Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . d along the scale. To operate the instrument,move the screen to such a point that the grease spotbecomes invisible from either side. The two candle-powers are now to each other as the squares of theirdistances from the screen. For instance, suppose thelamp A is a standard 16 candlepower lamp and at thepoint where the grease spot is invisible the distancefrom B to the screen is 20 in., and from A to thescreen 40 in., then B is to A as 20 squared is to 40squared, or as 400 is to 1,600 or one-fourth
Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . d along the scale. To operate the instrument,move the screen to such a point that the grease spotbecomes invisible from either side. The two candle-powers are now to each other as the squares of theirdistances from the screen. For instance, suppose thelamp A is a standard 16 candlepower lamp and at thepoint where the grease spot is invisible the distancefrom B to the screen is 20 in., and from A to thescreen 40 in., then B is to A as 20 squared is to 40squared, or as 400 is to 1,600 or one-fourth asgreat; therefore B is a 4 candlepower lamp. Care must ELECTRICITY FOR ENGINEERS 191 be taken that the two lamps are burning at just theproper voltage, otherwise the comparison will not beaccurate. Instruments of the kind just described aremade in a variety of different patterns, but their prin-ciple remains the same. Candlepowers may also becompared by a method known as Rumfords. Take apencil or other opaque rod and place it in front of awhite piece of paper or light-colored wall. Now place. in front of it, but separated so that there will be twoseparate shadows cast, the two lamps to be moving one of the lamps away from, or closer to,the screen, at some point the two shadows will becomeof the same density. Now measure the distances of thetwo lamps from the screen, and their candlepowerswill be to each other as the squares of the distances;(Fig. 107). 192 ELECTRICITY FOR ENGINEERS If the current consumed by a lamp and the voltagemaintained at its terminals are measured by an amme-ter and voltmeter, as shown in Fig. 106, we need onlyfind the watts (current times volts) and divide by thecandlepower to find the efficiency of the lamp. If, forinstance, the 4 candlepower lamp B is taking J ampereat no volts, we have 18^3 watts expended on it; thisdivided by the candlepower 4 shows an efficiency of4T\ watts per candle. CHAPTER XIII Storage Batteries. The storage battery
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