This capacitor was used in domestic radio receivers in the late 1930s and early 1940s. It filtered the raw rectified DC produced typically by valve rectifier type 80 or 5Y3G from a secondary transformer winding of 385 Volts. The rectifier had a directly-heated filament, so the rectifier began operating quickly. Other valves in the receiver were usually indirectly heated, and took a minute or more before conducting and forming a load on the DC supply. Consequently the DC voltage was the peak value of around 525 Volts during the initial warming up period. After warming up, the DC voltage dropped to around 350 Volts. The capacitor was made to handle the initial peak voltage.
The capacitor contains two sheets of aluminium. One sheet has an oxide layer, forming the dielectric in the capacitor. The conductive liquid between the sheets was a solution in water of an electrolyte such as ammonium phosphate, which maintained the oxide layer. As the capacitor aged, the liquid was electrolysed, producing gases and evaporating the water in the electrolyte. The capacitor no longer functioned, which was obvious because it no longer filtered the DC and hum could be heard in the receiver loudspeaker.
