THE CALIBRATION OF INSTRUMENTS 609 



ments, whose indications on direct-current circuits are compli- 

 cated by the effects of residual magnetism in the iron vane. 

 Also there may be errors due to wave form. These same remarks 

 apply to soft-iron voltmeters which are intended for alternating 

 currents. Induction ammeters and voltmeters must be cali- 

 brated with alternating current. 



The calibrations may be made by the alternating-current 

 potentiometer, used in connection with non-reactive volt boxes 

 and shunts, but this instrument is not yet in common use. 

 It would not be serviceable if wave-form errors were being 

 investigated. 



The arrangement shown in Fig. 371 does very well for ammeters. 



220 VoltS 



Transfer Inst. 



Hot Wire Ammeter 



aud Shunts 



FIG. 371. Connections used in calibrating alternating-current ammeter. 



The hot-wire ammeter, provided with an appropriate set of 

 shunts, is used as a transfer instrument. The ammeter under 

 calibration is compared with it, and then by means of the double- 

 throw switch the hot-wire instrument is transferred to the direct- 

 current circuit and calibrated at the proper point by means of a 

 potentiometer and standard resistances. This procedure avoids 

 all questions as to the permanence of the calibration of the hot- 

 wire instrument. A difficulty is that the range of a hot-wire 

 ammeter is short, the deflection depending upon the square of 

 the current. Consequently, the scale is of such a nature, ihat 

 even with care only about the upper 60 per cent of it is readable 

 with sufficient accuracy. Therefore the millivoltmeter part 

 should be sensitive, and the range extended by numerous shunts, 

 so that the deflection may be kept in the upper part of the scale. 



Soft-iron voltmeters may be compared with a dynamometer 

 instrument which has been calibrated with direct current. 



39 



