A thin-walled hollow wire of radius a lies with its axis along the z axis and carries current I in the positive z direction. A second identical wire is parallel to the first with its axis along the line x = d. The second wire carries current I in the negative z direction.

(a) Find the magnetic flux per unit length through the space in the xz plane between the wires.

(b) If the far ends of the wires are connected together so that the parallel wires form two sides of a loop, find the self-inductance per unit length of the loop.