POWER AND POWER-FACTOR. 



79 



F! Voltmeter measuring voltage in inductive 



portion of circuit. 

 V 2 Voltmeter measuring voltage in non-inductive 



portion of circuit. 



F 3 Voltmeter measuring voltage of total circuit. 

 A Ammeter reading current in circuit. 

 W Wattmeter reading either total power or power 



in inductive portion. 

 V S Voltmeter switch for changing connections of 



wattmeter shunt coil. 



Instructions.; Connect in series with an ammeter and 

 the series coil of a wattmeter an inductive and a non- 

 inductive resistance. Connect these to a source of alter- 

 nating current through a variable resistance in order to 

 enable the current in the circuit to be varied. 



Connect either three voltmeters, or a single voltmeter 

 with 3-way switch, to read the voltages of the whole 

 circuit, of the inductive and the non-inductive portions. 

 Connect the shunt coil of the wattmeter to a 2-way 

 switch so as to read the power in either the total circuit 

 or the inductive portion only, as desired. 



Take readings on all the instruments for .several 

 values of the current, noting both total watts and watts 

 in inductive resistance. Tabulate the results as shown 

 below, where a few readings actually determined in the 

 manner described are given as examples : 



Measurement of Power by Three Voltmeters. 



If the value of cos tf> used in calculating the power 

 from the readings of the ammeter and voltmeter is 

 obtained by actual measurement, a high degree of 

 accuracy is not possible. The power can be obtained 

 entirely by calculation from the ammeter and voltmeter 

 readings, as is shown by the following investigation : 



By the ordinary trigonometrical relations in a triangle 

 we have (see Fig. 14, page 31) : 



