An ohmic heater for heating a conductive fluid has a plurality of electrodes mounted to a structure with spaces between the electrodes. The electrodes (14) are selectively connect to poles (38, 40) of a power supply, so that some electrodes are connected to the poles and others remain isolated from the poles. Shunting switches are provided for connecting two or more of the isolated electrodes to one another. The shunting switches allow formation of a large number of different connection schemes having a variety of different electrical conduction paths through fluid in the spaces and a variety of resistances between the poles with relatively few electrodes and spaces.