I need to pull a neutral wire to my light switch for a smart switch. It's presently switching an outlet that I don't need switched, and the other side of the outlet already has unswitched power. My plan is to use a smart switch, which requires neutral, to control a different device wirelessly.
I can't feasibly run a new wire, because the wires aren't run through conduit. Instead, my plan is to repurpose the switched wire to get neutral from the outlet.
The switch, which controls an outlet has two wires, one black and one red. The switched outlet has a white neutral wire, plus the red and black wires that I assume run directly to the switch.
I've completely disconnected the red and black wires from the outlet and the switch. The black wire is measuring 120V AC when measured against ground using the box as ground, both at the outlet and at the switch. Using my DMM, the red wire is measuring 24V AC with respect to the box/ground at both sides. I assume this is a ghost voltage caused by induction from the charged black wire over to the floating red wire.
Our house is ~100 years old and this particular switch seems to be run using armored cable (note the paper wrap around each wire.) There are literally only two wires in the switch box; no wire nuts.
If relevant, we do have some old knob and tube wiring active, but I'm pretty confident there's none on this circuit.
Is there some way I can determine with enough confidence that the red wire isn't connected elsewhere such that I can know it's safe to connect the red wire at the outlet to netural?