Two long parallel straight wires carrying currents $I_1$ and $I_2$ (with $I_2 < I_1$) are placed a certain distance apart, both currents flowing in the same direction. The magnetic field at a point exactly midway between the wires is measured to be $10\ \mu T$. When the current $I_2$ is reversed in direction (keeping its magnitude the same), the field at that midpoint becomes $30\ \mu T$. What is the ratio $I_1/I_2$?