484 Mr. Rimington on the Measurement of the Power 

 Now for permanent currents, 



8=&~C, and eC = KS, 

 r 2 



where K is the constant of the instrument for watts. 

 Therefore 



fc- K , 



or _ K £ n(r 2 2 + a 2 L 2 2 ) 



In order to be able to make use of this result it is necessary 

 to know the coefficients of self-induction of P + A and B ; the 

 latter may be found once for all by any of the well-known 

 methods and its value marked on the instrument ; but the 

 former will require to be determined for the same values of 

 the currents in the primary and secondary coils of the trans- 

 former as are flowing when the power is measured, since the 

 apparent coefficient of self-induction of the primary coil de- 

 pends on the saturation of the iron of the transformer and 

 also on the current in the secondary coil. The best method 

 of measuring L x under these conditions is that due to Joubert. 



Connect an inductionless resistance R in series with the 



primary, and pass an alternating current of known period 

 through the two ; arrange the resistances of the primary and 

 secondary circuits so that the currents in them have about the 

 same values as when the power-measurement was made. Now 

 connect a high-resistance Siemens electrodynamometer* be- 

 tween V and Vj, and let the reading be o\ ; again connect it 

 between Y x and V 2 and let the reading be 8 2 . 

 Then 



r — ~ V S7 



* A Cardew voltmeter may be employed, in which case 



