DIFFERENTIAL GALVANOMETER. 333 



galvanometer;* it is then advantageous to pass the current alternately 

 and in opposite directions in two coils, and we have 



r r' gr 



P+ 5 



, 



g+r g' + r' 

 Putting 



DI = P (g+ r) (g f + r) +gr (g 1 + r'} +g' r' (g+r), 

 we get 



S = ki- Kf = j| \kr (4 + r')-k'/(g+r)}. 



As before, we have r = r for 8 = 0, when the galvanometer is 

 adjusted, and we might eliminate any defects of adjustment by 

 substitution. 



With an adjusted galvanometer, the deflection S x which a 

 difference dr produces between the two resistances is 



Comparing with the value obtained in the preceding arrange- 

 ment, we have 



D i 



it is seen that the second method is more sensitive than the rest, 

 when 



r + 2 P ) g> P (g+ r) + 2gr, 

 or 



r<g. 



If the ratio - is very small, we have sensibly 



o 



* HEAVISIDE. Journal of the Society of Telegraph Engineers, Vol. II., p. 115. 1873. 



