Moving Coil Galvanometer Sensitivity

In a moving coil galvanometer, equilibrium is reached when the magnetic torque $NIAB$ balances the restoring torque $k\varphi$ of the suspension, giving deflection $\varphi =\frac{NAB}{k}I$. The current sensitivity is therefore $\frac{\varphi }{I}=\frac{NAB}{k}$. To raise sensitivity at fixed current, one should increase the numerator factors $N$, $A$, or $B$, or decrease the torsional constant $k$. Increasing the number of turns $N$ directly and effectively boosts sensitivity, which is why galvanometer coils are wound with many turns. Increasing the torsional constant $k$ appears in the denominator and would reduce sensitivity, so that option is wrong. Decreasing the area $A$ lowers the numerator and reduces sensitivity. Decreasing the field $B$ likewise reduces the numerator. NCERT discusses how a radial field and a soft-iron core are also used to make $\varphi$ proportional to $I$ and to strengthen $B$. A consistency check confirms that every quantity which raises the magnetic torque relative to the restoring torque increases sensitivity. Designers therefore wind many turns and use a strong radial magnetic field produced by concave pole pieces and a soft-iron core; the radial field keeps the plane of the coil always parallel to the field, ensuring the deflection stays linearly proportional to current over the full scale. Voltage sensitivity, the deflection per unit voltage, additionally depends inversely on the coil resistance, which is why increasing turns does not automatically improve voltage sensitivity.