526 



SUBMARINE CABLE LAYING AND EEPAIRING. 



their number to any considerable extent or to sub-divide them 

 into a sufficiently large number of small coils. 



This difficulty has been met by the device known as the 

 Kelvin-Varley Slides, by means of which 10,000 equal divi- 

 sions of resistance are obtained with only 201 coils. 



In order to explain the action of the slides, take a simple 

 case of 21 coils, which can be divided in the same way into 100 

 equal parts. 



Let AB (Fig. 297) consist of 11 coils of equal resistance, and 

 CD 10 coils. Let the whole resistance between C and D be 

 equal to that of two coils on AB. Let there be two contact 



Test Battery 

 Fig. 296. — Comparison of E.M.F.'s. 



pointers, E and F, connected to the ends of CD, rigidly con- 

 nected together, but insulated from each other. These can be 

 moved along AB so as to enclose any pair of coils. It is evi- 

 dent that between the pointers E and F the resistance to any 

 current flowing between A and B is equal to one only of the 

 colls on AB, since the two coils between the pointers are 

 shunted by an equal resistance. 



The potentials produced by the main battery connected to 

 AB are shown by the dotted lines. Between E and F there 

 is the same fall of potential as on any single coil on AB 

 outside the pointers, and the fall of potential at the ends of 

 any pair of colls on AB between the pointers EF is sub- 

 divided into 10 parts along CD. And since the resistance 

 between the pointers in any position is equal to that of one 



