12 DIRECT CURRENT GENERATORS. [Exp. 



and without delay so as to minimize the effect of heating. The re- 

 sistance R is equal to E-t-I. 



Fig. 6 shows the usual arrangement of apparatus, in case the volt- 

 meter current is but a small part of the total current. The voltmeter 



leads should be connected di- 



Adjueting Resistance , . . , 



rectly to the resistance to be 

 measured (not including un- 

 necessary connectors, etc.) 

 or should be pressed firmly 

 against its terminals. The 

 resistance of an armature 



winding is taken by pressing 



FIG. 6. Measurement of resistance by 



fall-of-potential method. the voltmeter le *ds against 



the proper bars 180 or 90 



apart; if resistance of brushes and connections is to be included, the 

 voltmeter is connected outside of these connections. 



In case the ammeter current is very small, so that the voltmeter 

 current is a considerable part of the total current, the voltmeter should 

 be connected outside the ammeter so as to measure the combined 

 drop of potential through the ammeter and unknown resistance. 

 With the voltmeter connected either way, an error is introduced 

 which may often be neglected but can be corrected for when par- 

 ticular accuracy is desired. 



18. The voltmeter should always be disconnected before the cir- 

 cuit is made or broken, or any sudden change is made in the current, 

 to avoid damage to the instrument. 



If the resistance being measured is highly inductive, not only the 

 instrument but also the insulation of the apparatus under test may 

 be damaged by suddenly breaking the current through it on account 

 of the high electromotive force induced by the sudden collapse of 

 the magnetic field. This may be avoided by gradually reducing the 

 current before breaking the circuit. 



19. The value of an unknown resistance can be found in terms 

 of a known resistance placed in series with it by comparing the drops 

 in potential around the two resistances, the current in each having 

 the same value. 



