MOVING COIL GALVANOMETER: Principle, Construction and Theory | Physics Video Tutorials

Moving coil galvanometer is a device used for measuring the current in a circuit. The Principle of moving coil galvanometer is, 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. 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. 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. Let PQRS be a single turn of the coil. A current I flows through the coil. In a radial magnetic field, the plane of the coil is always parallel to the magnetic field. Hence the sides QR and SP are always parallel to the field. So, they do not experience any force. The sides PQ and RS are always perpendicular to the field. PQ = RS = l, length of the coil and PS = QR = b, breadth of the coil Force on PQ, F = BI (PQ) = BIl. According to Fleming’s left hand rule, this force is normal to the plane of the coil and acts outwards.