374 COMPARISON OF RESISTANCES. 



two dials and the supplementary unit form thus 100 units, which will 

 serve to compare the tens ; and in the same manner with the coils 

 of 100 units. 



In determining two sets of coils b and b', care must be taken 

 to determine the resistances of the connecting wires /3 and /3'. As 

 the resistances b and b' are almost equal and the values of f3 and /3' 

 very weak, then, if / is the ratio obtained directly, which is very 

 near unity, 



or 



.>-/* 



This precaution is particularly important in the comparison of 

 units. A table of the values of the coils is then made as a function 

 of one of them or of the mean value. 



975. THOMSON'S DOUBLE BRIDGE. Sir W. Thomson has de- 

 vised a modification of Wheatstone's bridge, which resembles the 



wire bridge in that equilibrium is established by means of sliding 

 contacts. This arrangement (Fig. 197) comprises nine conductors, 

 which may be regarded as forming the nine edges of a truncated 

 triangular pyramid. The galvanometer being inserted in the branch 

 CD, the current is null when the two points C and D are at the 

 same potential. This condition cannot be realised unless a certain 

 point C : of the branch AjBj is at the same potential as the two 

 points C and D. There would then be no current in a conductor 

 connecting the points C x and C. 



