The magnetic field due to a straight conductor of uniform cross-section of radius a, and carrying a steady current (uniform current density) is represented by

Magnetic field at a point outside the straight conductor,

$$\begin{gathered} (\mathrm{r}>\mathrm{a}) : \\ \mathrm{B}=\frac{{\mathrm{\mu }}_0\mathrm{I}}{2\mathrm{\pi r}}\Rightarrow \mathrm{B}\propto \frac{1}{\mathrm{r}} \end{gathered}$$

Magnetic field at a point lying inside the conductor,

$$\begin{gathered} (\mathrm{r}<\mathrm{a}) : \\ \mathrm{B}=\frac{{\mathrm{\mu }}_0\mathrm{Jr}}{2}\Rightarrow \mathrm{B}\propto \mathrm{r} (\mathrm{J}: \mathrm{current} \mathrm{density}) \end{gathered}$$