demagnetizing 4140?

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

This won't work. The extension cord contains two conductors with equal and opposite currents at all times, so the magnetic field produced by the cord is almost nil.

You *could* do this by making up an "extension cord" using two single conductors, and wrapping one conductor into a coil while the other conductor bypasses the coil. Or put both conductors in the same coil, but wind one wire clockwise and the other counterclockwise.

Dave