chap, xiii.] WHEATSTON&S BRIDGE. 139 



makes connection with the end B of the compensator 

 wire. To the same binding screw c of the resis- 

 tance box Re, by which r is connected with it, is con- 

 nected a wire leading to the galvanometer G, and the 

 other terminal of G is attached to the slider s of the long 

 compensator. Now when the current reaches A it will 

 split into two branches, of which one branch current 

 will pass along the long compensator to B, and the 

 other will pass up to r, c, and Re. But at s and c the 

 two branches will also split, part of the branch rcne 

 passing down to the galvanometer, part of the branch 

 ASB passing up to G. These two currents, being in 

 opposite directions, will deflect the needle in opposite 

 ways. If one is in excess it will deflect the needle in 

 one direction, if the other is in excess, it will deflect 

 in the opposite direction ; if both are equal they will 

 neutralise one another, and the needle will remain 

 at zero ; or, to put it more accurately, when the 

 potential at s is equal to the potential at c, no 

 current will pass through the galvanometer. 



Now, as already explained (page 33), the strength 

 of the current in the two branches depends on the resis- 

 tances in the two branches, and this can be altered by 

 the position of the slider s. Consequently, all that is 

 necessary to secure equal potentials at c and s is to 

 move the slider one way or the other till the needle 

 returns to zero, this indicating the desired equality. 

 Now, it can be shown that when no current traverses 

 the galvanometer, the resistance of r is to the resis- 

 tance of Re as the resistance of As is to the resistance 



of SB thus, 



r : ~R : : AS : SB 



Put in another way, this is 



r AS 



SB ' 



and therefore 



AS 



r R* X 



SB 



