Two infinitely long wires separated by distance d. Currents: I1 = -I2. Find potential vector as a function of r1 and r2 at a point P (r1 and r2 distances to P from wire one and wire two). Del cross A= B B = (mu I)/(2pi r) Using Ampere's, I get an expression for the magnetic field that involves two different distances, r1and r2. I see that integrating this expression with respect to distance will give me the vector potential. But with two distances to take into account, I don't know how to solve this.