346 



ELEMENTS OF ELECTRICAL ENGINEERING. 



the details of behavior of lamps ranging from 4.39 watts per candle 

 to 2.28 watts per candle. 



VARIATION OF CANDLE-POWER, EFFICIENCY AND LIFE,* AND 



COSTS. 



Life of 3.1 watt-per-candle lamp is 510 hours. 



The cost per hour of the light supplied by a glow lamp includes 

 the cost per hour of the electrical power, and the first cost of the 

 lamp divided by the life of the lamp in hours. Column A in the 

 above table is the cost per hour of the power for 1,000 candles ; 

 column .Z? gives the lamp cost multiplied by i ,000 and divided by 

 the product of candle-power and life in hours ; and column Ogives 

 the total cost per hour of i ,000 candles. In calculating these costs 

 electrical power is reckoned at 10 cents per kilowatt-hour, the 

 cost of a 1 6 candle-power lamp is taken as 20 cents, the life at 



*From General Electric Company, Harrison, N. J., March, 1905. Candle-powers 

 in this table are mean horizontal candle-powers. 



