The magnetic field inside a conductor with uniform current density \(J=\frac{I}{\pi R^{2} }\) can be found with Ampere’s Law.

Inside the conductor, the magnetic field B increases linearly with r. Outside the conductor, the magnetic field becomes that of a straight conductor and decreases with radius. Note that the expressions for inside and outside would approach the same value at the surface if the magnetic permeability were the same.