PRINCIPLES OF ALTERNATING CURRENT 215 



Now as the impressed force must be equal to the result- 

 ant of the reactions and opposite to this resultant in phase, 

 it is properly shown at OE. It is seen that the result of 

 such a construction is to show the current lagging behind 

 the impressed force and we know that this is really the con- 

 dition in an inductive circuit. 



Magnitude of Current in an Inductive Circuit Impedance. 

 It may be shown that if / is the frequency of the current 

 in the coil, L the coefficient of self-induction in henries and I 



notation 



\ 



I 



FIG. 129. Vector Diagram of Inductance Reaction. 



the current in amperes the inductance reaction is equal to 

 2-rc/L/ volts. 



Now evidently 



= VOA 2 +OB 2 



(47) 



(48) 



where 



The term 2x/L is called the reactance of the circuit and 

 the expression \//2 2 +2x/L 2 is called the impedance of the 

 circuit. It is sometimes designated by the letter Z. 



