"The Electrical Porcelain Industry predates the invention of the radio. The pioneers of the porcelain business could not and did not anticipate the problem of radio interference caused by the corona created by spaces or gaps between the body of the porcelain insulator and the attachment of the insulator to a pin or conductor. By the mid 1920's manufacturers began researching the dilemma. Although several varying solutions were offered, all boiled down to making at least a portion of the insulator conductive.
"The popularity of radio caused the industry to seek a solution to this noise maker. The electrical whine is most acute on the AM band, and is seldom heard on the FM band.
"Some of the most vocal early complaints about this static came from owners of two way radio systems such as business interests and law enforcement."