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

(Given : Permeability of vacuum ${\mathrm{\mu }}_0=4\mathrm{\pi }\times {10}^{-7} \mathrm{N} {\mathrm{A}}^{-2}$)