The reason for having a large number of turns in the coil of a moving coil galvanometer is to--------

Analyze the question: It asks why a moving coil galvanometer's coil has many turns.

Concept: The torque on a coil carrying current I in a magnetic field is

τ = N B I A

where N is the number of turns, B the magnetic flux density, I the current, and A the area of the coil.

This torque is balanced by the restoring torque of the hairsprings, kθ. So at equilibrium:

NBIA = kθ, which gives θ = (NBA / k) I

The current sensitivity is therefore:

θ / I = NBA / k

Working through the options:

• A. The deflection is proportional to current because of the radial magnetic field (concave pole pieces and soft iron core) and the linear restoring spring. The number of turns does not cause this, so A is wrong.
• B. Since θ/I = NBA/k, increasing N directly increases the deflection for a given current. This increases the sensitivity, so B is correct.
• C. The coil does not reduce the magnet's flux, and more turns do not change the flux produced by the magnet, so C is wrong.
• D. The coil has no effect on the strength of the permanent magnet, so D is wrong.

Answer: B. Increase the sensitivity of the galvanometer

Exam tip: Remember the sensitivity formula, NBA/k. To make a galvanometer more sensitive, you increase N, B and A, or use a weaker spring (smaller k). Also remember that the radial field is what makes the scale linear, which is the point of option A. WASSCE often tests this contrast.