Two concentric coplanar circular loops of radii ${r_1}$ and ${r_2}$ carry currents of respectively ${i_1}$ and ${i_2}$ in opposite directions (one clockwise and the other anticlockwise.) The magnetic induction at the centre of the loops is half that due to ${i_1}$ alone at the centre. If ${r_2} = 2{r_1}.$ the value of ${I_2}/{I_1}$ is