Mercury arc rectifier. In this example the rectifier is being used to convert alternating current (AC) to direct current (DC) to power a revolving stage. It was invented by Peter Cooper Hewitt, a US engineer, in 1902. The rectifier consisted of a bulbous glass vessel with a pool of liquid mercury at its base, whose vapours filled the vessel. When an electric arc is estabilshed between a cathode submerged in the mercury pool, an anode suspended above the mercury vapour is ionised and electrodes flow from the cathode to the anodes, but not back the other way. The light produced is a byproduct of the process. Photographed at the Lyric Theatre, London, UK.