The coil of a moving coil galvanometer is wound over a metal frame in order to

To provide electromagnetic dampening.

The coil is wound on a metal frame to instantly bring it to rest. In turn, this makes it possible for the coil to very immediately reach its equilibrium position. In the metal frame, eddy currents form. The coil cannot go any further due to these eddy currents. The galvanometer is referred to as a dead beat galvanometer because the oscillation of the coil abruptly ceases.