416 Prof. Ayrton and Mr. Medley on 



The price of a kilowatt-hour we took as k\d in fig. 12, and 

 as 9d. in fig. 13, the cost of anew lamp being taken as Is. 9d. 

 in both sets of curves. 



Of course, it will be seen from the formula quoted above, 

 that whenever a lamp breaks and a new one is put in its 

 place, there must be a sudden, though temporary, rise in the 

 cost per candle-hour due to the cost of the new lamp ; but 

 this rise becomes less marked as the number of lamps included 

 in the calculations increases, and would have become inappre- 

 ciable if the tests could have been carried on for a sufficient 

 length of time. 



The first three lamps to break in both the 102 and the 

 104 volt groups were replaced by the lamps which we had 

 previously run at 102 and 104 volts respectively for 110 hours ; 

 and in calculating the cost-curves we have added in the watt- 

 hours and the candle-hours due to these previous runs ; we 

 have also added to the abscissa? of the curves the 110 hours. 



On examining the total-cost curves on figs. 12 and 13, it is 

 at once apparent that they nowhere show any such rise in 

 the cost of producing light, as was seen in the similar curves 

 for American lamps (fig. 5). The curves in fig. 12, for 

 which the price of a Board-of-Trade unit was taken as 4|c?., 

 do not turn up at any point, not even for the three lamps run 

 at 101 volts, which lasted unbroken for over 1300 hours. 



In fig. 13, however, for drawing the curves on which the 

 price of a Board-of-Trade unit has been taken at 9d., there 

 are slight indications of the curves turning up : for example, 

 the curve for the 100-volt group rises a little at 850 hours, at 

 which time the lives of the three lamps being tested were 

 850, 850, and 278 hours ; and, again, at about 1600 hours, 

 when the lives of the three living lamps were 1028, 643, and 

 433 hours, the cost per candle-hour rises a little. In the 

 group run at 101 volts the cost of lighting began to rise a 

 little after the lamps had been lit for 1100 hours; and in the 

 102-volt group the curve (fig. 13) turns up a little at 1700 

 hours, at which time the ages of the lamps alight were 928, 

 678, and 552 hours. But at no one of these points is there 

 a sufficient rise to indicate that a time has been reached when 

 it would be good economy to discard lamps. 



The remaining total cost curve on fig. 13, viz. that for the 

 group of lamps run at 104 volts, when the price of the Board 

 of Trade unit is taken at dd., does not show any indication 

 of a minimum point. 



Hence, generally, we may conclude that when the modern 

 Edison-Swan 8 C.P. 100-volt lamps such as we have been 

 testing, costing Is. 9d. per lamp, are supplied with energy 



