in the Branches of a Wheatstone 9 s Bridge. 



165 



lines as drawn would be disturbed by the current which flows 

 across the new circuit. Before the connexion is made, how- 

 ever, it is evident that the potential-difference is represented 

 by the line PQ in the figure ; for the closed circuit ACD 

 (fig. 1) contains no impressed E.M.F. Hence the total fall of 

 potential around the circuit must be equal to zero. The three 

 E.M.F/s in these branches must therefore form a triangle, 

 and as the E.M.F.'s OP and OQ are determined, it follows 

 that the E.M.F. PQ must be that between the terminals 

 CD (5). 



We therefore have : 



That the condition for no current in the galvanometer is that 

 the points P and Q in the diagram shall coincide, or that the 

 line PQ shall be zero. 



Fig. 3. — Diagram the same as fig. 2, except that the points P and Q are 

 brought into coincidence ; a necessary condition for zero current in the 

 galvanometer of a Wheatstone's bridge. 



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D 



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f 



(T 7 --. 



In fig. 3 is represented another diagram constructed like 

 fig. 2, except that the points P and Q are brought into coin- 



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