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arcs in the balance-circuit, we reduce the galvanometer indication 

 to zero, that is to say, make qp = 0. We shall have by equating the 

 numerator of the preceding expression to zero, and resolving for TT', 



To interpret this expression, it may be remarked that if the second 

 term vanishes, that is to say, if 



^SPT' S'Q 



wehave 



and this is the condition aimed at in the arrangement. Now the 

 connexions at S and T" must be made so good that the resistance 

 SG in the first is inappreciable in comparison with GP, and the 

 resistance HT', in the second, inappreciable in comparison with PH ; 



so that we may have 



PT' PH 

 SP H GP' 



where a denotes an equality not perfect, but having no appreciable 



error : and hence 



pTT 



The condition 



SPF S'Q 



SP~ )~ 

 is to be secured by one or other of two ways or by both combined ; 



that is, by making 



RHO ... ........ (a) 



SPT'S'C) ,,. 



' ' ' ' 



or each as nearly as possible. If the connexion BC were quite perfect 

 and the marks S' and T were at the very ends of the conductors, 

 the condition (a) would be fulfilled and there would be no necessity 

 for the condition (6). We should then have a perfect Wheatstonn 



balance, the secondary testing-conductor S T f Q becoming merely 

 a part of the galvanometer electrode. Hence whenever the re- 



