Many experimental and theoretical efforts have been devoted to the study of the electrical properties of rocks (e.g., Brace et al., 1965; Madden, 1982; Parkhomenko, 1982; Yamazaki, 1966). Special attention has been paid to possible relationships between variations of mechanical stresses inside the rock and variations of its electrical properties, in reference to a possible technique of predicting earthquakes (Raleigh et al., 1977). These studies were generally performed on small (decimeter sized) samples, but the complexity of the physical processes involved and the heterogeneity of rocks in the field make it necessary to try to get results from larger blocks, with dimensions of several decimeters (Sobolev et al., 1983) or of meters (this study). It also appears quite desirable to look for information on the electrical properties of rocks at different scales (from a centimeter to a kilometer, say) to see if some scaling laws could be experimentally established. These scaling laws would allow inference of electrical properties at a given scale from the knowledge of the electrical properties at a smaller scale (Madden, 1976). Scaling‐law methods seem to be promising in seismogenesis studies (e.g., Allegre et al., 1984; Turcotte, 1986).