Rayon became popular on cords after 1930. Prior to this it was silk. The use of silk, rayon and cotton were used until the 1950's.
A bit of trivia:
"In 1830, Joseph Henry (1797-1878) was teaching a class. He said, in effect, that there is no connection between electricity and magnetism. To prove it, he energized a wire, running near a second wire with a crude meter that showed electrical current. When he energized the first circuit, the meter moved. He had failed to prove his point.
Not only that, he found that, if he coiled up one or both wires, he could intensify the effect. And if he wrapped the wire around a piece of iron, the intensity of the magnetism was enhanced further.
Of course the wires could not touch each other, or the iron, or they would short-circuit. So he took his wife's silk dress and covered the wires with silk. Silk-covered wire was the hottest thing in the wire biz until well into the 20th century." by Steve Lampton The History of Wire and Cable
