  For the physical process, Bennett made the molecule in the Szilard engine diamagnetic and the measuring device a one domain ferromagnet with a fixed initial polarisation. With careful manipulation, he argued it is possible to use the perturbation of the magnetic field by the diamagnet, to correlate the polarisation of the ferromagnet non-dissipatively to the diamagnet's location on one side or other of the engine. By the same kind of manipulation, he argued that to reset the polarisation of the ferromagnet to its initial state without using the correlated location of the diamagnet, there is a cost of k T ln 2 in heat generation, in accordance with Landauer's principle. Extracting work from the Szilard engine allows the diamagnetic molecule to move freely throughout the engine, so losing the correlation. The heat generating reset operation is then the only way to restore the ferromagnet to its initial polarisation.