330 ELECTRICAL MEASUREMENTS 



rents should enter when flowing from the generator to the load 

 should be determined and marked on the instruments once for all. 

 The proper marking may be determined by putting the instru- 

 ments in a single-phase circuit. Then, whenever the instruments 

 are used, the currents, as they flow from the generator to the 

 load, must enter both the current and the potential coils at 

 the marked terminals. When the instruments are so con- 

 nected, if the pointer deflects up the scale, the mean product vi 

 is positive; if the deflection is in the contrary direction, the mean 

 product is negative. To obtain its numerical value the current 

 coils must be reversed, and the reading so obtained regarded 

 as negative. This simple procedure avoids all uncertainty as 

 to the algebraic signs of the readings and renders unnecessary 

 any special tests for their determination. The terminals of 

 current and potential transformers should be similarly marked. 



Two-phase Three-wire System. By the theorem, two watt- 

 meters are required, the connections being as in Fig. 193. 



Load 



FIG. 193. Power measurement; two-phase three-wire system. 



If the two phases are separately loaded, it is obvious that 

 the power is the sum of the wattmeter readings. A load might, 

 however, be connected between leads 1 and 3 as indicated and 

 then the instantaneous power would be 



P = VizilZ + V23123 + ^31*31 

 Z>31 = t>32 + V Z l 

 P = VizilZ + #23^23 + Vzzln + 



= #12(^12 fc'si) + V3z(i 3 i 1*23) 

 i\ = iiz + ^13 = iiz ^31 



is = ^32 + ^31 = ^31 ^23 



:.P = ~f viiitdt + ^ I vvisdt 

 * Jo J- Jo 



(27) 



