Direct determination of the conductivity exponent in directed percolation

The backbone in a continuum system of diodes is determined by applying the conjecture that in directed percolation the backbone is the collection of diodes which belong to the source-to-drain percolating cluster, as well as to the reversed drain-to-source percolating cluster. Obtaining the backbone enables us to apply our systematic resistance-computation procedure, which has been previously used for nondirected percolating systems. We suggest then a method which is based on a more systematic, more general, and more direct procedure than those used previously for the directed-percolation problem. We also find that this method yields a more accurate value for the conductivity critical exponent t=0.63 0.05.