484 THE ELECTROMAGNETIC FIELD. [PT. III. CH. XII. 



where n and n 2 are the numbers of turns in the primary and 

 secondary coils. If w 2 /Wi is so small that its square may be 

 neglected, we have 



E ^ 

 (at) lfc 



The ratio w 2 /7ii is called the ratio of transformation. 



The argument of the ratio B/A, being the difference of the 

 arguments of the numerator and denominator, shows that the 

 secondary lags behind the primary current by the phase-angle 



which approaches two right angles as the ratio L^co/R^ increases. 



The efficiency of the transformation, or the ratio of the activity 

 in the secondary ^R z I 2 (0)a , to that in the primary, is, by (18) and (12), 



which, neglecting R^/M^co 2 , becomes 



(22) 



that is, in practical cases, nearly unity. 



239. Circuit containing a Condenser. In the cases 

 heretofore considered the only energy of the system has been 

 electrokinetic. If the circuits are connected with conductors 

 upon which charges of electricity can accumulate, we shall in 

 addition have electrostatic, or potential energy. 



As the simplest case let us consider a single circuit whose ends 

 are connected to a condenser of capacity K. If the charge of one 

 plate of the condenser at any instant is q, then the current flowing 

 into that plate is defined as 



The electrostatic energy of the system is ( 143) 

 (2) F 



