First you need to know what is moving coil - galvanometer ???Galvanometer is operated by the force exerted by an electric current flowing in a movable coil suspended in a magnetic field.................. !!!!WORKING of moving coil of galvanometer :- Principle: -Its working is based on the fact that when a current carrying coil is placed in a magnetic field, it EXPERIENCES a torque.Principle:Its working is based on the fact that when a current carrying coil is placed in a magnetic field, it experiences a torque.Working:Suppose the coil PQRS is suspended freely in the magnetic field.Letl = Length PQ or RS of the COILB = Breadth QR or SP of the coiln = Number of turns in the coilArea of each turn of the coil, A = l × bLet B = Strength of the magnetic field in which coil is suspendedI = Current passing through the coil in the direction PQRSLet, at any instant, α be the angle which the normal drawn on the plane of the coil makes with the direction of magnetic field.The rectangular coil carrying current when placed in the magnetic field experiences a torque whose magnitude is given by,τ = nIBA sinαDue to deflecting torque, the coil rotates and SUSPENSION wire gets twisted. A restoring torque is set up in the suspension wire.Let θbe the twist produced in the phosphor bronze strip due to rotation of the coil and K be the restoring torque per unit twist of the phosphor bronze strip. Then,Total restoring torque produced = kθIn equilibrium POSITION of the coil,Deflecting torque = Restoring torque∴ NIBA = kθ. So this is the working moeel of galvanometer