30 ALTERNATING CURRENTS 



r loads the voltage by 90. This is illustrated by Fig. 29, 

 in which the relation is shown vectorially. 



It will be seen from the foregoing that alternating current does 

 not actually flow conductively through the insulation of the con- 

 denser. A perfect condenser offers an infinite resistance to alter- 

 nating, as well as to direct current. However, with alternating 

 current the condenser is alternately charged and discharged, so 

 _ v/ that a quantity of electricity flows into the 

 positive plate, and then out again, etc. It is 

 this quantity of electricity which flows to charge 

 and to discharge the condenser which constitutes 

 the alternating current. An ammeter placed in 

 the line to such a condenser indicates a current. 

 It is clear that this current is proportional to 



FIG. 29 Vector 



diagram for circuit the frequency, for the more rapidly the voltage 

 containing capaci- alternates, the greater the quantity of electricity 



tance only. j j- u j j xu 



charged and discharged per second, and there- 

 fore the greater the flow of current. This current is also propor- 

 tional to the capacitance, C, and to the voltage E. 



The actual value of the current in amperes is given by 



I = 2irfCE (10) 



where C is in farads. 

 This equation may also be written 



E J2 



1 X c (11) 



27T/C 



Xc is called the condensive or capacitive reactance of the circuit 

 in ohms and is equal to l/(27r/C). 

 Also 



E - 



Example. What is the condensive reactance of a 10-microfarad condenser 

 at 60 cycles per second and how much current will it take from 110-volt, 

 60-cycle mains? 



10 mf. = 0.00001 farad. 



TO-i " L ST - 265 ohms - Ans ' 



, 110 



7 = pr = 0.415 amp. Ans. 

 Jbo 



