N Cook wrote:

I sliced open one from the back of a pack of batteries.
Other than a nice source of ferrous shim of 0.03 mm or 1.2 thou/mil thick ,
not much in there. 3 such pieces , 2 the same length and one a bit shorter.

At least some of these things are not magnetic at all, and I think that
applies to the one you cut open.

The two "wings" you saw are antennas. At the junction of the wings is a
tiny dot which is actually a subminiature diode. When you walk through
the detection loop, you enter an RF field that is set up (transmitted,
if you will) by one of the loops. The wing antennas pick up the RF, and
the diode creates harmonics, which are re-radiated through the wings.
The detection part of the operation involves looking for the 3rd
harmonic of the original frequency.

The tag is disabled at the checkout counter by placing it on a pad that
contains a transmit antenna. The pad transmits enough energy to burn out
the tiny diode. Once burned out, the diode does not generate harmonics
on the way out of the store, so the detector isn't triggered. The field
strength at the surface of the pad is also strong enough to scramble the
magstripe on your credit card, of course - hence the warning not to put
your card on the pad.

Note that everything happens in what the RF engineers call the "near
field", so the field strength drops off very rapidly with distance.

Finally, the little theft prevention thingies are not "RF ID" devices -
that is a different kettle of fish.

Bill