608 ELECTRICAL MEASUREMENTS 



By using proper resistances at R, currents of all magnitudes can 

 be' measured by this method. 



Calibration by Means of Potentiometer. Direct-current 

 ammeters and voltmeters are most readily calibrated by means 

 of the ordinary or the deflectional type of potentiometer. Cur- 

 rents are determined by measuring the P.D. between the 

 terminals of standard resistances. These resistances should be 

 certified by the Bureau of Standards. This should be done once 

 a year, since the resistances are subject to slight changes. 



For voltage measurements, the range of potentiometers may 

 be extended upward from 1.5 volts to any desired extent by the 

 use of volt boxes. 



As a source of current for ammeter calibrations, a 4-volt storage 

 battery is most convenient. The cells may be charged in series 

 and discharged in parallel. Variations in the current are 

 obtained by the use of rheostats. These may have metal grids 

 for the large steps and a carbon compression rheostat for the fine 

 adjustments. 



For potential differences a storage battery should also be em- 

 ployed. The cells must be large enough so that they may 

 be properly taken care of. A drop wire furnishes the most 

 convenient means of regulating the P.D. at the instruments. 



Alternating-current Ammeters and Voltmeters. The larger 

 part of the alternating-current voltmeters in daily use for engineer- 

 ing work are based on the electrodynamometer principle. Such 

 instruments may be calibrated with direct currents, using either 

 a standard direct-current voltmeter, whose errors are accurately 

 known, or a direct-current potentiometer and volt box. On 

 account of the effect of the local field, it is essential that two read- 

 ings be taken at each point, first with a voltage in a noted direc- 

 tion, and then with it reversed. The two results should be 

 averaged. This procedure ignores the existence of any fre- 

 quency error. The magnitude of this error may be calculated 

 from the measured inductance and resistance of the instrument. 



Except in the case of the thermal instruments, which give the 

 same reading with both direct and alternating currents, and 

 regular electrodynamometers, with the two coils in series, it is 

 necessary to use alternating currents when calibrating alternat- 

 ing-current ammeters; for generally they are soft-iron instru- 



