312 ELECTRICAL MEASUREMENTS 



indicating instruments, instead of maximum values, and remem- 

 bering that ordinarily 8 P is very small, 



P L + H c = Pw - hV tan d P sin B L = P w - I L V sin L (4) 



The effect of the phase displacement in the potential circuit 

 evidently depends on the frequency and on the characteristics 

 of the load as well as on the wattmeter itself. It increases 

 as the power factor of the load is decreased and at extremely 

 low power factors extraordinary precautions must be taken in 

 order to procure accurate results. 



If the resistance of the potential circuit is exceedingly high, 

 there may be capacity effects in the series resistance. Some in- 

 struments are so designed that a part of the potential circuit is 

 coiled on a metal bobbin and although this is split lengthwise it 

 is not entirely free from eddy currents. Both the capacity and 

 the eddy currents modify the phase displacement. 



If the load current is leading and the power factor very low, 

 the wattmeter readings tend toward zero and at some particular 

 value of the power factor the reading will reverse. For an 

 interesting case in point, see Journal of the Institution of Electrical 

 Engineers, vol. 30, 1901, p. 467. 



The ratio of the true to the apparent power is 



PL+H C = COS L 



P w cos BP cos (B L - Op) 



and the total power is obtained by multiplying the apparent 

 power by the correction factor F. 



The trigonometrical form of this expression may be changed 

 so that the tangents rather than the cosines of the angles may 

 be used, then 



w = 1 + tan 2 B P 

 1 + tan P tan B L 



This is the usual form of the correction factor. 



If there are no modifying causes such as capacity or eddy- 

 current effects, tan B P may be expressed in terms of the inductance 

 and resistance of the potential circuit. Then 



F = '" p/ (7) 



1 + tan 



