>He said it had to be small...  That implies a low magnetic field,
>considering that the magnets I work with weigh in at 17 metric tonnes, and
>they're only .35 Tesla, though the gap is fairly large, and for this, your
>gap could be quite small.  The higher the field strength, the higher the
>output signal.  I assume a permanent maget is the only practical solution
>for this application.

Oh I dunno ..... :)

I was reading the obituary the just the other day, for a guy who had been a
pilot in WW2, flying bombers which were used to find magnetic mines. The
article had a photo of one of the bombers they used, which had a massive
coil of wire on a former, about 3/4 of the wingspan in diameter. Apparently
there was a 48kW generator driven by a diesel motor slung in the bomb bay to
provide the current for the coil.

From all accounts it could be fairly hairy flying this thing at about 50 to
100 feet above the water, if the mine was close to the surface it would
shower the plane with all sorts of shrapnel and debris when it exploded.

They do not say what the field strength of the coil was. :)

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