Moving coil galvanometer works on the principle that a current carrying coil placed in a magnetic field experiences a torque.
It consists of a rectangular coil of a large number of turns of thin insulated copper wire wound over a light metallic frame (Figure: a). The coil is suspended between the pole pieces of a horse-shoe magnet by a fine phosphor – bronze strip from a movable torsion head. The lower end of the coil is connected to a hair spring (HS) of phosphor bronze having only a few turns. The other end of the spring is connected to a binding screw. A soft iron cylinder is placed symmetrically inside the coil. The hemispherical magnetic poles produce a radial magnetic field in which the plane of the coil is parallel to the magnetic field in all its positions (Figure: b).
Fig: (a) Moving coil galvanometer; Fig: (b) Radial magnetic field
A small plane mirror (m) attached to the suspension wire is used along with a lamp and scale arrangement to measure the deflection of the coil.