24 



Mr. R. S. Brough on Wheatstone's Bridge. 



meter resistance is greater than the battery resistance (a condi- 

 tion which should always be fulfilled in testing telegraph-lines), 

 the galvanometer should be made to connect the junction of the 

 two greater resistances with that of the two lesser, thus : — 



LTJIiC. 



M 



Now, considering the question of what resistance practically to 

 give a galvanometer intended to be used in a Wheatstone's 

 bridge for testing telegraph-lines, the condition of maximum 

 sensibility becomes 



y a+b+w+x 



Supposing the branches a and b to each consist of three coils of 

 10, 100, and 1000 ohms resistance respectively, and w to be 

 variable between 1 ohm and 10,000 ohms, and putting* 



a = 10 ohms 



b = 1000 „ 



w = 5500 „ 



#=550,000 „ 



then we have approximately 



g =5500 ohms. 



Government Telegraph Department, 

 Electrician's Office, Alipore, 

 November 21, 1873. 



* See Mr. Schwendler's paper in the Philosophical Magazine for Mav 

 1866. 



