THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



MA Y 1895. 



XL. Tests of Gloiv-Lamps, and Description of the Measuring 

 Instruments Employed. By Prof. W. E. Ayrton, F.R.S., 

 and E. A. Medley*. 



THE conditions under which the glow-lamp can be most 

 economically used have attracted considerable attention 

 since a paper entitled " The Most Economical Potential 

 Difference to employ with Incandescent Lamps " was read 

 before the Physical Society in February 1885 f by Professor 

 Perry and one of the authors of the present communication. 

 In this paper they stated that it was well known, from experi- 

 ments made by their students in 1880, and from results 

 published in 1881 by Lord Kelvin, then Sir William Thomson, 

 as well as from subsequent experiments, that the light obtained 

 from an incandescent lamp increased much more rapidly 

 than the power expended in it ; or, that the number of 

 candles produced per watt of power expended in the lamp 

 increased as the filament became hotter. But it was pointed 

 out in the paper in question that such experiments by them- 

 selves gave no idea of the commercial value of any particular 

 glow-lamp, because they afforded no indication of its life 

 when run at different efficiencies, and it was known that the 

 length of time a filament would last was the less the higher 

 its temperature. 



They then proceeded to show that the cost of lighting per 

 hour per candle could be divided into two parts, viz., the cost 



* Communicated by the Physical Society : read December 14, 1894, 

 and January 25, 1895. 



t Phil. Mag. April 1885, p. 305. 



Phil Mag. S. 5. Vol. 39. No. 240. May 1895. 2 D 



