316 

 tne main conductors from the marks S' and T to their ends. Lastly, 



f ^5'Tlf^T 1 "1 

 let R denote the resistance in the double channel I ,Q^ > between 



S f and T. By the well-known principles of electric conduction, we 

 have 



R= _ 1 - ...... (1) 



S'BCT + S'QT 



for the resistance in the double arc between S' and T. Then, by 



addition, we have 



SS' + R+TT', 



f S'BCT 1 

 for the resistance from S to T' by the channel SS' < 0^ > TT'. 



I o v^l J 



This whole resistance is divided, by Q and its equipotential point 

 in the direct channel S'BCT, into the parts 



Hence if, for simplicity, we suppose the potential at S to be 0, 

 and at T' to be E, and if we denote by q the potential at Q, we 

 have 



' ' ( 2 ) 



SS' + R + TT 



Again, since P divides the resistance between S and T', along the 

 channel SPT', into the parts SP and PT', we have 



(3) 



if we denote by p the potential at P. Hence 



SS' + |^.R-^ (SS' + R+TT') 

 q p=E SS + R+TT' 



SP PT' 

 or, since 1-- 



SP TT + R 



.T ^+R 7 ~' 



-- ~~SS4-R+TT' 



Now let us suppose that, by varying one or more of the component 



