A moving-coil galvanometer measures current by exploiting which physical principle?

1. NCERT §4.11 describes the moving-coil galvanometer. 2. CONSTRUCTION: a rectangular coil suspended in a UNIFORM RADIAL magnetic field (between curved pole pieces with a soft-iron core inside the coil). 3. PRINCIPLE: when current $I$ flows through the coil, the magnetic field exerts a TORQUE on it of magnitude $\tau_\text{mag} = NIAB$ (where $N$ is the number of turns). 4. The radial field ensures the magnetic force is always tangential — torque is proportional to $I$ regardless of the coil's orientation. The spring's restoring torque $\tau_\text{spring} = k\phi$ balances it; the deflection $\phi \propto I$. 5. So a linear scale reads the current directly. This is the heart of analog ammeters and voltmeters. 6. Options B–D describe physical effects that don't match the galvanometer's design. _Source: NCERT Class 12 Physics Part 1, Ch 4, §4.11 (Moving Coil Galvanometer), p. 24–25._