Feeder insulators were previously made from glass, but the strain they endured caused them to crack, making glass an unsuitable material. The insulators were later made from molded mica to be used on poles at corners and curves to better endure with the strain. This model can take a cable up to 500,000 circular mils cross-section.
International Correspondence Schools A Textbook on Electric Lighting and Railways (Scranton, PA: International Textbook Company, 1901)
Courtesy the private collection of Roy Winkelman