The current density inside a long, solid, cylindrical wire of radius a $=4\mathrm{.}0$ mm is in the direction of the central axis and its magnitude varies linearly with radial distance r from the axis according to J Jor$/$a$,$ where Jo $390$ A$/$m$^2.$ Find the magnitude of the magnetic field at a distance $($a$)$ r$=0,($b$)$ r$=2\mathrm{.}3$ mm and $($c$)$ r$-4\mathrm{.}0$ mm from the center.