POWER MEASUREMENT 313 



It is preferable to use the correction term of equation (4) lather 

 than the factor given in equation (7), for the latter becomes 

 practically indeterminate at low power factors. 



Compensation for the Inductance of the Potential Circuit. 3 

 It naturally suggests itself that the effect of inductance in the 

 potential circuit may be annulled by the use of capacity. Simple 

 tuning of the circuit is manifestly inadequate, for the arrange- 

 ment must be such as to be practically independent of the 

 frequency, in order that there may be no errors when dealing 

 with non-sinusoidal waves. 



The arrangement used for this purpose by Abraham and Rosa 

 is a condenser inserted in series with the movable coil and 

 shunted by a non-inductive resistance. Such an arrangement 

 may be adjusted so that, for all practical purposes, the net 

 reactance of the circuit is reduced to zero. 



The connections of the potential circuit then become those 

 shown in Fig. 181. 



R.L 



f-c-r, L=O->J 



k . '00000000 



\0 



c 



FIG. 181. Showing method of compensating the inductance of the potential 

 circuit of a wattmeter. 



The impedance between a and b is 



1 7- r - 



The impedance between b and c is 

 Z bc = R + j 

 Therefore the total impedance is 



= R + jo>L + = ( R + T+~co~W/ 



Cr 2 



