COMPARISON OF GALVANOMETERS. 257 



corrected by the graduation which correspond to the passage of 

 the same current in the two instruments, we have 



(37) i-fi-fr. 



When the sensitiveness of the two instruments are of the same 

 order, the deflections 8 and 8' are given directly by the common 

 current ; if not, a shunt is interposed in the most sensitive instru- 

 ment, and the corresponding deflection is equal to the product of 

 the deflection observed, by the multiplying power of the shunt. 



If the galvanometer of comparison the first, for instance is 

 an absolute instrument, this experiment at once gives the factors 

 by which the indications of the second galvanometer must be multi- 

 plied to deduce from it the intensity of the current in absolute 

 values. 



876. If the external field were the same for the two v instruments, 

 we should deduce from equation (37) the ratio of the galvanometric 

 constants of the two coils ; but local influences generally prevent this 

 identity, even for points which are pretty near. 



In order to determine directly the galvanometric constant G' of 

 a galvanometer, it is placed in the centre of the coil of a tangent 

 galvanometer, the mean planes of the windings being parallel, and 

 the two coils are shunted at two points of the same circuit, so as to 

 form a sort of differential galvanometer. If I is the intensity in the 

 tangent galvanometer, I' that in the other, and 8 the deflection, 

 we have 



H tanS = GI-GT. 



The action of the galvanometer will generally preponderate, but 

 it can be reduced by a suitable resistance ; if we bring the needle to 

 zero, we have 



The total resistances of the two instruments being g and g' t we 

 may also write 



VOL. II. 



