EFFECT OF LIGHT ON ELECTRICAL CONDUCTIVITY OF SELENIUM. 361 



OBSEVATIONS ON the EFFECT of LIGHT on the 

 ELECTRICAL CONDUCTIVITY OF SELENIUM. 



By O. U. VONWILLER, B.Sc. 



[Read before ihe Royal Society of N. S. Wales, November 3, 1909.] 



It is shown in this paper that on the incidence of light 

 the conductivity of the selenium nearest the surface is 

 increased most, the magnitude of the effect decreasing 

 as the depth below the surface increases. The effect how- 

 ever is not limited to an exceedingly thin surface layer, 

 but an appreciable change is produced in many cases at a 

 depth of several hundredths of a millimetre. 



The rate at which the effect falls off with the depth 

 penetrated varies with the nature of the light used, it being 

 fouDd in the cells tested that it is much greater with green 

 light than with red. The sensitiveness of these cells is 

 much less for green than for red light. 



These results indicate a greater absorption of green light 

 than of red as was found by A. H. Pfund, 1 and it is shown 

 that the smaller effect produced by the green light is not 

 necessarily inconsistent with the view that the effect 

 produced in any layer increases with the amount of energy 

 absorbed in it. 



The "cells" employed in the experiments here described 

 were made in the following way. Two platinum wires 

 0*004 cm. in diameter were stretched on a sheet of mica, 

 being fastened at the ends by means of holes in the mica; 

 the distance between the wires varied from one to four 

 millimetres in different cells. The space between the 

 wires for a length of two or three centimetres was filled 

 with molten selenium spread into a thin layer by means of 



' Phys. Rev., May 1909. 



