470 ELECTRICAL MEASUREMENTS 



Theoretically, the three- wire meter is subject to certain errors; 

 for instance, with the connection shown in Fig. 270 the potential 

 lead being on main No. 1, an error will occur if the voltages are 

 unbalanced, for the power given to the circuit is 



P = VJt + VJ t . 



The angular velocity of the armature is supposed to be propor- 

 tional to this quantity, while in reality it is proportional to 



r,(J, + 70. 



Therefore, with a steady load, the correction which must be 

 added to the reading, reduced to watts, to obtain the true power, 

 is 



C = 7 2 (F 2 - Fi). 



This correction will be positive or negative depending on whether 

 F 2 or Vi is the larger. With the potential lead connected to 

 main No. 2, 



C = I^Fx - F 2 ). 

 If the potential circuit is connected between the positive and 

 negative mains, the angular velocity of the disc will be propor- 

 tional to 



HF(7x + 7 2 ) 



where V is the potential difference between the positive and nega- 

 tive mains. The correction in watts which must be added to 

 the reading to obtain the true power, will be the difference be- 

 tween P and this quantity, or 



- FJ 



~~2T 



The error in the registration will be zero if the currents in the 

 current coils are equal, even though the voltages are unbalanced. 

 It will also be zero if the voltages are balanced, even though the 

 currents flowing in the two current coils are unequal. If the 

 leads to the meter and load be of considerable resistance and the 

 voltages are balanced before any current is drawn, the meter 

 will always read too high when unequal currents are taken by 

 the two sides of the load; for if /i is greater than 7 2 the quan- 

 tity (F 2 Fi) is positive and the correction negative. If 7 1 is 

 less than 7 2 , (F 2 FI) is negative and again the correction is 

 negative. 



