Formation resistivity factors were measured as a function of porosity for various rubber‐brine mixtures under uniaxial compression. Electrical measurements were made parallel and transverse to the direction of compression for regular crystal lattices (FCC and CsCl) and various random packings. Porosities from about 0.35 to as low as 0.03 were obtained. The formation factors ranged from about 5 to nearly 1 000. A new, effective media formula qualitatively, or in some cases even semiquantitatively, describes the results. The results agree reasonably well with calculations made on a model system, which the rubber‐brine mixture approximates. These calculations show, and the results from the rubber‐brine mixture confirm, a critical porosity below which the system is insulating.