Abstract: A water heater has a pair of concentric, relatively rotatable, electrically insulating supports between which the water to be heated can pass. Each support mounts three parts of equidistantly spaced electrodes. The spaced electrodes of the first set are electrically connected in pairs and each pair is connected to a respective phase of a regulatable three-phase current supply by connection means which may include a thyristor. The spaced electrodes of the second set are electrically connected together exclusively in pairs. The sets of electrodes are spaced from each other and the water flowing therebetween provides conductive paths so that the first and second sets of electrodes are connected in a delta configuration. Means are provided to rotate one of the supports to change the relative position of the electrode sets relative to each other thereby regulating the current flow through the water.