Known as a "ceiling board" these are porcelain insulator hangers used inside buildings to hang and electrically isolate carbon arc lamps from the ceiling or from overhead support beams in industrial and commercial buildings, Some have fuses, some do not. Some also come with mechanical switches or disconnects as part of the unit. There were a great variety of manufacturers and designs produced from the 1880's through the 1920's. Some early versions were also made of wood. All of them come with a large heavy duty hook in the center of the unit.