Question
Answers

In a moving coil galvanometer, a strong horseshoe magnet of concave shaped pole pieces is used?
A) Increase space for rotation.
B) Reduce weight of galvanometer.
C) Protect magnetic fields which are parallel to the plane of coil at any position.
D) Make magnetic induction weak at the centre.

Answer
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Hint : A moving coil galvanometer works on the principle that a torque acts on a current carrying coil when it is suspended in a uniform magnetic field. Under the action of this torque, the coil rotates and the deflection of the coil is directly proportional to the current flowing through the coil.

Complete step by step answer
The moving coil of a moving coil galvanometer moves in a magnetic field. It is produced by a permanent magnet. The magnet is perpendicular to the magnetic field. When current passes through the coil, the magnet experiences equal and opposite force. The width of the coil separates the forces.
Current carrying coil should be always perpendicular to the magnetic field even when it has rotated. The magnetic field produced by the concave shaped pole is always a radial magnetic field. Now, the radial magnetic field is that field, in which the plane of the coil in all positions is parallel to the direction of the magnetic field. A radial field is always perpendicular to a conductor rotating about an axis, passing through the centre of the concave shaped poles.
So, the correct answer is option C.

Note
A radial magnetic field has been achieved by properly cutting the magnetic pole pieces in the shape of concave faces. Due to the horse shoe magnet the magnetic flux is uniform and linear.