CBSE Class 12-science Answered

Faraday used a hollow paper cylinder as the core of the coil connected to a galvanometer while changing the magnetic flux linked with the coil by moving a bar magnet, as the core of the coil connected to the galvanometer  produces a flux linked with the coil. Due to this an e.m.f. is induced in the coil which results in induced current. This current flows such that the magnetic field created by current opposes its motion.  So when steady current passes through coil of galvanometer, it gets deflected. Normally the coil oscillates about its equilibrium position for some time before coming to rest. The coil attains equilibrium almost instantly, and hence, hollow paper cylinder comes to rest quickly when it oscillates.

If we use ferromagnetic material like iron which has high susceptibility and permeability. They get strongly magnetized even in weak field. Hence there is crowding of lines of force which results in more induced current. However, galvanometer is a current sensitive device. Hence we cannot  use ferromagnetic material like iron.