An ohmic heater comprising: (a) a structure (20; 220) defining a flow path (22; 222) extending in a downstream direction; (b) a first pair of electrodes (34a, 34b; 234a, 234b) disposed within the flow path (22; 222) adjacent one another in the downstream direction but spaced from one another in a direction perpendicular to the downstream direction; (c) a second pair of electrodes (36a, 36b; 236a, 236b) disposed within the flow path downstream from the first pair of electrodes (34a, 34b; 234a, 234b), the electrodes of the second pair being within the flow path (22; 222) adjacent one another in the downstream directions but spaced from one another in a direction perpendicular to the downstream direction, the structure (20; 222) including a dielectric wall defining an elongated passage (30; 230) forming a part of the flow path (22; 222) between the first pair of electrodes (34a, 34b; 234a, 234b) and the second pair of electrodes (36a, 36b; 236a, 236b); and (d) an electrical circuit operative in at least three states including: (i) a first state in which the circuit applies a voltage between the electrodes of the first pair; (ii) a second state in which the circuit applies a voltage between the electrodes of the second pair; and (iii) a third state in which the circuit applies a voltage between at least one electrode of the first pair and at least one electrode of the second pair so that in the third state, the only current path through any of the electrodes extends between an electrode of the second pair and an electrode of the first pair, through the elongated passage (30; 230) and current flows through liquid in the passage (30; 230). (a) a structure (20; 220) defining a flow path (22; 222) extending in a downstream direction; (b) a first pair of electrodes (34a, 34b; 234a, 234b) disposed within the flow path (22; 222) adjacent one another in the downstream direction but spaced from one another in a direction perpendicular to the downstream direction; (c) a second pair of electrodes (36a, 36b; 236a, 236b) disposed within the flow path downstream from the first pair of electrodes (34a, 34b; 234a, 234b), the electrodes of the second pair being within the flow path (22; 222) adjacent one another in the downstream directions but spaced from one another in a direction perpendicular to the downstream direction, the structure (20; 222) including a dielectric wall defining an elongated passage (30; 230) forming a part of the flow path (22; 222) between the first pair of electrodes (34a, 34b; 234a, 234b) and the second pair of electrodes (36a, 36b; 236a, 236b); and (d) an electrical circuit operative in at least three states including: (i) a first state in which the circuit applies a voltage between the electrodes of the first pair; (ii) a second state in which the circuit applies a voltage between the electrodes of the second pair; and (iii) a third state in which the circuit applies a voltage between at least one electrode of the first pair and at least one electrode of the second pair so that in the third state, the only current path through any of the electrodes extends between an electrode of the second pair and an electrode of the first pair, through the elongated passage (30; 230) and current flows through liquid in the passage (30; 230). (i) a first state in which the circuit applies a voltage between the electrodes of the first pair; (ii) a second state in which the circuit applies a voltage between the electrodes of the second pair; and (iii) a third state in which the circuit applies a voltage between at least one electrode of the first pair and at least one electrode of the second pair so that in the third state, the only current path through any of the electrodes extends between an electrode of the second pair and an electrode of the first pair, through the elongated passage (30; 230) and current flows through liquid in the passage (30; 230).