A long thin hollow metallic cylinder of radius R has a current i ampere. The magnetic induction $B$ away from the axis at a distance ‘$r$’ from the axis varies as shown in

From Ampere's Circuital Law

$\oint \mathrm{B}.\mathrm{dl}={\mathrm{\mu }}_0{\mathrm{I}}_{\mathrm{enclosed}}$

$\Rightarrow$There is no current enclosed until x = r from x = 0.

For x > r, 

$\oint \mathrm{B}.\mathrm{dl}={\mathrm{\mu }}_0{\mathrm{I}}_{\mathrm{enclosed}}$

$\left|\mathrm{B}\right|.\left(2\mathrm{\pi r}\right)={\mathrm{\mu }}_0{\mathrm{I}}_{\mathrm{enclosed}}$

$\left|\mathrm{B}\right|=\frac{{\mathrm{\mu }}_0{\mathrm{I}}_{\mathrm{enclosed}}}{\left(2\mathrm{\pi r}\right)}$

this is best represented by