The boiler comprises a vessel (10) having vertically extending walls (12, 13) defining a plurality of vertically extending passages (26, 28, 30, 32). The vessel includes an intake opening (44) in communication with a liquid inlet adjacent a lower end of each of the passages, and a steam exhaust opening (38) in communication with a liquid exhaust adjacent an upper end of each of the passages. The boiler also includes a set of electrodes (26-1A, 26-1B, 26-2A, 26-2B, 26-3A, 26-3B, 26-4A, 26-4B, 28-1A, 28-1B, 28-2A, 28-2B, 28-3A, 28-3B, 28-4A, 28-4B, 30-1A, 30-1B, 30-2A, 30-2B, 30-3A, 30-3B, 30-4A, 30-4B, 32-1A, 32-1B, 32-2A, 32-2B, 32-3A, 32-3B, 32-4A, 32-4B) located in the passages, with some of the electrodes in each passage being located at various heights. The boiler power supply (50) has at least two poles (51, 53) and is operable to supply different electrical potentials to the different poles. The circuit (52) is operable to selectively connect and disconnect the electrodes to and from the poles in a plurality of different connection schemes, in each of which a current flows between the connected electrodes along a current path through the liquid in at least one of the passages.