ELECTRICITY METERS 



467 



voltage does not produce a proportionate change in the speed of 

 the armature. At moderate and full loads an increase of voltage 

 will tend to make the meter register too little. At light loads, 

 where the light-load coil F r furnishes a considerable part of the 

 driving torque, an increase of voltage tends to make the meter 

 register too high, for the torque of the light-load coil, which in- 

 creases as the square of the current in the armature circuit, may 

 more than counterbalance the tendency of the meter to register 

 too low due to the increase in the resistance of the potential 

 circuit. Practically, the effect of voltage variations will depend 

 on the load on the meter, for the position of the drag magnets 



1.040 

 1.030 

 1.020 



a 1.010 



1.000 



.980 

 .970 



A = Commutator Meter 

 B = Mercury Met'er 



'JO 



100 

 Per Cent Normal Voltage 



110 



120 



FIG. 2<)9. -Showing effect of voltage variation on the registration of direct- 

 current watt-hour meters at full-load current. 



and of the light-load coil is adjusted at some standard voltage. 

 For direct-current meters, at full-load current, a variation of from 

 10 per cent above to 10 per cent below the normal voltage should 

 not affect the accuracy by more than 3 per cent and at 10 per 

 cent of the rated full-load current, the effect should not be more 

 than 5 per cent. 



Accuracy at light loads is of great importance, for it is seldom that 



the meter in an installation is worked at full capacity. Indeed, 



for a great portion of the time the load on the meter may be but a 



small fraction of its rated capacity, and under these conditions it 



'utial that the energy be measured as accurately as possible. 



The meter should rotate continuously with 2 per cent of 



