PHOTOMETRY AND ELECTRIC LIGHTING. 347 



3.1 watts per candle is taken as 510 hours, and the falling off of 

 the candle-power of a lamp with age is ignored. Under these 

 conditions the total cost of the light is a minimum for an efficiency 

 of about 2.8 watts per candle with a life of 290 hours. The costs 

 given in the above table take no account, however, of the trouble 

 involved in the replacing of old lamps by new ones, which is 

 sometimes very considerable, and therefore the most satisfactory 

 efficiency is somewhat more than 2.8 watts per candle. 



The remarkable property of the carbon -filament glow lamp, as 

 shown in the above table, is that great changes of candle-power, 

 efficiency, and life are produced by a slight change of voltage. 

 Thus a one per cent, increase of voltage steadily maintained causes 

 a five per cent, increase of candle-power, about three per cent, 

 decrease in the watts per candle, and about twenty per cent, 

 shortening of life. It is this great sensitiveness of the carbon- 

 filament glow lamp to changes of voltage that makes it necessary 

 to avoid great changes of voltage in the operation of such lamps. 



In the purchase of glow lamps the purchaser should specify 

 voltage, candle-power, and efficiency, and of course also the style 

 of base. In meeting these specifications the manufacturer must 

 of course be allowed some lee-way for it is impossible to make 

 large numbers of lamps exactly alike. It is the usual practice for 

 the manufacturer to submit to a purchaser a package of 200 lamps 

 rated at the desired voltage, candle-power and efficiency. The 

 purchaser may then select at random from this package ten lamps 

 to be tested at the rated voltage. If the tests show that any one 

 of the ten lamps gives a candle-power 7. 5 per cent, above or below 

 the rated candle-power, or if the average candle-power of the ten 

 lamps is more than 2.5 per cent, above or below the rated candle- 

 power, or if any one of the ten lamps consumes more than 5-5 

 per cent, above or below the rated watts, or if the average con- 

 sumption of the ten lamps is more than 2.5 per cent, above or 

 below the rated watts, the whole package of 200 lamps may be 

 rejected by the purchaser. The extent to which the individual 

 lamps in a lot deviate from rated candle-power and watts is shown 



