The metal-insulator transition observed in krypton-fluoride laser irradiated polyimide is used as a method to study the three dimensional percolation theory of random resistor networks. Values for the percolation threshold and conductivity critical exponent were measured and found to be 0.31 ± 0.06 for the percolation threshold of a three dimensional continuum of interpenetrating spheres and 2.05 ± 0.05 for the critical exponent. Both values are in excellent agreement with theoretical predictions. The metal-insulator transition was observed to have a nonlinear current voltage characteristic near the percolation threshold when a small applied voltage was used for measurement. Under these conditions the conductivity critical exponent was measured and found to be 2.7 ± 0.4. The nonlinear case corresponds to theories for nonlinear random resistor networks and is the first experimental demonstration of the effect of nonlinear conduction on the critical exponent. The value is in agreement with theoretical calculations.