352 COMPARISON OF RESISTANCES. 



and if y' is the differential of y in respect of ^', the condition of the 

 minimum is 



As the factor within the bracket cannot be zero, it follows that 

 y' = 0, or 



Formulae (30) give then 



(33) * 



R 



958. It will be seen that if R and r are very large compared 

 with 3, we have 



and the ratio of the arms of the bridge is very small. 

 a 



This ratio is also very small if the resistance r alone is very 

 great, which gives appreciably 



As the latter equation may be put in the form 



it shows that a>b. 



If we are desirous of realising the condition of maximum sensi- 

 tiveness with equal arms a and a', equations (33) will give 



