Question

Why are the pole pieces of galvanometer made concave?

Answer 1

The magnetic fields produced by the concave shaped poles are always radial. The current carrying wire must always be perpendicular to the magnetic field, even while rotating. This condition is satisfied using concave shaped poles in the galvanometer.

Complete step-by-step solution:
We all know that a galvanometer is a basic instrument used to measure / detect presence of current in a given circuit. It also helps to determine if the current in the circuit is large or small. It can also be used to measure a potential difference.
The coil in the galvanometer is continuously rotating due to the magnetic field around it. The needle (pointer) used in the galvanometer is pivot at a point, and due to the magnetic fields it shows deflection. The magnetic fields have to be perpendicular to the coil at all times to satisfy this condition. This condition is satisfied when concave poles are used, as they always produce magnetic fields radially.
Thus the concave shaped poles allow the magnetic lines to be parallel to the plane of the coil at every instance and the needle(pointer) would show the deflections.

Note:
We use mirrors not only to make the fields radial but also to remove parallax error.
The concave poles keep the magnetic field radial.
Due to the radial field at every point, a dipole moment is generated. And dipole moment and magnetic fields are perpendicular to each other. Hence there is a torque generated in the galvanometer and rotates the coil continuously.