62 



On a Modified Form of Wheatstone's Bridge. 



Here, again, the second term on the right-hand side is of the 

 second order, so we get simply 



5 = 



c' + c" 



or, the resistance of b is the mean of the two values of c and 

 its shunt obtained by reversing. 



The switch- board used by S. P. Thompson in carrying out 

 Carey Foster's method of comparing resistances may easily be 

 adapted to the shunt method. But a switchboard designed 

 for the purpose is more convenient : this form may be 

 suggested. 



In fig. 2 the shaded parts represent solid brass or copper pieces: 



Fig. 2. 



the dark circles binding screws, the light ones mercury-cups. 

 The coils to be compared are put between 1 and 2 and 1' and 

 2'. The terminals of the resistance-box are connected to 

 H, K, so that by inserting a plug it can be put in parallel 

 with either of the coils. 



1 and 1' are then connected by copper bars either to 3 and 

 4' or to 4 and 3'. The equal resistances /3 and y are of course 

 wound on the same reel, and may be soldered permanently in 

 place. M, N are the battery terminals, 0, P those of the 

 galvanometer. 



If the coils to be compared differ as much as a B.A. unit 

 does from a legal ohm, the resistances taken from the box 

 will give the difference to '0002 ohm, and interpolation by 

 means of the galvanometer deflexions need only be resorted 

 to beyond that. The sensitiveness of the method is of course 

 limited by that of the galvanometer. 



