Space between two concentric conducting spheres of radii a and b (b > a) is filled with a medium of
resistivity $\rho $. The resistance between the two spheres will be :
${\rho \over {2\pi }}\left( {{1 \over a} + {1 \over b}} \right)$
${\rho \over {4\pi }}\left( {{1 \over a} + {1 \over b}} \right)$
${\rho \over {2\pi }}\left( {{1 \over a} - {1 \over b}} \right)$
${\rho \over {4\pi }}\left( {{1 \over a} - {1 \over b}} \right)$
