106 SINGLE-PHASE CURRENTS. [Exp. 



of one complete period or cycle, 90 indicating J period, etc. 

 A degree is, therefore, a unit of time, being sometimes designated 

 a "time-degree" or "electrical degree." This designation is, how- 

 ever, unnecessary except in discussions where " space-degrees " 

 are also used. 



12. Series Circuit with Reactance Only. In an alternating 

 current circuit containing only a reactance of X ohms, the electro- 

 motive force required to make flow a current I, is 



E X = XI; and I = E x ~X f 



as shown in 14-17. 



When the reactance X, is due to inductance, the electromotive 

 force to overcome reactance is 



Reactance is the same as resistance in that an electromotive 

 force proportional to it is required to cause a current to flow, 

 the electromotive force being XI for reactance 

 and RI for resistance. Reactance is, however, 

 different from resistance in that it consumes 

 no energy; when the current is increasing, 

 energy is stored* in the magnetic field (as in 

 a fly-wheel), this energy being returned to 

 the circuit when the current is decreasing. 



In a reactance, the current and electromotive 

 FIG. i. Vector 



diagram for circuit force are no * in P ha *e but are in quadrature 

 with inductive re- with each other, i. e. f the current and electro- 

 motive force differ in phase by a quarter of a 

 cycle or 90, and when one is a maximum the other is zero. 



*(i2a). The energy of the magnetic field is equal to r / 2 LI* t cor- 

 responding to the energy of a moving body, y 2 MV*. 



