3i8 ELEMENTARY LESSONS ON [CHAP. vi. 



ascertaining the accumulated charge by discharging it 

 through a ballistic galvanometer (Art. 204). 



357. Fall of Potential along a Wire. To under- 

 stand the principle of Wheatstone's Bridge we must 

 explain a preliminary point. If the electric potential of 

 different points of a circuit be examined by means of an 

 electrometer, as explained in Art. 263, it is found to de- 

 crease all the way round the circuit from the + pole of 

 the battery, where it is highest, down to pole, where 

 it is lowest. If the circuit consist of one wire of uniform 

 thickness, which offers, consequently, a uniform resistance 

 to the current, it is found that the potential falls uniformly ; 

 if, however, part of the circuit resists more than another, 

 it is found that the potential falls most rapidly along the 

 conductor of greatest resistance. But in every case the 

 fall of potential between any two points is proportional to 

 the resistance between those two points ; and we know, for 

 example, that when we have gone round the circuit to 

 a point where the potential has fallen through half its 

 value, the current has at that point gone through half 

 the resistances. 



358. \Kfteatstone's Bridge. This instrument, 

 invented by Christie, and applied by Wheatstone to 

 measure resistances, consists of a system of conductors 

 shown in diagram in Fig. 1 30. The circuit of a constant 

 battery is made to branch at P into two parts, which 

 re-unite at Q, so that part of the current flows through 

 the point M, the other part through the point N. The 

 four conductors D, C, B, A, are spoken of as the " arms " 

 of the " balance" or "bridge;" it is by the proportion 

 subsisting between their resistances that the resistance 

 of one of them can be calculated when the resistances of 

 the other three are known. When the current which 

 starts from C at the battery arrives at P, the potential 

 will have fallen to a certain value. The potential of the 

 current in the upper branch falls again to M, and 

 continues to fall to Q. The potential of the lower 



