The potential which is produced when a liquid is forced to flow through a capillary or a porous solid. G. H. Quincke (1859) found that the electromotive force produced by the streaming of pure water under a given pressure through a clay plate is independent of the size and thickness of the diaphragm and of the amount of water forced through the diaphragm; the electromotive force is, however, proportional to the pressure.