THE CALIBRATION OF INSTRUMENTS 611 



When calibrating high-capacity wattmeters, in order to save 

 power and bring the work within the range of the apparatus 

 found in a well-equipped laboratory, it is necessary to resort to 

 fictitious loading; that is, to supplying the current and potential 

 coils from two distinct sources. 



The connections for a calibration are then as shown in Fig. 373. 



It is convenient to use storage cells for supplying both the 

 current and potential circuits. Two readings are made at each 

 point, both the current and the potential switches being reversed. 

 The results are averaged to eliminate the effect of the local field. 



FIG. 373. Connections for fictitious loading of wattmeter. 



In this procedure eddy current and frequency errors are 

 assumed to be negligible. If there is doubt as to this, the watt- 

 meter must be compared with one known to be free of them 

 using alternating currents of the proper frequency. 



If an induction meter is being tested, alternating current of the 

 proper frequency must be used and the instrument compared 

 with an electrodynamometer wattmeter which has been calibrated 

 with direct current. 



In case it is necessary to test at different power factors, the 

 supply may be derived from two alternating-current machines, 

 having the same number of poles, with their armatures on the 

 same shaft, one field being arranged so that it may be given any 

 desired angular displacement about the axis of rotation. The 

 wave form of both machines should be sinusoidal. A phase 

 shifting transformer (see page 290) may also be used and is 



more convenient. 



Reference 



"Testing of Electrical Measuring Instruments," Circular No. 20, U. S. 

 Bureau of Standards. 



