420 Prof. Ayrton and Mr. Medley on 



From these tests it is seen that, in the case of lamps 

 Nos. XI. and XIV., there was practically no change either in 

 the light or in the power absorbed during the first 80 hours 

 of the running at the constant pressure of 100 volts ; and 

 that in the case of the remainder of this batch of lamps the 

 increase in the light and in the power absorbed was compa- 

 ratively small, the greatest change being with lamp No. XL, 

 where the light rose by about 11 per cent, and the power by 

 about 4 per cent. This is a very different result from that 

 obtained with the Edison-Swan lamps previously tested, and 

 run at a constant pressure of 100 volts, for there the total 

 light given out by the three lamps together rose by 35 per 

 cent, in the first 100 hours of running, and the power by 

 7*55 per cent. 



Since, then, as already stated, the vacuum, when tested 

 periodically with the induction-coil, appeared to improve 

 steadily with each of the six lamps Nos. IX. to XIV. during 

 the run of 80 hours, it would appear that improvement in the 

 vacuum was not the sole cause, as stated by Mr. Howell, for the 

 great rise in the candle-power such as we observed with all 

 the Edison-Swan lamps which we tested in 1893 and 1894. 



It is possible that this rise in candle-power may have been 

 due to a change in the surface of the filament causing the 

 emissivity for heat to decrease, since that would raise the light 

 emitted, as well as the number of candles per watt. Whether 

 such a change in the heat- emissivity of the filament occurs 

 with time we have not yet found out. 



Another point of difference between this batch of six lamps 

 Nos. IX. to XIV. and those which we tested during the main 

 part of our investigation, is the relative inefficiency of these 

 six lamps. For in no single test at any time during the 80 

 hours run with these six lamps were the watts per candle less 

 than 4*25, and in many cases they were over 5 ; while with 

 lamp No. XIII. the watts per candle, as seen in the last 

 table, were as high as 5*15 after this lamp had been run for 

 13 hours 50 minutes. It is possible, then, that when in the 

 body of the paper we spoke of the group of three lamps 

 which we ran at a constant pressure of 101 volts as being 

 " quite abnormal," because the average watts per candle 

 required with this group during the run of 1340 hours were 

 as high as 4" 68, we ought rather to have spoken of the 

 various othe* groups of lamps which we ran at the constant 

 pressures of 100, 102, and 104 volts respectively as being 

 abnormally good Edison-Swan lamps. 



At any rate it is clear that it is at present impossible to 

 state the most economical potential difference to employ with 



