Obtain the expression for current sensitivity of moving coil galvanometer.

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Solution

If $d θ$ is the change in deflection corresponding to the small change $d I$ in the current through a galvanometer, then its current sensitivity is
$S_{I} = \frac{d θ}{d I}$
In moving coil galvanometer, the deflection $θ$ is given by
$θ = \frac{N A B}{d C} I$
or $θ = K I$
where $K = \frac{N A B}{C}$ is a meter constant.
Differentiating the above equation with respect to $I$ .
$\frac{d θ}{d I} = K$
So $S_{I} = K = \frac{N A B}{C}$ . So, for a moving coil galvanometer with a radial field, its current sensitivity depends upon $N$ , $A$ and $B$ and inversely proportional to $C$ .

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