132 ELECTRICAL ENGINEERING 



(2) The reactance of a circuit consumes a component of e.m.f. 

 in quadrature with the current, leading in the case of circuits of 

 large inductance and lagging in circuits of large electrostatic 

 capacity, but it does not consume any power. 



The inductive reactance of a circuit is 



X = 27T/L ohms, 



where / is the frequency of the impressed e.m.f. and L is the in- 

 ductance in henrys. Commercial circuits are operated at a fixed 

 frequency and so / is constant. 



The inductance of a circuit in air or any non-magnetic material 

 is constant but in an iron-clad circuit it varies with the current, 

 decreasing as the current increases since the permeability of the 

 iron decreases as the flux density in it increases. 



Since inductive reactance consumes a component of e.m.f. in 

 quadrature ahead of the current it is taken as a positive reactance. 



The condensive reactance of a circuit is 



where C is the capacity of the circuit in farads. The capacity of 

 a circuit does not vary with the current or e.m.f. and thus the 

 condensive reactance is constant so long as the frequency is con- 

 stant. 



Condensive reactance is taken as a negative reactance since it 

 consumes a component of e.m.f. in quadrature behind the cur- 

 rent. Thus when inductive reactance and condensive reactance 

 are connected in series they oppose and the reactance of the 

 circuit is 



X= XL - X c = 



Z 7T/C 



In series-parallel circuits the reactance is a complex function of 

 the resistances and reactances of the various branches. 

 The reactance of any circuit is 



^ _ wattless component of impressed e.m.f. 

 current 



_ quadrature component of impressed e.m.f. , 

 current. 



