DIFFERENTIAL GALVANOMETER FOR MEASURING RESISTANCES. 383 



The way in which the contacts are made has no influence provided 

 the resistance of the galvanometer is considerable. 



The use of the differential galvanometer in the conditions 

 pointed out in 939 appears still the most exact method. 



The two conductors are traversed by the same current ; the 

 ends of the wires of the two coils form for each of them the 

 electrodes 2 and 3 (977). The galvanometer being regulated so 

 that the galvanometric constants of the two coils are equal, the 

 resistances g and g' of the two circuits, are adjusted so that the 

 needle remains at zero. 



The expression for the difference of potential between the two 

 electrodes A and A' is 



on the other hand we have 



and therefore 



The other coil, connected with the points B and B' of the second 

 conductor, gives also 



from this follows 



g' - 



A fresh experiment is then made by giving new values g l and g' lt 

 very different from the first, to the resistances of the galvanometer, 

 and we have 



x _gi-y g-y gi-g 



x> g'i-y' g'-y' g'i-g'' 



982. MEASUREMENT OF VERY GREAT RESISTANCES. The 

 methods given in 902 and 940 enable us to compare very 

 high resistances with the standard resistances with which we 

 ordinarily work. We may attain the same results by other 

 arrangements. 



