Aluminium Electrolytic Capacitors: Wound, Soaked and Sealed

An aluminium electrolytic capacitor is two aluminium foils wound up with a paper spacer between them. The anode foil is etched rough to multiply its surface and then "formed": a voltage in an electrolytic bath grows an insulating oxide layer whose thickness follows the forming voltage, and that oxide is the dielectric. The liquid electrolyte soaked into the paper, commonly ethylene glycol with boric acid, is the real cathode; the second foil only connects to it. The foils and paper are cut to width, wound with their terminals welded on, soaked with electrolyte under vacuum, put in an aluminium can, sealed with a rubber disc by curling the can's edge, sleeved, and finally run at rated voltage in heat to heal any oxide damaged by cutting and winding.