POWER MEASUREMENT 



315 



The adjustment is then complete, provided there are no eddy- 

 current effects in the fixed coil or in the frame of the instrument 

 (see page 316). These effects are discussed in the original 

 paper by Rosa. 



Effect of Mutual Induction between Fixed and Movable- 

 coil Circuits. In the previous discussion no reference has been 

 made to the possible effects of mutual induction between the 

 current and potential circuits. In commercial instruments 

 these effects are so small as to be negligible. They might, how- 

 ever, be worthy of consideration in special low-voltage wattmeters 

 where an attempt is made to attain a maximum of sensitivity. 



When the instrument is read by a torsion head the mutual 

 inductance can be made zero, but if the movable coil is allowed 



FIG. 182. Pertaining to effect of mutual induction in wattmeter. 



to deflect, the mutual inductance will have an effect dependent 

 on the reading of the instrument, being zero and changing sign 

 when the planes of the coils are perpendicular. 



Take the connections shown in Fig. 1827 which are the same 

 as those assumed in Fig. 180. 



Equating the values of the P.D. between 1 and 7 reckoned 

 via the load and via the potential circuit gives 



[R 



/66 = 



R F (RL 



Zc) ' 



r(i 



Re) + j<*(L 



R c ) 



Rp+ju(L P m) 

 (L L + L c m) (L P 



. 



J 



23 



The current in the potential coil is seen to consist of two 

 components, one in phase and one in quadrature with the current 

 in the fixed coils, /23, which is also the load current I L . 



