A moving coil galvanometer consists of a strong horse - shoe magnet with concave shaped poles N and S . A rectangular coil of insulated copper wire wound on a non - conducting frame is suspended between the magnetic poles by means of a phosphor - bronze thread P from a torsion head T1 . A small mirror M is fitted to the thread a little above the coil . The phosphor - bronze has high Young's modulus and very low rigidity modulus . Hence , it can easily be twisted but cannot be elongated . The lower end of coil is connected to a binding spring T2 . A soft iron cylinder I is a placed inside the coil without any connection with the coil .