A long cylinder of uniform cross section and radius R is carrying a current i along its length and current density is uniform cross section and radius r in the cylinder parallel to its length. The axis of the cylinderical cavity is separated by a distance d from the axis of the cylinder. Find the magnetic field at the axis of cylinder.

Answer

`[(mu_(0)I r^(2))/(2 pi d (R^(2) - r^(2)))]`