PRINCIPLES OF ALTERNATING CURRENT 219 



current running into, or out of, the condenser continually. 

 It was stated above that when the e.m.f. on the terminals 

 of a condenser changes, current will, flow either into, or out 

 of, the condenser. If the voltage increases the current will 

 be into the condenser and when the voltage decreases current 

 will flow out of the condenser. 



Current in a Condenser in an A-C. Circuit. It may be 

 shown mathematically that if the voltage impressed on a 

 condenser is a sine wave the current flowing into the con- 



.006 



FIG. 130. Charge and Discharge Curves of Condensers. 



denser will be similar in shape but will be 90 ahead of 

 the e.m.f. wave in phase; this is generally expressed by 

 saying that a condenser draws a leading current- from the 

 line in contrast to an inductance, which draws a lagging 

 current from the line. In Fig. 131 are shown three curves, 

 one a sine wave of e.m.f., another the rate of change of this 

 e.m.f., and in phase with this second curve is shown the 

 current taken by the condenser. It is seen that this cur- 

 rent wave is 90 ahead of the voltage wave. 



Power Used in a Condenser. Now if the voltage and 

 current in a circuit are 90 apart in phase the power used 

 in the circuit must be zero, as power = El cos cj> and cos 90 



