Tests of Glow-Lamps. 409 



was from 30*6 to 35, or a rise in 120 hours of 14*4 per cent., 

 the current in the same time rose from 1/26 to 1*31 amperes, 

 or by 3*97 per cent., so that the watts per candle diminished 

 from 4-22 to 3*82. 



The average consumption of energy during the first 500 

 hours was at the rate of 3' 9 watts per candle, during the 

 second 500 hours 4'06 watts per candle, during the third 500 

 hours 4*2 watts per candle, and over the whole run the 

 average was 4*12 watts per candle, corresponding with an 

 efficiency of 0-243 candle per watt. 



Turning to the group run at 104 volts, the curves show 

 that the breakages were very frequent. Altogether five 

 lamps broke, their lives being 94, 175, 210, 242, and 254 

 hours ; the three lamps left unbroken at the end of the test 

 had been run for' 165, 222, and 364 hours. 



The initial rise in candle-power was from 33*8 to 40*5 

 candles, that is 19*8 per cent.; and the current rose from 

 1*264 to 1*307 amperes, or by 3*4 per cent. 



Over the whole run the average watts per candle, for the 

 lamps run at 104 volts, were 3*6, corresponding with an 

 efficiency of 0*278 candle per watt. 



It is known from preceding tests (see the 6 Electrician/ 

 July 15th, 1892, for example) that the light given out by a 

 new glow-lamp varies approximately as the seventh power of 

 the pressure when the pressure is something like the normal 

 pressure for the lamp. Now, as the groups of lamps that we 

 tested at 100, 102, and 104 volts respectively were selected 

 so that each group gave practically the same light initially 

 when tested at the same pressure of 100 volts, it was to be 

 expected that the three groups run at the three different 

 pressures would follow the law for the light given out by the 

 same lamp when used at different pressures. 



And this is practically the case, for if Lioo, Lio2, and L104 

 be the number of candles emitted by the three groups of three 

 lamps each, we see from the curves on fig. 10 that at the start 

 L 1 oo=26*3, Lio2=30*6, Li 4=32'5 ; 



.*. ^ 2 = 1*16, and^ = 1*24. 



Hence the light is roughly as the seventh power of the pres- 

 sure at the start. 



But after the lamps have been glowing for 100 hours this 

 relationship no longer holds, for after 100 hours from the 

 start we see from the curves on fig. 10 that 



Lioo = 33*9, Lio2=34*l, Li 4=39*8, 



■L102 -, L104 -, 17 

 or -j — =1, 1 — = 117, 



JU100 Moo 



