Updating knob and tube wiring
Knob and tube electrical wiring involved stringing individual insulated electrical wires across porcelain insulators ("knobs") which were nailed to the surface of wooden framing members.When electrical wires needed to actually pass thru wooden framing members, the electrical wires were strung through porcelain insulators called "tubes" which protected the wires.
Both the he knob and tube pieces served as insulation to prevent hot wires from heating the wood and starting a fire.
Grounding is arguably the most important part of a sound electrical system.
A tell-tale sign that an electrical outlet might be fed with Knob & Tube is that the receptacle is only a “2-prong” version, missing the 3rd prong on the bottom which is for the grounding conductor.
How Knob-and-Tube Wiring Works: K&T wiring consists of insulated copper conductors passing through lumber framing drill-holes via protective porcelain insulating tubes.
They are supported along their length by nailed-down porcelain knobs.
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The linked video below from the Edison Tech Center on You Tube will give you the ability to see more about it for a better understanding.