1. The problem statement, all variables and given/known data Question 3 of http://www.damtp.cam.ac.uk/user/examples/B10b.pdf . 2. Relevant equations [itex]\nabla \times B = \mu_0 J[/itex] 3. The attempt at a solution I know for a cylinder, J (vector) = J (scalar) * k (vector), where unit k is the vector in the z-direction. So [itex]\nabla \times B = (0, 0, \mu_0 J)[/itex], somewhere (not quite sure where this is true in terms of a, b, d). I think I need to find B for the big cylinder, then B for the small cylinder, and combine them somehow? I don't see why the field in between the cylinders is important; we're only interested in B for x² + y² < a². Thanks!