Sean Breheny wrote:

> I never understood why non-ferromagnetic metals should have any effect
> on metal detectors.  Ferromagnetism is such a VERY MUCH larger effect
> than diamagnetism (exhibited by all materials) or paramagnetism
> (exhibited my some materials especially at low temperatures),

  Well, if you are looking for iron bolts (or by the same token,
reinforcing rod), you may be right.  If you are looking for rust
(minerals) or trying to look *through* it, you are probably lucky these
have so little ferromagnetism.

> The only thing I can think of is that the conductivity of the metal
> causes some of the signal to be coupled to it and thereby alters the
> inductance by creating a mutual inductive effect, or causing resistive
> losses in the signal, or by acting as a resonant object.

  Now you«re getting it.  The "shorted turn" effect.  A gold wedding
ring should be *very* detectable (*if* it is broadside!).  Apart from
saturation generating harmonics as also mentioned in this thread, and
using twin coils with cancelled mutual inductance, you appear to have
ably summarised the effects.

  Remeber the "tuning wand" used to trim RF coils, including striplines?
A fiberglass wand with a ferrite slug (chosen for the frequency, one
hopes) at one end and a brass(, copper, silver) slug at the other?  A
shorted turn lowers the inductance.  Ground resistivity lowers the "Q",
but if you can actually compensate for that, so much the better.

  They say the good ones, using secret circuitry (ICs with the markings
ground off; supposedly may use microprocessors) really do discriminate.

  I«ve often thought however, that working in the sub-megahertz range, a
process similar to the capacitance measurement using the PIC«s input
hysteresis could quite likely be used to measure the inductance of a
coil which is after all, what you are doing metal detecting.  It could
perhaps by extension assess "Q" and balance.

  There is a really nifty "stud finder" (generally capacitive, though
some of these use both capacitive and inductive sensors, and include an
AC voltage detector as well) which has an "auto balance" button instead
of adjusting the knob till the light goes out.  I said "hey, a PIC could
do that for sure" (e.g. 12C508).

  Cheers,
        Paul B.