336 



ELECTRICAL MEASUREMENTS 



When the potential coil currents in equation (31) are replaced by 

 these new values the turning moment becomes 



K( ri + r) -K'rJl C T . 1 f. 1 



M - 77: : r "I ml Wlltf* + jf, \ l^V^dt 



r(2r 1 + r) \TJ Q TJ 



K'( Tl + r) - KrJl C T . !_ C T . ] 



r(2r, + r) [Tj h^J^s^^J. 



The second term on the right-hand side disappears if 7*1 is 

 adjusted so that 



K , = Kn 



M' then reduces to 

 M' = 



That is, the turning moment becomes the same as if there were 

 no interference. If the instrument is of the deflectional type 

 the correctness of this adjustment will vary for different points 

 on the scale but the error will be small. 



The present practice of the Weston Instrument Co. is to use 

 laminated iron shields between the two elements. 



Three-phase Power Measurement by Three Wattmeters. 

 If the neutral is accessible, the sum of the readings of three 

 wattmeters connected as in Fig. 199 will give the power. 



FIG. 199. Connections for measurement of three-phase power by three 



wattmeters. 



With this scheme of connections, no question can arise as to 

 the algebraic sign of the readings; their arithmetical sum gives 

 the desired result. 



