Define the current sensitivity of a galvanometer. Write S.I unit. Figure shows two circuits each having a galvanometer and a battery of 3V. When the galvanometer in each arrangement do not show any deflection obtain the ratio R1/R2.

The deflecting torque in a radial magnetic field, on a coil of galvanometer is given by , τ=nBIA, where n=number of turns, I=current in coil, A=area of coil, B=magnetic field in which coil is placed, in equilibrium, kθ=nBIA, where k=restoring torque/unit twist, θ=deflection in coil, therefore, θ/I=nBIA/k, this θ/I is called current sensitivity i.e. deflection in galvanometer for per unit current. Its S.I. unit is radian/A. As there is no deflection in galvanometer in both arrangements therefore both are in balanced state, for first arrangement, 4/R1=6/9, or R1=36/6=6Ω, for second arrangement, R2/8=6/12, or R2=48/12=4Ω, therefore R1/R2=6/4=3/2