90 



THE ROYAL SOCIETY OF CANADA 



It is desirable for reference in explaining subsequent details, to 

 describe, and to explain briefly, the instrument itself. This Slide is 

 devised from a special form of the Double Bridge designed by Kelvin. 

 If nine conductors are placed in the form of a network, one branch 

 containing a galvanometer and another an electromotive force, and 

 arranged as shown in Fig. 1, we have the general form of the Double 

 Bridge. 



Let ri, r 2 , r s , r 4 , r b , r 6 , t- 7 , r 8 , r 9 be the nine conductors and let the 

 galvanometer g be contained in r b and the battery b in r 6 . If no current 

 passes through the galvanometer it can easily be shown that 



r s + 



7-77-9 



f7+ 7-8 + 7-9 



n + 



rsr<> 



(1) 



7-7 + 7-8 + 7-9 



The Kelvin-Varley Slide consists of an -arrangement of this kind 

 in which 7-7 + 7-8 = 7-9, and therefore equation (1) reduces to 



7-3 + 



7-4 + 



(2) 



Figures 2 and 3 show the arrangement of this system in the K.-V. 

 Slides, the latter being an actual diagram. The same letters are used 

 in each of the three figures for the corresponding junctions. It will 

 be apparent how connections can also be made for using the instru- 

 ment as a potentiometer in the usual way. 



Fi s . 2 



