304 



Steady Currents in Linear Conductors [CH. ix 



the points DE. Denoting the resistances of the coil and plug by E C) R p , the 

 resistance between D and E will be 



1 



-- + -i-' 

 Re R P 



and since R p is very small, this may be neglected. When the plug is 

 removed, the resistance from D to E may be taken to be the resistance of 



FIG. 97. 



the coil. Thus the resistance of the whole box will be the sum of the 

 resistances of all the coils of which the plugs have been removed. 



351. The Wheatstone Bridge. This is an arrangement by which it is 

 possible to compare the resistances of conductors, and so determine an 

 unknown resistance in terms of known resistances. 



The " bridge " is represented diagrammatically in fig. 98. The current 

 enters it at A and leaves it at D, these points being connected by the lines 



ABD, A CD arranged in parallel. The line AD is composed of two con- 

 ductors AB, BD of resistances R l9 R 2 , and the line AGD is similarly composed 

 of two conductors AC, CD of resistances R 3 , R. 



If current is allowed to flow through this arrangement of conductors, it 

 will not in general happen that the points B and C will be at the same 

 potential, so that if B and C are connected by a new conductor, there will 

 usually be a current flowing through BC. The method of using the 

 Wheatstone bridge consists in varying the resistances of one or more of the 

 conductors R l} R ZJ R 3) R 4 until no current flows through the conductor BC. 



When the bridge is adjusted in this way, the points B, C must be at the 

 same potential, say v. Let V A , V D denote the potentials at A and D, and 



