the first industrially realized electrolytic capacitors consisted of a metallic box used as the cathode. it was filled with a borax electrolyte dissolved in water, in which a folded aluminum anode plate was inserted. applying a dc voltage from outside, an oxide layer was formed on the surface of the anode. the advantage of these capacitors was that they were significantly smaller and cheaper than all other capacitors at this time relative to the realized capacitance value. this construction with different styles of anode construction but with a case as cathode and container for the electrolyte was used up to the 1930s and was called a "wet" electrolytic capacitor, in the sense of its having a high water content.