLE-PowER is a verj" misleading term, inas- much as the luminosity varies with the angle at which the light is emitted ; in the case of arc lamps it is greatest at an angle of 40° below the horizon- tal. It is spoken of as mean horizontal and mean spherical, according to the direction of the light- rays, whose average is taken, or else the maxi- mum value is ascertained and used. The mean spherical candle-power is the most, and the mean horizontal is the least, significant; but since the latter is the easiest and the former the hardest to measure, and as they frequently approximate in value, the horizontal is the one most commonly observed. Soiniiial cundk-ijoivir is an ordinary and generally misleading term. It is assiuned that an arc consuming 450 watts yields 2000 candle-power, which may not vary consideral)ly from the trvith if maximum instead of spherical candle-power is meant ; but it cannot in any case be considered as anything more than an ap])roxi- mation. It is usually far better that lighting contracts be based on the energy consumed, ex- |)ressed in watts, which may be readily and auto- matically measured and recorded, rather than on candle-power, a method which is uncertain at best and requires frecpient |)hotometric measure- ments to insure a fulfillment of contract. On the other hand, if the lamps, including the globes of arc lamps, are poorly maintained, nuich of the cnergj' 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 arc-lamp carbon is S to 10 hours ; under favor- able conditions this, in inclosed arc lamps, may amoimt to from 100 to 120 hours, or perhaps more. The life of incandescent lamps — that is, the useful lite — ranges from 400 to t)00 hours for lamps having an initial efficiency of 3.1 watts per candle-power to IGOO hours for 4 watts. The Nerxst Lamp, invented by Professor Xernst. of Gottingen. employs a small rod or strip of refractory material which is an insulator at ordinary temperatures, but becomes a good conductor and luminant at high temperatures. This is used in place of the carbon filament of HOLDER FOR GLOWER OF XERNST LAMP. the incandescent lamp, and is not inclosed in a vacuum. 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 lamp is first lighted, a temper- ature of 700° F. being necessary. Vaci'I'M TfBEs for lighting have been pro- posed by Nikola Tesla, P. Cooper Hewitt, and
