466 



ELECTRICAL MEASUREMENTS 



Trouble may be experienced from improper wiring, leads carry- 

 ing large currents being placed too near the meter. This would 

 be likely to occur in heavy direct-current switchboard work, 

 for in this case the meter coils consist of only a few turns and the 

 busbars at the back of the switchboard may be very near the 

 meter. To obviate this trouble a special astatic wattmeter has 

 been designed. It is shown diagrammatically in Fig. 26&A. 



FIG. 268A. Diagram for astatic watt-hour meter. 



The spindle carries two equal armatures, one operating in the 

 field above and the other in the field below a straight conductor. 

 In consequence of this arrangement, variations of the local field, 

 which affect its strength equally at the upper and lower armatures, 

 have no effect on the registration. The drag magnets are so 

 placed that if the strength of one is increased by the extraneous 

 field, that of the other is diminished. The whole retarding 

 device is enclosed in an iron shield. 



Any watt-hour meter should maintain its accuracy under 

 varying conditions of voltage and load. In general, in the neigh- 

 borhood of the station, the voltage on a system of electrical 

 supply will remain nearly constant, especially if the system be 

 large ; but at a distance, owing to insufficient copper in the con- 

 ductors, the voltage variations may be considerable and the 

 accuracy of the meter should not be affected by them. Because 

 of heating, the resistance of the potential circuit of the meter is 

 dependent on the line voltage. Consequently, a change in line 





