454 ELECTRICAL MEASUREMENTS 



potential-coil circuit so that when the circuit of R 2 is open the 

 instrument is under lagged, that is, the angle between the ap- 

 plied voltage and the useful potential-coil flux is less than 90. 

 Referring to the diagram, Fig. 263, (in which a 1/1 ratio is 

 assumed), the flux which threads the voltage winding and the 

 secondary winding, S, (shown in Fig. 261) is the mutual flux 

 3>Af. This flux is also the useful potential-coil flux; it crosses 

 the air gap in the horizontal direction and cuts the hollow 

 aluminum cylinder which is pivoted in the gap. This cylinder 

 forms the rotor or movable member of the wattmeter. $ M 

 induces an fe.m.f., E, in both the voltage and the secondary 

 windings. The applied voltage, V, must overcome the voltage, 

 E, induced by & M , the ohmic drop in the primary circuit IiRi, 

 and the reactive drop in the primary circuit, I\Xi, due to the 



Secondary Open 



FIG. 263. 



primary leakage flux. As indicated in Fig. 263 A, the angle A 

 between the applied voltage, V, und the useful potential-coil 

 flux, $> M , is less than 90. 



When the secondary circuit is closed through the resistance, 

 R 2 , (see Fig. 263) a current, 7 2 , flows in it. The total pri- 

 mary current, 7i, is the vector sum of 7 2 and the no load 

 current 7 . It is seen that closing the secondary circuit rotates 

 1 1 counter-clockwise so that when I\Xi and IiRi are added to 

 E the vector, V, which represents the applied voltage, is also 

 rotated counter-clockwise and the angle A between the applied 

 voltage, V, and the useful flux, $ M , is increased. 



In order to adjust the lagging so that A is 90 it is necessary 

 to calibrate the instrument with a load of unity power factor, 

 and then to calibrate it with a load of low power factor, about 

 0.5. If the two results do not agree, the resistance R 2 (Fig. 261) 



