Moving coil galvanometer: A galvanometer is used to detect current in a circuit.
Construction: It consists of a rectangular coil wound on a non-conducting metallic frame and is suspended by phosphor bronze strip between the pole-pieces (N and S) of a strong perm an ent magnet. A soft iron core in cylindrical form is placed between the coil. One end of coil is attached to suspension wire which also serves as one terminal (T1) of galvanometer. The other end of coil is connected to a loosely coiled strip, which serves as the other terminal (T2). The other end of the suspension is attached to a torsion head which can be rotated to set the coil in zero position. A mirror (M) is fixed on the phosphor bronze strip by means of which the deflection of the coil is measured by the lamp and scale arrangement. The levelling screws are also provided at the base of the instrument.
The pole pieces of the permanent magnet are cylindrical so that the magnetic field is radial at any position of the coil.
Principle and working: When current (I) is passed in the coil, torque τ acts on the coil, given by
τ = NIABsinθ
where θ is the angle between the normal to plane of coil and the magnetic field of strength B, N is the number of turns in a coil.
When the magnetic field is radial, as in the case of cylindrical pole pieces and soft iron core, then in every position of coil the plane of the coil, is parallel to the magnetic field lines, so that θ = 90° and sin90° =1
Deflecting torque, τ = NIAB
If C is the torsional rigidity of the wire and θ is the twist of suspension strip, then restoring torque =Cθ
For equilibrium, deflecting torque = restoring torque
deflection of coil is directly proportional to current flowing in the coil and hence we can construct a linear scale.
