  We have simulated numerically an automated version of Maxwell's demon
inspired by Smoluchowski's ideas of 1912. Two gas chambers of equal volume are
connected via an opening that is covered by a trapdoor. The trapdoor can open
to the left but not to the right, and is intended to rectify naturally
occurring fluctuations in density between the two chambers. Our results confirm
that though the trapdoor behaves as a rectifier when large density differences
are imposed by external means, it can not extract useful work from the thermal
motion of the molecules when left on its own.
