1876.] 



On the Action of Light on Selenium. 



113 



V. ^' The Action of Liglit on Selenium.-'^ By Prof. W. G. Adams, 

 F.R.S., and Mr. E. E. Day. Received May 18, 1876. 



(Abstract.) 



The paper contains an account of a series of experiments which have 

 been carried on during the past year, and which have had for their ob- 

 ject the investigation of the electrical behaviour of selenium, especially 

 as regards its sensitiveness to light. The first part contains a short 

 summary of the results obtained by Professor Adams, which have been 

 communicated to the Society *. 



It has been already shown that the action is due principally, if not 

 entirely, to those rays of the spectrum which are luminous, and that the 

 ultra-red or the ultra-violet rays have little or no effect ; also that the 

 intensity of the action depends on the illuminating power of the light, 

 being directly as the square root of that illuminating power. 



It M^as also observed that with the same piece of selenium at the same 

 temperature, the resistance diminished as the battery-power was 

 increased. Also it was found that the electrical resistance of the rod of 

 selenium was different for currents going through it in opposite direc- 

 tions. Thus if two platinum wires be melted into the selenium at two 

 points, A and B, and the resistance of the selenium be balanced by the 

 Wheatstone's bridge arrangement, the positive pole of the battery being 

 connected to the electrode A, then on reversing the current so that the 

 negative pole of the battery was now connected to the electrode A, the 

 numerical value of the balancing resistance was always found to be 

 different from that previously obtained. 



If the electrical conductivity of selenium followed the ordinary law of 

 metalhc conduction this would not be the case ; and hence it seemed 

 probable that a careful investigation of these points would lead to some 

 important results. 



In the experiments recorded in this paper, the objects we have had 

 especially in view have been: — 



(i) To examine the character of the electrical conductivity of selenium 

 when kept in the dark. 



(ii) To determine whether light could actually generate an electric 

 current in the selenium. 



Several pieces of selenium were prepared as follows : — A small 

 piece varying from a quarter of an inch to an inch in length was 

 broken off a stick of vitreous selenium. A platinum wire was then 

 taken and bent round into a small ring at one end, and the re- 

 mainder of the wire turned up at right angles to the plane of this 

 ring. The rings of two such wires were then heated in the flame of a 

 spirit-lamp, and pressed into the ends of the little cylinder of selenium, 



* See ' Proceedings,' vol. xxiii. p. 535, and vol. xxiv, p. 163. 



