with Crystalline Selenium. 413 



before. Then either 



2E , 2E 

 Ci= changes to c 2 = * 



*=2E^^j (III.) 



C 2 Ci 



or 



i, + 2E +# 



C! changes to c 2 = ^ ? 



y=8H^=2s. ..... (IV.) 



It is plain that, if the values of x calculated by (I.) and 

 (III.) agree better than those of y calculated by (II.) and 

 (IV.), the change must be due to resistance. 



The battery consisted of twelve similar Darnell's cells of 

 large surface. They were connected with a commutator, by 

 which, with a single movement, they could be connected up 

 either in series or six and six parallel. In this way, by using 

 the same elements in each measurement, the effect of any 

 slight accidental difference in their electromotive forces would 

 be, to a great extent, neutralized. No difference could be 

 observed by the discharge of an accumulator ; and therefore I 

 assume they were in every respect equal. The currents were 

 measured by means of a reflecting galvanometer. 



A selenium plate with six elements in the dark gave a cur- 

 rent c= 0*498 micro weber. When the battery was doubled 

 and the constant resistance i\ added, the current observed was 

 Cj = 0*508 micro weber. 



Diffused daylight admitted to the selenium, with the six 

 double surface elements the current increased to c/ = 0*860 

 microweber ; with the twelve elements in series and resistance 

 ri the current was c 2 = 0*643 microweber. 



Putting E = 6*72 volts, the values of x and y, calculated 

 from the above data, are as follows :— 



x y 



..A_. 



I. HL II. IV? 



5*68 meg. 5*56 meg. 4*89 volts. 3*57 volts. 



Similar measurements were made with two other plates, 

 which gave the following results : — 



£~ lii. ff. iv? 



6*39 meg. 6*43 meg. 5*76 volt. 3*86 volt. 



55*2 „ 53*0 „ 0*75 „ 0*53 „ 



