The current density inside a long, solid, cylindrical wire of radius a $=1\mathrm{.}6$ 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 $=420$ A$/$m$^2.$ Find the magnitude of the magnetic field at a distance $($a$)$ r$=0,($b$)$ r $=1\mathrm{.}1$ mm and $($c$)$ r$=1\mathrm{.}6$ mm from the center