70 ELEMENTS OF ELECTRICAL ENGINEERING, 



in equations (8) and (9) we have : 



or 



7 = 



and 



tan*-* (12) 



Resistance, reactance, and impedance are all expressed in ohms. 

 It is to be remembered however that reactance, and of course 

 impedance also, depend upon frequency as well as upon the con- 

 ditions actually existent in the circuit. 



29. Conductance, susceptance, and admittance. The factor g by which the 

 electromotive force E is multiplied to give the component of I parallel to E is 

 called the conductance of an alternating current circuit. 



The factor b by which the electromotive force E is miiltiplied to give the compo- 

 nent of I which is 90 behind E, is called the susceptance of an alternating cur- 

 rent circuit, and 



The factor y ' g* -\- b 2 by which the value of the electromotive force must be mul- 

 tiplied to give the value of I is called the admittance of an alternating current 

 circuit. 



The quantities conductance, susceptance, and admittance are related to resistance, 

 reactance, and impedance as follows : 



R . 



& >2 I J(l 



r '=|- () 



in which y is the admittance, and 2 is the impedance. 



The use of the quantities conductance, susceptance, and admittance simplifies the 

 algebraic treatment of the problem of coils in parallel as exemplified in Art. 46. 



30. Special cases of the problem of Art. 27. (a) Non-inductive 

 circuit. In this case the electromotive force must overcome 

 resistance only, and therefore the electromotive force is equal to 

 Ri at each instant, that is the electromotive force is in phase 

 with i and its effective value is equal to RI. 



