METHOD OF MEASUKING POWER IN TBI-PHASE CIRCUITS. 193 



a 3 , and the volt coil is connected across 3 and 2. Suppose that w l 

 is the reading on one meter and that w 2 is the reading on the 



Fia. 81. 



other, and suppose that w\ is greater than w%. Then the watts 

 given to the circuit are w\ w%. If the phase difference between 

 3 and v 3 . 2 is less than 90, w% is positive, but if greater than 90 

 w% is negative. It is easy to find out experimentally whether w 2 

 is positive or negative. 



The second method is to use three wattmeters, their ampere 

 coils being put in the main circuits and theu? volt coils across 

 and 1, and 2, and and 3 respectively. 



These methods also apply when the windings are as in Fig. 82, 

 where 1, 2, and 3 are the terminals for the mains. 



Let i, 62, and e 3 denote the P.D.s between 1 and L, 2 and M, 

 and 3 and N respectively. 



Then 



w = eidi + ^2 4- 



= ^i.2i 4- %-2^3 as before. 

 Similarly 



w = eii 4- e^a* 4- C 3 a 3 4- V LO I 4- v 



4" 



It is to be noted that if all the mains are equally loaded, one 

 meter is sufficient. If the volt coil be connected across two of the 



o 



