The principle of moving coil galvanometer is that when a conductor carrying current is placed in a magnetic field, it experience a force. The construction of a moving coil galvanometer is as shown in the figure. A rectangular coil is suspended in between the poles of a powerful magnet by means a phosphor bronze wire. The magnet is bored out cylindrically so that the field is radial. A soft iron core is placed inside the coil to induce large number of magnetic lines. A mirror M is fixed on the phosphor bronze wire. A spring is fixed at the bottom of the coil.
Let B be the strength of the magnetic field, n be the number of turns in the coil, A be the area of the coil and I be the current flowing through the coil.
The deflecting couple produced in the coil
Due to this deflecting couple, the coil rotates and this couple is opposed by the torsional couple produced in the phosphor bronze wire.
The restoring couple produced in the coil
Where, C is couple per unit twist of the wire and θ is the angle of twist which can be measured by means of a lamp and scale arrangement.
BIAn = Cθ
i.e., I = (C/BAn) * θ
since, C,B,A and n are constants, the defelction in the coil is directional in the coil is directly proportional to the current through the coil.