An electrolyte chamber has an opening on the side facing the foil to be measured for letting the electrolyte through to the foil, a hollow cathode in the chamber, a supply line connected to the chamber through the hollow cathode for supplying the electrolyte to the chamber, and a negative pressure flow-off line connected to the chamber located at a distance from the mouth of the supply line. A negative pressure generator is connected to the flow-off line through a separator vessel which has a level switch which shuts off the generator when the electrolyte reaches a predetermined level. The supply line is connected to a supply vessel containing the electrolyte and runs continuously into the liquid volume in the vessel. The vessel has a connection to the atmosphere.