A rod with circular cross-section area $2 \mathrm{~cm}^2$ and length 40 cm is wound uniformly with 400 turns of an insulated wire. If a current of 0.4 A flows in the wire windings, the total magnetic flux produced inside windings is $4 \pi \times 10^{-6} \mathrm{~Wb}$. The relative permeability of the rod is (Given Permeability of vacuum $\mu_0=4 \pi \times 10^{-7} \mathrm{NA}^{-2}$ )