Resistance of Selenium. 



163 



cent.; hence the quantity transmitted by the glass cell and 

 the alum solution it contained was as much as 95*2 per cent., as 

 measured by the rod of selenium. 



The same, measured by the thermopile, was found to be 7*06 

 per cent. 



Having satisfied myself as to the comparative, if not the abso- 

 lute insensibility of a bar of selenium to radiant heat of low 

 refrangibility, and therefore as to its being unsuitable for repla- 

 cing the thermopile in the measurement of those rays, I made a 

 few experiments with the view of finding whether it was likely 

 to be a suitable instrument in photometry. 



To obtain a more constant light I substituted a paraffin lamp 

 for the candle ; and by placing in front of the horizontal sele- 

 nium bar a vertical slit whose breadth could be varied at pleasure, 

 and which could be closed before and during every alternate 

 measurement of the resistance, also by varying the distance of 

 the lamp, it was found that while the decrease of resistance varied 

 as the breadth of the slit, and therefore as the length of the por- 

 tion of the bar exposed to light, it was far more nearly propor- 

 tional to the reciprocal of the distance, and therefore to the square 

 root of the intensity of the incident light than to the intensity 

 simply, between the limits within which the observations were 

 made. The decrease of resistance observed was : — 



Distance from lamp 

 to centre of bar. 









Decrease. 



inches. 





per cent. 



H 





. ... 38 



5 



. 











27 



7 



. 











22 



9 



, 











21 



131 



. 











15 



19 



, 











11 



22i 



. 











10 



441 













3-77 



75 













1-57 



ength of the b 



ar was 



%i 



- in 



che 



3. 





No experiments were made to ascertain how far the sensibility 

 was affected by the temperature of the room ; but the presence 

 of moisture rendered the action feeble and uncertain as the dew- 

 point was approached, probably owing to the deposit of a very 

 slight film of moisture on the surface of the bar. Another effect, 

 probably to be ascribed to the same cause, was noticed on two 

 occasions — a diminution of the change of resistance after a certain 

 duration of exposure to a screening from the light. 



M2 



