With a labelled diagram, explain the principle and working of a moving coil galvanometer. Obtain the expression for its current sensitivity.

1. 1 · State the principle. A current-carrying coil in a magnetic field feels a torque.
2. 2 · Draw and label. [Coil between concave poles N and S; soft iron core; radial field lines; spring; pointer and scale]
3. 3 · Deflecting torque. The radial field keeps the coil's plane along B, so τ = NIAB at every position.
4. 4 · Restoring torque and balance. The spring gives kφ; at rest kφ = NIAB, so φ = (NAB/k) I and the scale is linear.
5. 5 · Current sensitivity. φ/I = NAB/k; raise it with more turns, a larger area, a stronger magnet or a weaker spring.
6. 6 · Real life. Needle meters in voltage stabilisers and battery chargers work this way.
7. 7 · Close in one line. So, the deflection of a moving coil galvanometer is directly proportional to the current.

Scoring tip: Write 'radial field' and say why it matters: sin θ = 1 at all positions, so the scale is linear.