366 



COMPARISON OF RESISTANCES. 



mercury cups, two of which are of ebonite, and by a commu- 

 tator D. Their function may be interchanged in the equilibrium 

 of the bridge without displacing them. 



\ 967. With the ordinary arrangement of wire bridges, the re- 

 sistance of the copper bands which serve as junctions may be 



neglected, and the ratios - - and of the resistances to be 



a b 



compared should be so close that the difference might be com- 

 pensated by two portions of the wire. As these resistances are 

 very small about one ohm, for instance a galvanometer with a 

 high resistance would, so to speak, be short-circuited. In this 



Fig. 192. 



case a galvanometer of small resistance, satisfying the conditions 

 Of (959), should be used 



If the breaks P and Q (Fig. 191) are themselves formed of 

 resistances which can be neglected, the equation of equilibrium 

 reduces to 



a' l-x 



As the second member of 'this equation may have all positive 

 values, any given resistances might be compared by this method ; 

 but it is easy to see that the sensitiveness diminishes as the point 

 of equilibrium comes near one or other end of the wire. If we 

 call AJC the displacement of the contact corresponding to a variation 

 A3' of b', we have 



b 1 



' 



