In a uniform magnetic field, the magnetic needle has a magnetic moment $9.85 \times 10^{-2} \mathrm{~A} / \mathrm{m}^2$ and moment of inertia $5 \times 10^{-6} \mathrm{~kg} \mathrm{~m}^2$. If it performs 10 complete oscillations in 5 seconds then the magnitude of the magnetic field is $\_\_\_\_$ mThange $\pi^2$ as 9.85]