TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The apparatus is constructed to generate an electrical potential difference between the two master electrodes by charging or discharging of the master and floating electrodes to generate electricity. The floating electrode comprises a non-selective ion barrier layer which is electrically conductive but impermeable for anions and cations. The apparatus: comprises a controller provided with a first switch to direct the electrical potential from the master electrodes either to a first pole or a second pole of a power terminal; and is being provided with a water flow adjuster to create a water flow along the electrodes with the water containing sequentially relatively high and relatively low concentrations of ions. The water flow adjuster is a valve or a pump, and the apparatus comprises an inlet and a water outlet, where the water flows from the water inlet to the water outlet at least partially along the electrodes. The water flow adjuster is provided in the water inlet for closing and opening a water stream to allow the flow of water with a relatively high ion concentration along the electrodes and/or for closing and opening a water stream to allow the flow of water with a relatively low ion concentration. The floating electrode is provided with the selective ion barrier layer. The selective ion barrier layer is provided along the surface of the master and/or floating electrode extending parallel to the surface of the floating and/or master electrodes. In the apparatus, a thickness of the selective ion barrier layer is 5-1000 mu m, preferably 20-250 mu m, and an electrical insulating material is provided to the floating electrode extending outwardly from an edge in a longitudinal direction of the floating electrode. The insulating material extends from the edge at least 0.5 mm, preferably 3-20 mm. The controller controls the water flow adjuster so that the controller alternatively allows the flow of water with a relatively high ion concentration with the flow of water with a relatively low ion concentration along the electrodes. The master electrodes are provided with the selective ion barrier layer.