Tests of Gloio-Lamps. 391 



therefore the total cost per hour per candle equalled 



p JQO-07545 v -11-697 _|_ H(3*7 + 10 8007_0 ' 07667t '). 



From this equation, using the same values for the price of 

 lamps and power as before, it was shown that to make the 

 cost a minimum the P.D. maintained between the lamp- 

 terminals should be 101*1 ; a result in close agreement with 

 that obtained graphically. 



At the next meeting of the Physical Society a paper was 

 read by Dr. Fleming *, in which he considered what proportion 

 the cost of lamp renewals should bear to the cost of power in 

 order that the total cost should be a minimum. As the result 

 of the examination of various experiments, he showed that for 

 any particular type of lamp the average life could be ex- 

 pressed as an exponential function of either the watts per 

 candle, or of the candle-power, or of the P.D.; that is, he 

 showed, for the lamps with which he was dealing, that 



T- A. 5 I 



W a "~ c* ~ vy' 



where L was the life, W the watts per candle, C ihe candle- 

 power, v the P.D., and A, B, a, /3, and <y constants for any 

 particular type of lamp. Then, by substitution in the cost 

 equation already referred to, he obtained the result that for 

 maximum economy the cost of power must be to the cost of 



6 — 1 

 renewals as a to unity. The values for « and /3 he gave 



for Edison lamps as 6J and 4J respectively ; so that he finally 

 arrived at the result that, whatever the price of lamps, or of 

 electrical energy, the pressure used must be such that the 

 annual cost of renewals should be about 17 per cent, of the 

 total annual cost. 



In April 1888, in a paper read before the American Institute 

 of Electrical Engineers, Mr. Howell gave numerous examples 

 of the application of Ayrton and Perry's graphical method to 

 the finding of the efficiency at which various types of lamps 

 should be run to obtain maximum economy for various prices 

 of lamps and power. 



He gave a curve showing the results of his tests of Edison 

 lamps when run at different efficiencies, but the lamps he 

 used were so abnormally good, compared with any with which 

 we are acquainted, that his experiments have no practical 

 value. For example, his lamps run at 4*5 watts a candle 

 lasted, he says, nearly 12,000 hours. 



Further, Mr. Howell's paper can hardly be said to have 



* Phil. Mag. May 1885, p. 368. 

 2 D 2 



