34 



ELEMENTS OF ELECTRICAL ENGINEERING. 



circuit (ei positive), and two smaller pulses of power taken back 

 by the alternator from the receiving circuit (ei negative) during 

 each complete cycle of electromotive force and current. 



The full-line curve in Fig. 32 is the electromotive force curve 

 of an alternator, the dotted curve is the current curve for the 

 case in which the alternator delivers current to a circuit which 

 has inductance but no appreciable resistance, and the ordinates 

 of the dot-dash curve represent the successive instantaneous values 

 of ei. In this case there are two pulses of power delivered by 



Fig. 32. Current lagging 90 behind voltage. 



the alternator and two similar pulses of power taken back by the 

 alternator from the receiving circuit during each complete cycle. 

 In this case, therefore, the average value of ei is zero. A 

 mechanical analogue of this third case is afforded by the move- 

 ment of the balance wheel of a watch, assuming it to be friction- 

 less. The hair spring does work upon the wheel setting it in 

 motion, and the spring takes all of the energy back again in stop- 

 ping the wheel. Then work is delivered to the wheel once more 

 in reversing its motion, and this energy is all taken back by the 

 spring as the wheel returns to its starting point, thus completing 

 one cycle of the motion during which two pulses of power have 

 been delivered to the wheel by the hair spring, and two similar 

 pulses of power have been taken back from the wheel. 



