and Electrical Conductivity in Selenium. 485 



where C is the "dark" current and C the current corre- 

 sponding to the intensity of illumination I. The accuracy with 



Fiar. 1. 



50 Current: In Selenium. 100 



which the law holds may be seen in the following table, where 

 ^stands for the distance between the selenium and the source 

 of light. P is proportional to d~\ so that the constancy of 

 the numbers in the last column is a test of the law. The 

 value of C is 23'1 microamperes, and is deduced by drawing 



distance d 



i 



Current 



Extra 





(metres). 



rf-2. 



C. 



current 



c-c . 



S(c- 



4-50 



0-472 



36-2 



131 



27-8 



3-00 



(W>77 



398 



16-7 



290 



2-00 



0-707 



43-5 



20-4 



28-9 



1-50 



0817 



46-3 



23-2 



28-4 



TOO 



1-000 



51-5 



28-4 



28-4 



070 



1-195 



56-9 



33-8 



28-3 



0-50 



1-415 



62 9 



39-8 



28-1 



0-35 



1-690 



70-6 



47-5 



28-1 



0-25 



2-000 



79-8 



56-7 



28-4 



0-20 



2-237 



86-0 



62-9 



28-1 



