I assume this is some USA wiring system, am I right?
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Knob and tube wiring uses two separate copper conductors, each isolated by cotton cloth or soft rubber. The conductors are strung tightly through the cavities of your walls and floors, and depend on being separated by air in order to avoid overheating. When going through joists, the wires pass through porcelain tubes. In the wall and floor cavities, they are attached intermittently by porcelain knobs.
Knob and tube wiring was commonly used in North America from the 1880s through the 1930s.
Vebjorn Ljosa
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Knob and tube was the standard long ago. Now it's considered verboten by many insurance companies and is often replaced when a home comes under contract (either the seller replaces it or drops the house price by a significant amount and the buyer replaces it).