With a labelled diagram, explain the principle and working of a moving coil galvanometer. Why is a radial magnetic field used, and how can its sensitivity be increased?

1. 1 · State the principle. A current-carrying coil placed in a magnetic field feels a torque.
2. 2 · Draw the galvanometer. [Horseshoe magnet with concave poles N and S; rectangular coil between them on pivots; soft iron core inside; spring; pointer over a scale; radial field lines; all labelled]
3. 3 · Describe the parts. A coil of fine wire on a light frame, a strong magnet with curved poles, a soft iron core, springs, a pointer and a scale.
4. 4 · Explain the working. Deflecting torque NBIA; restoring torque kα; at rest NBIA = kα, so I = (k/NBA)α and α ∝ I.
5. 5 · Explain the radial field. The plane of the coil always lies along the field, so sin θ = 1 everywhere and the scale is evenly marked.
6. 6 · Give ways to raise sensitivity. α/I = NBA/k: more turns, a larger area, a stronger magnet and springs with a small k.
7. 7 · Link to life. An electrician's analogue multimeter is a galvanometer with shunts and series resistances.
8. 8 · Close in one line. So, the deflection measures the current directly, on an evenly marked scale.

Scoring tip: Write NBIA = kα and then α ∝ I on two separate lines. Those two lines often carry two marks.