EFFECT OF LIGHT ON ELECTRICAL CONDUCTIVITY OF SELENIUM. 377 



expected. We may imagine the selenium to be made up of 

 molecules, some of which have temporarily lost an electron, 

 there being as many free electrons as there are molecules 

 which have lost an electron, and the conductivity being 

 proportional to the number of free electrons. The free 

 electrons will often com bine again with a molecule which has 

 lost an electron, the rate of combination being assumed to 

 be proportional at any instant to n 2 where n is the number of 

 free electrons at that instant, and the rate of production 

 of fresh electrons may be regarded as a constant depending 

 on the density of the molecules, their relative position, 

 temperature, etc. 



The absorption of energy may well be supposed to 

 increase the readiness of the molecule to part with an 

 electron, and so, in such a case there would be an increase 

 in conductivity, the rate of change of n being given by 



S=' J -«" 2 • « 



b being the rate of production and an 2 the rate of combin- 

 ation; b is equal to ant, n x being the number of free elec- 

 trons when a steady state is attained. In the dark 6 has 

 a value b = mil , n being the number of free electrons in 

 the dark. The greater the increase in b the more rapidly 

 does the change occur, as is easily seen from (1). 



A. H. Pfund investigated the sensitiveness of a cell for 

 light of different colours and found the maximum sensitive- 

 ness with light of wave length 700 fifi, i.e., in the red. It 

 might be expected that the maximum effect would be pro- 

 duced by the light which is most strongly absorbed, and 

 Pfund advances a theory to explain why this is not so. He 

 suggests that the effect is limited to an extremely thin 

 surface layer the thickness of which is of the order of 

 lO -6 cm. for blue light, and that though this film is made 

 up of material which possesses a high conductivity when 



