532 



ELECTRICAL MEASUREMENTS 



From the vector diagram it is seen that each wattmeter gives 

 a deflection proportional to El cos (90 6) El sin 6. If the 

 two wattmeters in Fig. 315 are the two elements of a polyphase 

 wattmeter, the reading of that instrument will give 2EI sin 6. 

 As the total idle volt-amperes in the load is \/3EI sin 6, 



Idle volt-amperes = (reading) 



Z 



The scale of the instrument may be graduated so that the idle 

 volt-amperes may be read directly. (Compare with two watt- 

 meter method for measuring three-phase power, page 331). 



Tuma Phase Meter. In America, power-factor meters are 

 much more frequently used than idle current meters. Power- 

 factor meters, as well as various forms of synchroscopes, are 

 developments from the Tuma Phase Meter, the essential portions 

 of which are shown in Fig. 316. 



FIG. 316. Diagram of Tuma phase meter. 



In its original form the ideally perfect Tuma phase meter is 

 applicable only to single-phase circuits and gives a deflection equal 

 to the power-factor angle of the load. By a trifling alteration it 

 may be adapted to polyphase circuits, as will be seen later. 



The fixed coil F is traversed by the load current. The coils 

 A and B are of equal magnetic strength and are firmly lashed 

 together to form a single movable system which is pivoted in 

 the field due to F; in the ideal instrument A and B are inclined 

 at an angle of 90 to each other. 



