ELECTRICITY METERS 



479 



errors inherent in the construction of the instrument, which are 

 not found in instruments based on the electrodynamometer prin- 

 ciple. In the main, these errors are due to incorrect phase rela- 

 tions of the various fluxes and to saturation effects in the iron. 

 They are most troublesome at low power factors and with badly 

 distorted wave forms. The following curves apply to a single 

 meter, in which the errors were much exaggerated. Unless 

 otherwise specified the wave forms contained no irregularities or 

 peaks. 



Temperature Errors. The temperature of the instrument may 

 be altered either through self-heating or change of room tempera- 



i.o4 



1.02 

 1.00 



H.98 

 1% 



3 



*.94 

 .92 

 .90 



Induction Watt-hour Meter 

 Effect of Temperature 

 Frequency=60 Cycles 

 E =110 Volts 

 Smooth Waves 



1C 20 30 40 50 60 70 80 90 100 110 120 130 

 Per Cent K.V.A.Load 



FIG. 273. Showing effect of temperature on induction watt-hour meter. 



ture. If the temperature rises, the resistance of the disc increases 

 so that while the driving torque is decreased, the retarding torque 

 is lessened in about the same proportion, the two effects thus 

 tending toward compensation. There are certain other effects; 

 for instance, the drag magnets decrease in strength and as their 

 effect depends on the square of their strength, a 1 per cent change 

 will change the retarding torque 2 per cent. The resistances of 

 the potential and lag coils change and disturb the lag adjust- 

 ment; the permeability of the iron and the iron losses are also 

 changed. The net effect is very different in different meters. 



The effect may be of importance in the use of portable rotat- 

 ing standard watt-hour meters. With some types of such meters 



