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From: John De Armond
Subject: Re: Mystery wire
Date: Wed, 14 Nov 2007 20:12:20 -0500
Message-ID: <>

On Thu, 15 Nov 2007 01:26:58 +0100, Trygve Lillefosse <>

>They are still connected to the mains, but there are no load(lamp or
>whatever) at the end.
>>You may have to turn the gain to max on the probe to penetrate the walls
>>Most likely you have one end of  a three-way switch .
>I do not quite understand how this gadget work. Do you have any link
>that explains the workings?

The little black thing is a tone generator that connects to the wire.  The probe
contains an inductive pickup that signals when it picks up the radiated field from
the wire.

The one cited is designed for de-energized wiring. Telephone, network and other
similar wiring.  One probably more suited for your application is this:

This unit is similar to the above one except that it works with live circuits.  The
tone generator connects to the live circuit and is powered from it.

None of these work very well when tracing Romex because the two parallel wires act
like a transmission line and thus don't radiate much signal.  If you're tracing a
single wire then either should work well.

You probably don't need either of these to find an energized power conductor.  All
you need is a capacitive probe feeding a reasonably high gain audio amplifier.  I
have a Rat Shack amplified speaker with a fairly high impedance microphone input that
I've used to find hidden wiring.  A phono preamp is ideal.  The probe can be as
simple as a length of wire or perhaps a plate of metal.  It picks up the e-field
radiated by the wire.  Simply move the probe over the area and home in on the spot
that produces the most hum.


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