78 



POWER AND POWER-FACTOR. 



Consequently, in the diagram Fig. 14 we have 

 Energy in whole circuit = C X A B X cos CAB. 

 Energy in inductive part 



of circuit = C X C B X cos DC A. 



Energy in non-inductive 



part = C X AC. 



Where in each case the lines are taken as the voltages 

 which they represent. 



If, therefore, a non-inductive resistance forms part of 

 a circuit, or if a non-inductive resistance can be inserted 

 in the circuit for the purposes of the measurement, three 

 readings of a voltmeter and a reading of the current 

 enable us to measure the power of the circuit. This is 

 frequently of great advantage where an accurate watt- 

 meter is not available. 



The diagram of connections necessary for such a 

 measurement is Fig. 13. In the following experiment a 

 wattmeter is introduced into the circuit so that a com- 

 parison may be made between the readings obtained with 

 the wattmeter and by the 3-voltmeter method. 



The ammeter is unnecessary if the value of the non- 

 inductive resistance is known, as will be seen from the 

 calculation given below. 



EXPERIMENT XIII. MEASUREMENT OF POWER BY THE 

 3- VOLTMETER METHOD. 



DIAGRAM OF CONNECTIONS. 



PIG. 37. 



MI M 2 



Source of alternating current. 



Inductive resistance. 



Non-inductive resistance. 



Kesistance for varying current in circuit. 



