396 



Prof. Ayrton and Mr. Medley on 



Exactly what these changes in the equation mean will be 

 best shown by working out the results for a particular type 

 of lamp, and for this purpose we have chosen the American 

 make tested by Professor Thomas, the life curves of which are 

 shown marked M in fig. 3. 



It is clear, from these curves, that knowing the initial 

 candle-power, in this case about 12'5, we can calculate at 

 any time during the life of a lamp the total candle-hours 

 obtained and the average watts consumed per candle during 

 that time, and by substituting these quantities in the equation 

 of cost we can find the cost per hour per candle averaged over 

 the number of hours considered. 



Fig. 5.— Cost Curves calculated for American make M in fig. 3. 



0-05 



cm oo-4 



c 



W 



o 

 O 



o-o.°- 



o-o: 



1000 



400 600 



Time in Hours. 



The curves in fig. 5 show the results we have obtained 

 when these calculations are made at different times in the 

 life of a lamp, and for different prices of lamps, and of power. 



