A wire of resistance ‘4R’ is bent in the form of a circle. What is the effective resistance between the ends of the diameter ?

If a wire of resistance ‘4R’ is bent in the form a circle. The resistance of upper and lower part’s becomes 2R, 2R respectively. They are in parallel i.e ${\mathrm{R}}_1={\mathrm{R}}_2=2\mathrm{R}$

$\begin{array}{l}\frac{1}{{\mathrm{R}}_{\mathrm{P}}}=\frac{1}{{\mathrm{R}}_1}+\frac{1}{{\mathrm{R}}_2};\frac{1}{{\mathrm{R}}_{\mathrm{P}}}=\frac{1}{2\mathrm{R}}+\frac{1}{2\mathrm{R}}\\ \frac{1}{{\mathrm{R}}_{\mathrm{P}}}=\frac{2}{2\mathrm{R}}\Rightarrow \frac{1}{\mathrm{R}},{\mathrm{R}}_{\mathrm{P}}=\mathrm{R\Omega }\end{array}$