This image is of a strain insulator and turnbuckle which is used for trolley wire. A strain insulator is able to withstand the tension of the wire. This device helps support the trolley wire. A turnbuckle adjusts the tension accordingly. A turnbuckle is often used for corners of a wresting arena.
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International Correspondence Schools A Textbook on Electric Lighting and Railways (Scranton, PA: International Textbook Company, 1901)