A copper rod of diameter 1 cm and length

The radius of the copper rod is 0.5 cm and length is 8 cm. Therefore, the volume of the copper rod is

$V=\pi \times(0.5)^{2} \times 8 \mathrm{~cm}^{3}$

Let the radius of the wire is r cm. The length of the wire is 18 m=1800 cm. Therefore, the volume of the wire is

$V_{1}=\pi \times(r)^{2} \times 1800 \mathrm{~cm}^{3}$

Since, the volume of the copper rod is equal to the volume of the wire; we have

$V_{1}=V$

$\Rightarrow \pi r^{2} \times 1800=\pi \times(0.5)^{2} \times 8$

$\Rightarrow r^{2}=\frac{0.25 \times 8}{1800}=\frac{1}{900}$

$\Rightarrow r=\frac{1}{30}=0.033 \mathrm{~cm}$

So, thickness $=0.33 \times 2=0.66 \mathrm{~mm}$