A long wire is identified to encompass a radius greater than 5.7 mm and to carry a current which is uniformly distributed over its cross section. The magnitude of magnetic field due to that current is 0.58 mT at a point 4.0 mm from the axis of the wire and 0.30 mT at a point 38 mm from the axis of the wire. Determine the radius (in mm) of the wire?