32 TREATISE ON ALTERNATING CURRENTS. 



where Q is the quantity of electricity on the condenser at any 

 instant. Therefore by 21 and 23 



Famfrt-tf) 



- 



where tan 9 = - 

 and tan <6 = *r- 



Equations (12) and (13) show that for high frequencies (i.e. p 

 is large) nearly the whole of the current goes through the con- 

 denser branch. 



The current i in the main circuit is the resultant of the 

 currents i\ and i^ and is given by 



i* - ^2 + i/ 4. 2^ 2 cos (0 + ^) 



If 



L = CVi?'2 



and i'i, ^2 differ in phase by a right angle. 



MEASUREMENTS OF ALTERNATING CURRENTS AND ELECTROMOTIVE 



FORCES. 



26. In taking electrical measurements in alternating current 

 circuits special types of instruments are necessary. 



Instruments which can be used for alternating current measure- 

 ments can usually be employed for direct current measurements, 

 but the converse is not true. 



All instruments for the measurement of alternating currents 

 and E.M.F.s depend upon a square law and the deflections are 

 proportional to the mean square of the quantity to be measured. 

 Amongst instruments for the measurement of alternating currents 

 may be mentioned eleetpo-dynamometeps, and Kelvin 



