Inductive coupling, house wiring, DMMs?

Folks:
I was going over an old circuit of mixed knob & tube and early NM, replacing old receptacles and switches (ancient P&S) and upgrading some wall-mounted fixtures (early 30s Markel Electric). The splices are well-soldered and taped; the K&T has intact insulation, and for once there was a reasonable amount of slack left to pull into new plastic boxes -- and capped-off gas pipes from the original lighting system...
With all the loads removed from the circuit, and the fuse removed, I measured no continuity from hot to system ground (good), and excellent continuity from neutral to system ground (also good). (I had a 3-wire extension cord plugged into a newer circuit in the same house). Nice to see the neutrals were still properly color coded.
Now, one of the Markel lights was fed by a run of K&T, which was soldered to the fixture feeders and a run of 1950s NM, said splice being buried in some kind of AWFUL white plastery substance as hard and unmerciful as death and taxes (my best guess is it was white lead putty). Into a plastic box this all went, ready to join the brand spanking new fixture leads.
For good measure, I took my DMM and measured AC volts between another circuit's hot side (from the extension cord) to the K&T neutral. 120 V, fine. Then I measured between the cord's hot to the K&T hot. 120V? Wha? I rechecked with a neon lamp. The lamp lit. The NM conductors, which ran through two 3-way switches to another light fixture, which was off, read 79v each. Wha?
I don't get it. The circuit is definitely dead. I have accounted for all the other branches, some of which are multiwire, but this one is not. Everything is turned off, and indeed there seems to be no DC path to ground through the K&T hot, and certainly not through the disconnected run to the 3-way switched fixture!
What does everyone think? Inductive coupling? Would that light the neon bulb? Should I go back to my old analog meter?
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Definitely try an analog meter or wiggy (http://www.twacomm.com/catalog/model_69115.htm )
DMM's are often misleading when checking house wiring.
autobus snipped-for-privacy@yahoo.com wrote:

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autobus snipped-for-privacy@yahoo.com wrote:

Without following exactly what you are doing - there is capacitance between wires (not much with K&T, a lot more with ungrounded Romex). Digital meters are very high resistance and will register voltage due to capacitive currents. Neon lights operate on low current and can also indicate on a dead circuit. Analog meters are a lot lower resistance (might be similar to neon?), but may still register. You could connect a small light bulb where you want to measure and measure the voltage across it.
bud--
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In K&T, inductive/capacitive coupling is a smaller effect than in Romex because of conductor spacing.
If I've followed your scenario correctly, the 79v DMM readings could well be inductive/EM coupling. "not zero and not 120V" is more indicative of that than zero or 120V. One way to check out the 79v readings is to turn the lights _on_. If I read your posting right, the lights are downstream of the unpowered K&T section. The lights shouldn't light when you switch 'em on, and the voltage will drop to 0v (presuming the light's neutral is still connected).
You're measuring from the cord hot to the dead circuit's hot? Is the dead circuit's hot touching anything grounded, including the neutral, including going through any switched on device like a light bulb? That'd explain it. The circuit _is_ dead, the hot wire is acting as a neutral to the cord's hot.
Testing against an adjacent hot circuit just makes things confusing.
Neon testers don't take much current (touch one lead to a hot wire, hold the other one in your hand, the bulb will light up even if you're totally insulated, and you won't feel a thing).
But I really don't expect a neon to light up from _just_ inductive coupling to a disconnected wire.
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Chris Lewis, Una confibula non set est
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