THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



-*, 



[FOURTH SERIES.] 



MARCH 1874. 



XXI. On the Electric Resistance of Selenium. 

 By the Earl of Rosse, D.C.L., F.R.S. #c* 



THE recently discovered fact of the diminution of the electric 

 resistance of selenium in the crystalline state when exposed 

 to the action of light or, possibly, of radiant heat, is one which 

 naturally excites some interest beyond that arising from the 

 curious and unexpected nature of the phenomenon considered 

 by itself; for the possibility of selenium being applied to the 

 measurement of light or radiant heat invests the discovery with 

 a very general importance. 



Mr. Willoughby Smith seems to have satisfied himself that 

 light, not heat, is the active agent; but I have spoken of the 

 latter as possibly the cause of the effect observed, as Lieutenant 

 Sale's paper in the Proceedings of the Royal Society (although 

 it is therein stated that selenium is affected by light, and again, 

 that the change of resistance is not due to an alteration of tem- 

 perature) might lead one to infer that the observed effect was 

 due to radiant heat, not to light ; for he says that the actinic 

 rays produce no effect, but that it is at a maximum in the red 

 rays, or beyond them, near the maximum of the heat- rays ; and 

 inasmuch as he appears not to have determined by means of the 

 thermopile the relative calorific power of the various rays of his 

 spectrum, nor even to have reduced his results to what they 

 would have been if the normal or diffraction spectrum had been 

 employed, the experiment is inconclusive as to the comparative 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 47. No. 311. March 1874. M 



