ELECTRICITY METERS 



469 



When the consumer uses energy the voltage at the load is 

 diminished by the 1R drop in the current coils of the meter. This 

 of course varies with the load. For a small direct-current meter 

 (5 amp.) the drop in the field coils may be about 1 per cent of the 

 line voltage when the rated full-load current is used. The ex- 

 pense of energizing the field coils falls on the consumer. 



Use of Watt -hour Meters on Three-wire Circuits. For meter- 

 ing on low-tension, three-wire, direct-current circuits such as are 

 used in congested districts in cities, two ordinary two-wire meters 

 may be used, one with the current coils in the positive lead, the 

 potential circuit being connected between this lead and the neu- 

 tral wire, while the other is similarly connected on the negative 



\nn-t : 



Neutral 



Fio. 270. Diagram for three-wire direct-current watt-hour meter. 



side of the circuit. Such an arrangement is free from errors due 

 to the unbalancing of the voltage of the circuit and to unequal 

 currents in the positive and negative leads; it is, therefore, a 

 desirable arrangement when the circumstances are such that these 

 effects may become very large. 



Ordinarily, a single three-wire meter is employed. In this 

 instrument, one of the current coils is in the positive, while the 

 other is in the negative lead. 



When traversed by equal currents, the two current coils should 

 have equal effects on the armature. The potential circuit may 

 be connected between the positive or the negative lead and the 

 neutral or between the positive and negative leads. Fig. 270 

 shows diagrammatically a three-wire meter with the first con- 

 nection. 



