308 THE POLYPHASE CIRCUIT. 



Method 111. The method consists in connecting together 

 three non-inductive resistances for the purpose of measure- 

 ment, so as to form a star system, the neutral point of 

 which will be at the same potential as the neutral point 

 of the generator winding, and which will consequently 

 answer the same purpose for measurement. The watt- 

 meter is then connected so as to read the current in 

 one line, and the voltage from this star point to the 

 same line. (See Fig. 154b.) The voltage applied 

 to the volt coil of the wattmeter is consequently the 

 same as would be generated in one phase of a star- 

 wound generator ( = e). Also the current coil carries the 

 same current as would be carried by the winding of the 

 generator in the same case ( = c). Hence (employing 

 the previous notation) the reading of the wattmeter will 

 be ce, or the total power of the circuit. 



Usually a wattmeter intended for permanent con- 

 nection to a circuit in this manner would have its scale so 

 calibrated that the readings would be three times the 

 power actually producing the deflection. In this case 

 the power of the circuit would be registered directly. 



If the neutral point of the generator winding, or the 

 neutral point of the motors or branch circuits supplied, is 



(a) Circuit with neutral point (b) Wattmeter connected to 



available. artificial neutral point. 



FHJ. 154. WATTMETER CONNECTIONS IN THREE-PHASE CIRCUIT. 



available, the resistances shown in Fig. 154b are not 

 necessary, and connections are then made as in Fig. 154a. 

 The resistances are only required in order to create a 

 neutral point when this is not otherwise available. 



In cases where the load on the three wires is not 

 balanced, the usual way to determine the power of the 

 system is to insert a wattmeter in each branch circuit, 



