ELECTRICITY METERS 



405 



the brake is at a considerable distance from both the current and 

 potential coils, the error is practically that due to the change in 

 temperature of the potential circuit. 



The net result of the heating due to the current coils is that if 

 the meter is adjusted at full-load by changing the position of the 

 drag magnets and then at light load by means of the light-load 

 coil, the registration will be correct at light and at full-load, 

 will be a little too great at intermediate loads and a little too 

 small at overloads, the general form of the percent-registration 

 curve being that shown at AHF in Fig. 268. 



20 



60 80 100 



Per Gent Full Load 



120 



140 



ICO 



Fir,. 268. Pertaining to self-heating error due to current coils of direct- 

 current watt-hour meter. 



In detail, if there were no errors due to the heat from the cur- 

 rent coils the per cent-registration curve, if the meter were adj usted 

 to register correctly at full-load and at light load, would be that 

 derived from equation (4), all the coefficients being constant. 

 If the adjustments were made at 10 per cent load and full-load, 

 the curve would be AHB. 



The effect of the heating is to increase R. This causes the 

 upper parts of the curve to droop, the result being the curve AC. 

 By moving the drag magnets the curve AC may be shifted bodily 

 so that it takes the position DGE and by means of the light-load 

 coil the registration at some low load (10 per cent load) may be 

 made correct. The percent-registration curve is then AHF. 

 The registration is correct at 10 per cent load and at full-load. 

 These self-heating errors are important in portable rotating 

 standard watt-hour meters such as are referred to on page 495. 



The meter should be unaffected by local magnetic fields. 



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