DIFFERENTIAL GALVANOMETER, 331 



If we adjust the resistance / so that the needle stops at zero, 

 we get 



k g+r 



When the differential galvanometer is adjusted, we have k = k' 

 and g=g f ', consequently 



It is easy to get rid of errors of adjustment by substitution, as in 

 a double weighing; a variable resistance which is not standardised 

 is used for r' ; r is then replaced by a standard resistance r^ which 

 re-establishes equilibrium, and we have 



If, instead of bringing the needle to zero in each experiment, 

 it was observed at the same point corresponding to a deflection S, 

 the method of substitution would only be exact provided the 

 principal current I, and therefore the electromotive force, remained 

 unchanged. 



938. The absolute sensitiveness of the method is determined 

 experimentally by ascertaining what deflection is produced by a 

 known variation of one of the resistances. 



Suppose that, the differential galvanometer being adjusted, the 

 two resistances r and r' differ by dr. 



The deflection 8 is given by the equation 



and the absolute sensitiveness is measured by the ratio 



8 kE 



k 

 for a given electromotive force, it is proportional to the factor 



