Universal material property in conductivity of planar random microstructures

We study scatter involved in finite-size scaling of the conductivity and re sistivity tensors resulting, respectively, from uniform essential and natur al boundary conditions applied to domains that are finite relative to the s ize of a heterogeneity. For various types of planar microstructures generat ed from Poisson processes (multiphase Voronoi mosaics, composites with circ ular or needlelike inclusions, etc.) we report a universal property: the co efficient of variation of the second invariant stays practically constant a t about 0.55+/-0.1, irrespective of the domain size, the boundary condition s applied to it, the contrast, and the volume fraction of either phase.