In 1925 Samuel Ruben patented a much more compact and
lower ESR form of electrolytic capacitor. Ruben’s capacitor adopted
the stacked construction of a silver mica capacitor and
inserted a second aluminum foil called the “cathode foil” as
connection to the electrolyte. This cathode foil reduced substantially
the thickness of electrolyte between the dielectric and the
cathode terminal of the capacitor, which reduced losses (ESR)
significantly, from about 30 to 3 ohms. The cylindrical case,
which previously served as the electrolyte cathode connection,
no longer had an electrical function.