Two parallel long wires carry currents ${\mathrm{i}}_1\mathrm{and}{\mathrm{i}}_2$   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 $10\mathrm{\mu T}$ .  If the direction of i₂ is reversed, the field becomes  $30\mathrm{\mu T}$.  The ratio ${\mathrm{i}}_1/{\mathrm{i}}_2$  is,