604 



ELECTRICAL MEASUREMENTS 



Direct-current Instruments. As electrical measuring instru- 

 ments cannot be relied upon to give absolutely accurate results, 

 it is necessary to have methods for calibrating them. It should 

 be possible to assemble the apparatus necessary for dealing 

 with direct-current ammeters and voltmeters from the instru- 

 ments found in any laboratory devoted to general electrical 

 measurements. 



Voltmeter Calibration by Standard Cell. The method here 

 given is an application of Poggendorfs method (see page 269). 



An electromagnetic voltmeter is in reality a galvanometer in 

 series with a high resistance. The scale of the galvanometer is 

 graduated, not in current strengths, but in the voltages which it 

 is necessary to apply to the terminals of the instrument in order 

 to obtain the various deflections; that is, in values of I V R V 

 where Iv and R v are the current through, and resistance of, the 



w 



Std. Cell 



FIG. 369. Connections for voltmeter calibration. 



voltmeter. Therefore, if R v and I v have been measured, one 

 may find the true value of the P.D. applied to the instrument, 

 and this may be compared with its nominal value as read from 

 the scale. The difference will be the correction to be added to 

 the observed reading to obtain the true value of the P.D. 



The resistance of the voltmeter is determined by a Wheatstone 

 bridge. 



The necessary connections for the measurement of I v are 

 shown in Fig. 369. 



