628 



ELECTRICAL MEASUREMENTS 



The deflection may be controlled by varying C. Currents may 

 be dealt with in the usual way by determining the P.D. between 

 the terminals of a non-reactive resistance through which the 

 current flows. 



Determination of the Average Values of Potential Difference 

 and Current Waves. If the commutator is arranged as in Fig. 

 392, average values of the potential difference and current may 

 be determined. To do this the brushes must be so set that the 

 reversals are at the zero points of the wave. When the switch 



FIG. 392. Connections for determining average value of alternating-cur- 

 rent wave. 



is on 1 the galvanometer is subjected to a series of unidirectional 

 impulses and gives a deflection proportional to their average 

 value. The instrument may be calibrated by transferring it to 

 the line side of the commutator and applying a known direct- 

 current voltage. 



To properly set the brushes the switch is placed on 2 so that the 

 condenser C is in circuit and the position of the brushes altered 

 until the galvanometer stands at zero. Currents are dealt with 

 by using a non-reactive resistance as in the previous method. 



D.O. 



FIG. 393. Connections for determining form factor. 



Determination of Form Factor. To determine the form factor, 

 an electrodynamometer voltmeter and a direct-current voltmeter 

 are placed in parallel as shown in Fig. 393. 



