ELECTRICITY METERS 471 



If alternating currents are used and the loads are reactive, 

 these relations are still further complicated by the phase dis- 

 placements of the currents. The corresponding corrections are 

 readily deduced. 



Practically it is impossible to make allowance for these errors, 

 for the loads on the two sides of the installation are continually 

 shifting; the best that can be done is to see that the load distribu- 

 tion is such that the two sides are well balanced. 



The Use of Commutating Watt-hour Meters on Alternating- 

 current Circuits. Before the introduction of induction watt- 

 hour meters, it was customary to employ commutating meters 

 on reactive circuits. In this case the reactance of the potential- 

 coil circuit introduces an error, for the potential-coil current in 

 it is not in time phase with the voltage applied to the load and 

 the mean product of the currents in the fixed and movable ele- 

 ments of the watt-hour meter will not be proportional to the 

 power delivered to the circuit. The error so arising will be in- 

 significant when the instrument is used on a non-inductive load, 

 but when the power factor is low, the error may become of im- 

 portance. A general discussion of this phase-angle error will be 

 found in the section on the " Electrodynamometer Wattmeter," 

 page 309. 



Lag Coil. It is necessary to adjust the phase difference be- 

 tween the current in the armature and the current in the field 

 coil so that this phase difference is the same as that between the 

 voltage applied to and the current in the load, and to do this 

 without greatly altering the current in the field coils. This is 

 accomplished by the use of the "lag coil," which is a non-inductive 

 shunt placed around the field coils of the instrument. 



Its action may be made clear by the following: 



Let Rp = resistance of potential-coil circuit. 

 Lp = inductance of potential-coil circuit. 

 Re = resistance of current coils. 

 Lc = inductance of current coils. 



Ra = resistance of "lag coil" or shunt around current coils. 

 / = load current. 



Is = current in lag coil. 



Ic = current in current coil. 



IP = current in potential coil. 



V = line voltage. 



