two ground balls deep in the ground close to each other

$\varphi = \frac{I}{4 \pi \sigma} \frac{1}{r}$

$U = \varphi_2 - \varphi_1$

$R = \frac{1}{2 \pi \sigma} \Big( \frac{1}{a} - \frac{1}{s} \Big)$