If you have an old house, you know this refers to the wiring that’s probably coursing current to your lights and plugs.
The knobs are ceramic clips (ceramic–imagine the expense installing those today!) that secure and protect the fabric-jacketed wire parallel to the joist and/or serve as a pivot point to change the wire’s direction. The tubes are ceramic sleeves that protect the wire as they pass perpendicular through a joist or stud.
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