POWER AND POWER-FACTOR. 65 



Each set of readings obtained in the experiment just 

 described would enable us to construct a voltage diagram for 

 the circuit. 



The energy voltage E e is obtained by dividing the watts 

 by the current, since 



watts = energy voltage x current. 



The following construction gives the idle voltage over- 

 coming the self-induction of the circuit. 



Draw a horizontal line (see Fig. 29) E to represent the 

 total voltage E. On E describe a semicircle, and from E 



TOTAL VOLTAGE^lds'5 



10 20 30 40 50 60 70 80 90 tOO 11( 



SCALE OF VOLTS 



FIG. 29. DIAGRAM OF ELECTROMOTIVE FORCE. 



with radius equal to the energy voltage describe a circle 

 cutting the semicircle in F. Then OF represents the 

 magnitude and phase of the self-induction electromotive 

 force, as the sides F E, E represent in phase and magnitude 

 the energy voltage and total voltage respectively. 



The angle of lag is the angle F E between F E and E, 

 since OE is the voltage of the circuit and F E is in phase 

 with the current. 



Fig. 29 represents such a diagram for the point on the 

 curve in Fig. 28, for which total voltage of circuit was 105-5 

 the current 9 amperes, and the true watts 875. The energy 



ft7^k 



voltage is consequently - - = 97*2. The curve in Fig. 28 



y 



shows the value of the power factor or cos q> to be -921, 

 whence <p = 22 45'. This value for <p is seen to correspond 

 with that of the angle in Fig. 29. 



