Below, a derivation is given based on Gauss's law and Coulomb's law. All conductivity interfaces, denoted by S, are discretized into planar triangular facets t m with centers r m . Assume that an m -th facet with the normal vector n m and area A m carries a uniform surface charge density ρ ρ m . If a volumetric tetrahedral mesh were present, the charged facets would belong to tetrahedra with different conductivity values. We first compute the electric field E m + at the point r m + δ δ n m , for δ δ → → 0 + i.e., just outside facet 𝑚 at its center. This field contains three contributions: