60 



ALTERNATING CURRENTS 



reading. (See page 91, par. 48.) This necessitates reversing 

 the connections of one of the instruments, which is often incon- 

 venient. If both wattmeters be combined in one, that is, if both 

 moving coils be mounted on the same spindle, the turning 

 moments for each element add or subtract automatically, and 

 the total power is read on a single scale. 



FIG. 57. Connections for polyphase wattmeter on 3-phase circuit. 



Figure 56 shows the construction of a Weston polyphase watt- 

 meter in which the two elements are clearly shown. Figure 57 

 shows one methad for connecting a polyphase wattmeter in a 

 three-phase circuit. 



To 



Potentiometer 



FIG. 58. Connections for calibrating a wattmeter. 



Although it is often more convenient to use a polyphase watt- 

 meter, two single instruments are better adapted to precision 

 work, as there is no mutually inductive action between the ele- 

 ments of the instruments such as may occur between the elements 



