PRINCIPLES OF ALTERNATING CURRENT 221 



opposite to E in phase is shown the reaction of the con- 

 denser which is equal to . It is noticed that the 



reaction in a condenser leads the current by 90; but with 

 respect to the impressed force it 

 is the current which leads. 



Ordinary Unit of Capacity. 

 The farad is too large a unit 

 to be useful for ordinary con- 

 densers; their capacity is gener- 

 ally given in microfarads, the 

 microfarad being one-millionth 

 part of a farad. Condensers are 

 used extensively in telephone 

 service. The capacities used in 

 different parts of the system 

 range from one-tenth to a few FlG . 132. Vector Diagram 

 microfarads. Condensers are of Condenser Reaction, 

 also used largely in wireless teleg- 

 raphy, but the capacities employed are very small, being 

 only a small fraction of a microfarad for small outfits, and 

 about one microfarad for very high-powered stations. 



From the equation, 



it is seen that, for a given voltage, the current increases 

 directly with the frequency. For instance, a 10-microfarad 

 condenser, on which is impressed an e.m.f. of 100 volts 

 and 60 cycles frequency will have a charging current of 

 2xX60X10XlO~ 6 X100 = 0.377 ampere. If the frequency 

 is increased to 1000 cycles, the current will be 6.28 amperes. 

 In wireless telegraphy the frequencies used are very high 

 and, therefore, large currents are obtained with small 

 condensers. One of the transatlantic outfits has a capacity 

 of 1.16 microfarads and is used on circuit of 75000 cycles 



