Thegalvanometerconsists of a coil of wire often rectangular, carrying the current to be measured.There are generally many turns in the coil to increase its sensitivity.The coil is placed in a magnetic field such that the lines of B remain nearly parallel to the plane of wire as it turns.This is achieved by having a soft iron cylinder placed at the center of the coil.Magnetic field lines tend to pass through the iron cylinder, producing the field configuration.The moving coil is hung from a spring which winds up as the coil rotates; this winding up produces a restoring torque proportional to the winding up (or twisting) of the spring, i.e. to the angular deflection of the coil.The coil comes to equilibrium when this restoring torque k balances the torque due to the magnetic field balances the torque due to the magnetic field.Since by design field lines are radial,

we have sinq-1 ,so that for equilibriumkø = INBA

Thus the deflectionøof the galvanometer is proportional to the electric currentIpassing through it.