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longitudinal conductance (S) The sum of all the thickness/resistivity ratios of n − 1 layers which overlie a semi-infinite substratum of resistivity ρn, such that S = h1/ρ1 + h2/ρ2 + h3/ρ3 + … + hn−1/ρn−1 (mho), where h1, h2, etc. are the depths and ρ1, ρ2, etc. the resistivities, of successive layers. A knowledge of hi/ρi for the ith layer when it is sandwiched between two layers of much higher resistivity is of importance in resolving the problem of equivalence.