These are [58]: (a) The positive, negative and overall average potentials on the surface: V þ S ¼ 1 m ∑ m i ¼ 1 V þ S ðriÞ; V S ¼ 1 n ∑ n i ¼ 1 V S ðriÞ; V s ¼ 1 mþn ∑ m i ¼ 1 V þ S ðriÞþ ∑ n i ¼ 1 V S ðriÞ ! ð3Þ here, the summations are over the m positive and n negative points of a square surface grid, with V þ S and V S being their averages. Fig. 1.