The magnetic field at the centre of a wire loop formed by two semicircular wires of radii $R_1=2 \pi \mathrm{m}$ and $R_2=4 \pi \mathrm{m}$, carrying current $\mathrm{I}=4 \mathrm{~A}$ as per figure given below is $\alpha \times 10^{-7} \mathrm{~T}$. The value of $\alpha$ is ________. (Centre $\mathrm{O}$ is common for all segments)