POWER MEASUREMENT 317 



by use of the torsion head and the compensation tested at various 

 points along the scale. If the eddy-current error is absent, the 

 adjustment of the potential circuit for zero effective inductance 

 will be the same at all points. 



OTHER METHODS OF MEASURING POWER 



In consequence of the development of the wattmeter, the 

 three methods now to be described are merely of historical 

 interest as methods for power measurement. However, two 

 of them have other applications which are still of practical value. 



The Three Dynamometer Method. The connections are as 

 shown in Fig. 183. At DI, Z) 2 and D 3 are three electrodyna- 



Fn;. 183. Connections for three dynamometer method for power 

 measurement. 



mometers each with its fixed and movable coils in series. R 

 is the total resistance of the circuit of D^. The instan- 

 taneous values of the currents are as indicated. In general 



1 C T 



P = g -,*. 



K is assumed to be non-inductive so, 



P = R i I iiiidt. 



1 Jo 

 At any instant 



i\ = ii + is 

 and 



ii 2 = * 2 2 + 2i 2 i 3 + 'a 2 

 so 



