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 .