EFFECT OF LIGHT ON ELECTRICAL CONDUCTIVITY OF SELENIUM. 363 



opened and the change in conductivity produced by one 

 side of the cell being illuminated was observed. Then 

 the door was closed, and when the conductivity had fallen 

 to its original value the other door was opened and the 

 change produced thereby noted, and then this door being 

 closed and the conductivity having again fallen to the 

 original value in the dark, both doors were opened together 

 and the change in conductivity produced by the simultane- 

 ous illumination of the two sides of the cell was observed. 

 On other occasions, one door was opened and after the 

 conductivity had attained its maximum value the other 

 was opened, the first remaining open and the increase in 

 conductivity observed. The doors were then closed, and 

 after the conductivity had fallen to its original value the 

 experiment was repeated, the doors being now opened in 

 the inverse order. 



The conductivities were measured by a direct deflection 

 method, a battery of accumulators, the cell and a galvano- 

 meter being joined in series; with low resistance cells 

 fractions of the E.M.P. of one accumulator were obtained 

 by means of a potentiometer. 



Let (Di) and (D 2 ) be the increases in conductivity 

 observed when the doors £1 and D 2 are opened separately 

 and (Di + D 2 ) the increase when both are opened together; 

 if the effect is limited to a thin surface layer of the selenium 

 we should have (D 3 + D 2 ) = (D,) + (D 2 ). 



The cells tested varied considerably in their conductivities 

 and in their sensitiveness to light (in consequence of the 

 different temperatures employed in forming them) but in 

 all tested it was found that (Di + D 2 ) was less than (jD x ) + 



(D 2 ) the ratio ^ , "' generally becoming smaller as 



the intensity of the light increased. In tables I and II are 

 given the results obtained with two cells of which the 

 conductivities were very different. 



