Tests of Glow -Lamps. 413 



with the lamps which we tested at 100, 101, and 102 



volts the light given out by a group was never as low 



as it was at the beginning when the lamps were new. 



(3) In spite of the first rise in candle-power recorded in 



the curves on fig. 11, it was only for the lamps whose 



behaviour was recorded in curves P and Z that the 



power expended per candle diminished at first with 



time. And even in the case of these two sets the 



power expended per candle increased again, and was 



greater when the filaments broke .than it was when 



the lamps were new. Whereas in our tests the power 



expended per candle not only diminished considerably 



during the early life of the lamp, but it never rose 



again as high as it was when the lamps were new. 



Although, then, the rise in candle-power during the early 



part of the life of a glow-lamp is apparently not an absolutely 



new fact, the magnitude of the rise and the effects resulting from 



it were, in the older lamps, so trifling that no special attention 



seems to have been devoted to this important subject in the 



former reports of tests of lamps. Indeed, even in a prominent 



book connected with glow-lamps which has quite recently 



been brought out in this country, no mention whatever is 



made of the fact in question. And yet, as we have already 



shown, and as will become more apparent from what follows, 



this remarkable rise in the candle-power during the earlier 



part of the life of an Edison-Swan lamp has a very important 



effect on the economy of lighting with glow-lamps. 



The next point toconsider is the way in which the cost of 

 lighting with the modern 100-8 Edison-Sw T an lamps depends 

 on the pressure at which they are run, and on the cost of 

 a Board of Trade unit. The curves on figs. 12 and 13 give 

 the cost of obtaining light with the various groups of lamps, 

 including the cost of replacing lamps with broken filaments, 

 calculated day by day from our tests as the experiment 

 went on. 



The ordinates of the curves show the cost of 100 candle-hours 

 at any time during the test, time being measured horizon- 

 tally, and the values of the ordinates have been calculated from 

 the following formula : — 



Total cost per candle per 100 hours at the end of any 

 number of hours t equals 



1 00 v -Board °f Trade units used during time t X Price of one Board of Trade unit 



Total Candle-hours during time t 



i aa Number of lamps used during time t X Price of one lamp 

 Total Candle-hours during time t 



