Two isolated metallic solid spheres of radii R and 2R are charged such that both have same charge density $\mathrm{\sigma }$ . The spheres are then connected by a thin conducting wire. If the new charge density of the bigger sphere is ${\mathrm{\sigma }}^{\text{'}}$ . The radio  $\frac{{\mathrm{\sigma }}^{\text{'}}}{\mathrm{\sigma }}$ is: