the first more common application of wet aluminum electrolytic capacitors was in large telephone exchanges, to reduce relay hash (noise) on the 48 volt dc power supply. the development of ac-operated domestic radio receivers in the late 1920s created a demand for large-capacitance (for the time) and high-voltage capacitors for the valve amplifier technique, typically at least 4 microfarads and rated at around 500 volts dc. waxed paper and oiled silk film capacitors were available, but devices with that order of capacitance and voltage rating were bulky and prohibitively expensive.