238 POLYPHASE CURRENTS. [Exp. 



PART II. MEASUREMENTS. 



44. Many of the methods just described for measuring poly- 

 phase power and power factor can best be taken up as occasion 

 arises for their use. Without undertaking in the present experi- 

 ment to subject all of these methods to test, it will be well to 

 select a few of them for trial in the laboratory in order to illus- 

 trate and make clear the methods as a whole. For this the 

 following tests are suggested. 



45. Verification of Methods for Measuring Polyphase Power. 

 With a single-phase non-inductive load, forming a 2-wire sys- 

 tem, measure the total power with two wattmeters. Each line 

 is to contain the current coil of one wattmeter, the potential coil 

 of which is connected from the line to a common point P, as in 

 21. The experiment consists in connecting P to different 

 parts of the circuit, of various potentials, and noting that the 

 algebraic sum of the two wattmeter readings is constant. 



When the power indicated by one wattmeter becomes greater, 

 as P is changed, the power indicated by the other wattmeter be- 

 comes less.* 



46. For example, let the supply lines be c^au, as in Fig. 6, 

 Exp. 6-A. Connect P, successively, to points of different po- 

 tential, as a lt a z , the neutral O, A^ and A 2 , these points being 

 all on phase A. When phase B of a two phase supply is avail- 

 able, proceed, also, to connect P successively to points B lt b ly 

 b 2J B 2 on phase B. 



47. Repeat with an inductive load. 



48. Repeat in some modified manner, as by using ajj^ as 

 supply lines and connecting P, successively, to various points as 

 described above. 



49. When points, as in Fig. 6, Exp. 6-A, are not available, 

 a resistance can be bridged across the circuit and the point P 



* A positive reading decreases ; a negative reading increases. 



