A rod with circular cross–section area \( 2\,\text{cm}^2 \) and length \( 40\,\text{cm} \) is wound uniformly with \( 400 \) turns of an insulated wire. If a current of \( 0.4\,\text{A} \) flows in the wire windings, the total magnetic flux produced inside windings is \( 4\pi\times10^{-6}\,\text{Wb} \). The relative permeability of the rod is:
(Given: Permeability of vacuum \( \mu_0 = 4\pi\times10^{-7}\,\text{N}\,\text{A}^{-2} \))