The magnetic field at the centre of a wire loop formed by two semicircular wires of radii $\mathrm{R}_1=2 \pi \mathrm{~m}$ and $R_2=4 \pi \mathrm{~m}$ carrying current $I=4 \mathrm{~A}$ as per figure given below is $\times 10^{-7} \mathrm{~T}$. The value of $\alpha$ is. (Centre O is common for all segments)