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Hi --
In honor of Ben's birthday:
I bought a multimeter at the local home center. After I got
it home I was pleased to discover that it has an "NCV" feature:
Non-Contact Voltage.
If you push the NCV button and hold the instrument close to a wire
carrying AC voltage, it chirps.
Such things have been around for a while, but I didn't expect
the feature to be "bundled" in an under-$40 instrument.
-- It is not a metal detector; if the wire has no voltage you
get no indication.
-- It is not a current detector: it cares about voltage, not
current.
-- It is not a field mill: it can't see DC fields ... although
I imagine you could make a sensitive (but imprecise) field mill
by putting a windmill-style interrupter in front of it.
-- I assume it's just an AC voltage detector, capacitively coupled,
with a reeeeally high input impedance. You can get one or two
bits of "resolution" on the magnitude of the voltage by moving
the instrument nearer-to/farther-from the source.
It can see a wire carrying 120VAC from at least ten inches
away ... and it can see right through gypsum wallboard ...
which is exceedingly useful if you are trying to trace
wires buried inside a typical residential interior wall.
It is marginal-to-poor for tracing wires embedded in thick
adobe walls, and of course hopeless for wires inside metal
conduits.
In addition to tracing, it is useful for waving in front of
a mess of wiring to make sure it is all "cold" before you
start working on it.