230 Prof. C. Niven on Determining and Comparing 



For example, if c= 600, B = 20, G=300, 



2 _ 20x600x1200 

 a ~~ 1220 ; 



and a may be chosen about 100 ohms. 



(2) If c and b be chosen, A will be a minimum when 



W. a + c + Gr=aGc.(B.a + c + ac). 



As an illustration of these methods, the field-magnets of an 

 Elwell- Parker dynamo were measured by a condenser having 

 a capacity of J microfarad. The resistance of the field- 

 magnets was found to be 23*5 ohms, and when shunted the 

 combined resistance of the magnets and shunt was 7*6. a,b y d 

 were taken = 100, 100, 1031 ; and by the aid of a rheostat 

 and coils the resistance c was made up to 1031, when no 

 permanent current was observed. 



When R= 1011, the kick due to the self-induction appeared 

 to be balanced by the condenser ; but the observation was not 

 quite satisfactory, as the current of self-induction seemed to 

 last longer than the other. 



ThuS ' /7-6\« 1 



L = 3*256 earth-quadrants. 

 Method II. — The condenser may be placed in BC, one pole 



D 



being movable along BC. Calling BP = R, and using the 

 same symbols as before, we find 



1 bA 



A having the same meaning as above. 

 When 7 = 0, 



CWc-Lb = 0, 



This method really constitutes an extension of Maxwell's ; but 

 it does not appear to possess any advantage over the one given 

 above. 



