Two parallel long wires carry currents ${\mathrm{i}}_1$ and ${\mathrm{i}}_2$ with ${\mathrm{i}}_1>{\mathrm{i}}_2$. When the currents are in the same direction, the magnetic field at a point midway between the wires is $10\mathrm{ }\mathrm{\mu }\mathrm{T}$ and when the direction of ${\mathrm{i}}_2$ is reversed, the field becomes $30\mathrm{ }\mathrm{\mu }\mathrm{T}$. The ratio $\frac{{\mathrm{i}}_1}{{\mathrm{i}}_2}$ is