Moving Coil Galvanometer

NCERT Class 12, Chapter 4 (Moving Charges and Magnetism) explains that in a moving coil galvanometer the coil sits in a radial magnetic field, so the deflecting torque is $NIAB$ regardless of coil orientation. A spring provides a restoring torque $k\varphi$ proportional to the twist angle $\varphi$. At equilibrium the two torques balance: $NIAB=k\varphi$, giving $\varphi =\frac{NAB}{k}I$, which is linear in current. The linearity relies specifically on the restoring couple being proportional to the deflection angle, making the scale uniform. The option 'proportional to the square of deflection' would give a non-linear, cramped scale. The option 'independent of the deflection' would allow uncontrolled rotation with no fixed reading. The option 'proportional to the current' is incorrect because the restoring couple is a mechanical property of the spring and cannot depend on the electrical current. Plausibility check: only a linear restoring law yields a uniform, evenly spaced scale, exactly what practical galvanometers display, confirming the first option. The radial field arrangement, produced by a cylindrical soft-iron core between concave pole pieces, is what removes the usual $\sin \theta$ factor from the deflecting torque, so the entire non-linearity would otherwise have come from the geometry; keeping the restoring couple strictly proportional to deflection is therefore essential for a trustworthy, calibrated current reading.