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The paper presents the first stage of a new field of development of aluminium electrolytic capacitors. Much of the paper describes the author's own experiments and development work which originated in a search for methods of obtaining lower loss angles at mains frequencies in low-voltage electrolytic capacitors. This new field of development, involving performance at audio and higher frequencies and under pulse conditions, has been made possible by the introduction of electrolytes which are often as much as seventy times more conducting than the conventional electrolytes used in aluminium electrolytic capacitors. The use of highly conductive electrolytes not only gives the obvious advantage of lower internal resistance, but permits the use of `high ratioÂ¿ etched foil at the lower end of the voltage range, even down to 3 V d.c. working, without objectionably high loss angle. In turn, the use of `high ratioÂ¿ etched foil results in the use of smaller foil areas and shorter lengths and thus lower inductances. A few preliminary life test results are given, and in the light of these encouraging results, it is concluded that further advances are probable, but use at higher temperatures will become practicable only with the advent of improved sealing methods and electrolytes of lower vapour pressure.