An iron rod of susceptibility 599 is subjected to a magnetising field of 1200 $\mathrm{A} {\mathrm{m}}^{-1}$ . The permeability of the material of the rod is $\left({\mathrm{\mu }}_0=4\mathrm{\pi }\times {10}^{-7}\mathrm{T}\mathrm{m}{\mathrm{A}}^{-1}\right)$