THE MEASVRRMENT OF CURRENT 17 



or, if the ratio of the area of the bare to that of the covered wire 

 be u, 



G = K 3 ^/y^ (4) 



The Best Resistance for a Thomson Galvanometer. It is well 

 known that in the practice of most methods of electrical testing, 

 the precision obtainable depends on the proper adjustment of the 

 galvanometer resistance to the work in hand. 



Consider the simple case of a galvanometer in series with a 

 definite resistance, R, and a battery of electromotive force E. 

 It will be assumed that the bobbin on which the galvanometer 

 coil is wound is of fixed dimensions. 



It is easy to see that if a very large wire is used the current 

 will be considerable, on account of the low resistance, but as 

 there are only a few turns, the ampere-turns or the magnetic 

 effect at the needle, to which the deflection is proportional, will 

 be small. If a very fine wire is used, the resistance will be large 

 and the current small and while the number of turns is great, the 

 ampere-turns and the resulting deflection will again be small. 

 Between these two extremes there will be a size of wire which 

 will correspond to a maximum deflection of the galvanometer. 



The "best galvanometer resistance" is that obtained when the 

 coil is wound with the size of wire which renders the deflection 

 a maximum. 



The deflection of the instrument, 1), will !><> proportional to the 

 magnetic force at the nccdlr, or 



D = Kl,(! 

 and 



I 



'< 



Ro 



!) = KJlVuRo 



R is the resistance external to the galvanometer. It is to be 

 noticed that R Q is a function of u. The deflection is to be made 

 a maximum by varying R a . If V is the fixed volume of the coil 

 and w is the resistance per unit volume of the winding, 



A', = Vw 



