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