228 POLYPHASE CURRENTS. [Exp. 



20. General Method, n Wattmeters. In any star-connected 

 system, if a wattmeter is connected in each line the current 

 coil connected in series with the line and the two ends of the 

 potential coil connected, respectively, to the line wire and to the 

 junction or neutral point of the system the total power of the 

 system will be the sum of the separate wattmeter readings, as 

 discussed in 2. 



21. This arrangement of wattmeters, however, is not limited 

 to star-connected circuits ; nor is it necessary to have the neutral 

 point accessible. The true power of any system whatsoever may 

 be measured by connecting one wattmeter in each line, with cur- 

 rent coil in series with the line and potential coil with one end 

 connected to the line and the other end to any point P of the 

 system, which may or may not be the neutral. To this potential 

 point P is connected the potential coil of every wattmeter. The 

 algebraic sum of the wattmeter readings gives the true power. 

 A general proof of this is given in 53 ; it can be verified by ex- 

 periment, 45-49- 



22. The fact that any point of the system may be taken as 

 the potential point P leads to the practical simplification by which 

 one wattmeter is omitted. In a system of line wires, a, b, c n, 

 let the line wire n be taken as the potential point. Wattmeters 

 A, B, C, etc., will have current coils connected in series with 

 a, b, c, etc., and potential coils connected from a to n, from b to n, 

 etc. Wattmeter N would, accordingly, have its potential coii 

 connected from n to n; as both ends of the pressure coil would 

 thus be connected to the same point, this wattmeter would always 

 read zero and, accordingly, can be omitted. The method of 

 n i wattmeters, 16, is thus established. 



23. Two Wattmeter Method for any 3-wire system. This 

 is the method generally used for measuring 3-phase power. Be- 

 ing a particular application of the n I wattmeter method, 

 1 6, the two- wattmeter method can be applied to any 3-wire 



