392 Prof. Ayrton and Mr. Medley on 



advanced the matter beyond the point at which it was left 

 by the considerations contained in the paper on " The Most 

 Economical Potential Difference, &c," read before this Society 

 in 1885 ; for he took no account of the changes which more 

 recent experiments had shown to occur in lamps during their 

 life, and which render any results obtained by methods that 

 disregard these changes useless for practical purposes. 



Numerous experimenters had found out that there was a 

 serious diminution in the light emitted by a lamp as it grew 

 old, and also a very considerable rise in the watts consumed 

 per candle. Further, there was evidence to show that the 

 shape of the curve between candle-power and time was not the 

 same for different pressures ; so that it had come to be recog- 

 nized that, in order to determine the P.D. at which lamps 

 should be run, it was useless to make a comparison merely 

 between the efficiency of lamps when new and the number of 

 hours they will last at various voltages. 



Moreover, this falling-off in the quality of a lamp as it ages 

 led people to consider another point, viz., Might it not be 

 possible, in consequence of this deterioration, for a lamp to be 

 economically dead before the filament had actually broken ? 

 For the increased cost of the current required, compared 

 with the light given out, might more than overbalance the 

 expense of replacing the lamp with a new and, therefore, more 

 brilliant one. 



Before describing the results which we have obtained re- 

 garding this interesting question of the existence of a point, 

 called by the Americans the " smashing point," beyond which 

 a lamp cannot be economically used, it will be well to shortly 

 indicate the general conclusions to which previous experimen- 

 ters have been led from tests on the modern glow-lamp. 



In November 1892 Mr. Feldman published a table compiled 

 from the results of tests made by Prof. Thomas and Messrs. 

 Martin and Hassler in America,- by M. Haubtmann in 

 France, and from some of his own measurements, which con- 

 tained mean values for more than 500 lamps taken from 28 

 different factories and representing 49 different types. He 

 divided the lamps into five groups according to their initial 

 efficiencies, and gave for each group the average candle-power 

 in per cent, of the initial candle-power and the average watts 

 per candle at every hundred hours in the lives of the lamps. 

 The curves in figs. 1 and 2 are drawn from the figures given 

 in this table, fig. 1 showing the candle-power and fig. 2 the 

 watts per candle as the lamps grow old. The numbers L, II., 

 111., IV., and V. on the curves refer to the initial efficiencies 



