A simple model is proposed and a numerical simulation is carried out to calculate the electrical resistivity of a composite of carbon black in an insulating matrix. From load of carbon black and aggregate size and distribution, it is possible to know the resistivity of the compound assuming a random lattice where the resistance between sites (aggregates) varies exponentially with the gap. A computer program was developed and several samples were prepared and measured to obtain the model parameters. Numerical results are compared with experimental data.