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writes:

An AC coil is the answer in any case. Preferably one that you can hold
the part in and then draw it away from while the coil is still active.

For small items, the frame les the rotor of a cheap fan or blower motor
will work fine, just place the part oin the feild between the poles and
withdraw it slowly away.

Another source: a magnetic tape demagnetizer. Radio Shack used to sell
a handheld version of this; it was a coil with an iron core and the core
face was just inside the bottom of a plastic case. You turned it on,
placed it against the tape to be demagnetized, moved the tape around a
bit, then pulled the tape away before turning off power. I've used
this for demagnetizing all sort of tools, and if anything it's too
strong - large masses of metal are attracted with a lot of force.

They used to be something like $30, but I recently found one at a thrift
shop for $2.

Then there are industrial tape demagnetizers with a coil large enough to
pass a whole open reel of magtape through.

Dave