THE MEASUREMENT OF CURRENT 



19 



To illustrate the influence of the size of wire on the deflection 

 of a galvanometer suppose the volume of the coil to be 5 cu. in. 

 and the external resistance 1,900 ohms. The relative deflections 

 calculated by aid of formula (5) and the table are plotted in 

 Fig. 9. 



The figure shows that it is better to err on the side of too great 

 resistance and that a considerable departure from the ideal 



No.34 



'NolffiX 

 No.36 



50 



F No.30 



40 



>30 



No.20 



10 



External Resi 



tance-1900 Ohms 



No.40 



1000 2000 3000 4000 5000 6000 



Galvanometer Resistance- Ohms 



FIG. 9. Illustrating effect of galvanometer resistance on the deflection. 



resistance does not greatly affect the sensitiveness of the 

 arrangement. 



An important use of the reflecting galvanometer is in insula- 

 tion testing, where the current is mpasured in a circuit of exceed- 

 ingly high resistance. The galvanometer resistance cannot be 

 more than a few thousand ohms so the current is practically 

 independent of it. This case corresponds to that of the ordinary 

 ammeter; every turn adds something to the deflection, the 

 amount becoming smaller and smaller as the diameter of the coils 

 increases. In this case, there is no "best resistance." 



The Sensitiveness of Reflecting Galvanometers. The sensi- 

 tiveness of a Thomson galvanometer depends on the arrangement 



