6-B] MEASUREMENT OF POWER. 227 



this common return. No wattmeter is required for a return 

 circuit; thus, for a 2-wire system, one wattmeter only is needed, 

 no wattmeter being needed in the return conductor ; in a 3-wire 

 system, two wattmeters are used, none being needed in the re- 

 turn conductor, etc. If n is the number of line conductors, 

 n i wattmeters are, accordingly, required. For a 3-wire sys- 

 tem, the connections are shown in Fig. i. 



To measure power in any system, connect a wattmeter in every 

 line circuit except one (considered as the return conductor), 

 each wattmeter having its current coil in series with one of the 

 lines and its potential coil connected from this line to the return 

 conductor. One less wattmeter is required than the number of 

 line wires; the total power is the algebraic sum of the individual 

 wattmeter readings. 



17. To read positive power each wattmeter is to be connected 

 in the positive sense, that is, connected in the same way as for 

 measuring power in a 2-wire system, direct or alternating. If, 

 when connected in this manner, the needle of any wattmeter 

 deflects the wrong way, the connections of its potential or current 

 coil are to be reversed and its reading is to be considered negative. 

 Compare 25. 



18. This method of measuring power is absolutely general; 

 the current may be direct or alternating and may vary by any 

 law whatsoever ; the system may be single-phase or polyphase, 

 balanced or unbalanced, symmetrical or unsymmetrical. 



As a particular case, the two-wattmeter method for a 3-wire 

 system is of special importance with reference to 3-phase circuits 

 and will be considered later ( 23) in detail. 



19. The foregoing method has been explained by considering 

 one conductor as a common return for all the others, and for 

 most purposes this explanation is sufficient. The method with 

 n i wattmeters can be rigorously established (22) by first 

 developing the method with n wattmeters, 20. 



