A cylindrical wire of radius 0 . 5   mm and conductivity 5 × 10 7   S   m - 1 is…

Conductivity $\sigma =5\times {10}^7 \mathrm{S} {\mathrm{m}}^{-1}$

Radius $r=0.5 \mathrm{mm}=5\times {10}^{-4} \mathrm{m}$

$E=10\times {10}^{-3} \mathrm{V} {\mathrm{m}}^{-1}$

$J=\sigma E=10\times {10}^{-3}\times 5\times {10}^7$

$\Rightarrow J=5\times {10}^5$

$\Rightarrow \frac{i}{A}=5\times {10}^5$

$i=5\times {10}^5\times \pi r^2$

$=5\times {10}^5\times \pi \times {\left(5\times {10}^{-4}\right)}^2$

$=125\pi \times {10}^{-3} \mathrm{A}$

Therefore, $i=125 \pi \mathrm{mA}$

Hence, $x=5$