A polarised capacitor exhibiting a very high capacitance/volume ratio that consists of two electrodes immersed in an electrolyte, with a chemical film that acts as a dielectric on one or both electrodes. Electrolytic capacitors are made by winding either plain or etched foils on which an oxide has been formed on the surface of one (either anode or cathode) film. The etching of the foil increases the surface area and a considerable increase in capacitance can thus be obtained.