The current density inside a long, solid, cylindrical wire of radius a $=2\mathrm{.}0$ mm is in the

direction of the central axis and its magnitude varies linearly with radial distancer from

the axis according to J $=$ Jor$/$a$,$ where Jo $=360$ A$/$m$2.$

$.$ Find the magnitude of the magnetic

field at a distance $($a$)$ r$=0,($b$)$ r $=1\mathrm{.}2$ mm and $($c$)$ r$=2\mathrm{.}0$ mm from the center.