58 ALTERNATING CURRENTS 



since this would represent the sum of the two wattmeter readings if 

 these instruments were employed. There is, however, another way 

 of expressing the power. A reference to Fig. 37 shows that OA, OB, 



FIG. 40. Vector Diagram of Unbalanced Non-inductive Three-phase Load. 



and 00 give the star voltages of the equivalent star-connected load. 

 Hence, the load being non-inductive, and the currents in phase with 

 the star voltages, we have for the total power 



OA x Ii + OB x I 2 + 00 x I 3 



30. Measurement of Energy 



For the measurement of the energy supplied to a three-phase 

 -^ circuit, two single-phase energy meters 



I ^ suitable construction such as 



the Aron clock meter or the Elihu 

 Thomson meter may be employed, the 

 total energy being given by the sum 

 of the readings of the two instruments. 

 The connections of the current and 

 pressure coils of these instruments are 

 precisely similar to those used in the 

 two-wattmeter method of power mea- 

 surement shown in Fig. 36. For a 

 balanced three-phase circuit, a single 

 instrument will suffice if, by the crea- 

 FIG. 41. Connections of Energy tion of an artificial neutral point, this 



Meter for Balanced Load. point ig ma( j e accessible . the instm _ 



ment then has its current coil in series with one of the mains, and 



