Magnetize/De-magnetize

Close. All ferromagnetic materials (many iron alloys, cobalt, nickel, hematitie etc.) spontaneously magnetize at the microscopic level, but the 'relaxed' state has a mix of domains oriented N,S,E,W and the external field averages to zero. When a field is applied, like Earth's magnetic field, the co-aligned domains grow a bit (and the antialigned and orthogonal domains shrink). But tools aren't pure and unstrained crystalline material, there are internal strains and such (and hard steel has multiple crystal forms with boundaries between the martensite and austenite allotropes). The domain-growth phenomenon has to occur by moving the magnetic domain boundary out into new territory, and the strains and impurities and crystal- structure transitions are all blockages that can halt the movement.

A shock wave going through the hard metal can alter the blockages and has the net effect of allowing bulk magnetization (if the pressure peak enhanced the blockage, the next pressure trough will lessen it, and that means the net effect is to enhance the domain growth).

A degaussing coil doesn't exactly demagnetize, it just re-remagnetizes in different polarities as the AC reverses, and does so to less depth as you pull the tool away from the coil, resulting in a magnetization inside the steel that has thousands of different polarities- random enough to pass as 'unmagnetized' from a distance.

Just as a solder-gun can demagnetize your screwdriver (pull the screwdriver slowly from the loop as you hold the trigger) it can also magnetize it; just tap the trigger instead of pulling the screwdriver away.

Well ... it sounds sort of like the first tape degausser (bulk eraser) which I got used several decades ago.

It was two transformer cores (E-sections only) mounted in a shallow steel box with the open ends of the transformer cores pointing up, a black plastic top, holding a 1/4" diameter smooth stud to serve as the pivot for rotating a reel of tape around. The two transformer cores were located so one covered a 7" (175mm?) tape (given a full rotation of the tape), and the other covered the extra area which a 10-1/2" (266mm?) reel added. There was a switch on the near edge, and you placed the tape reel there, switched on, rotated the tape through a full 360 degrees or so (I usually gave it three full turns), then picked up the reel and moved it slowly away, switching off the power when you reached the limits of your arm's travel.

There was a fuse for each coil.

I added another switch to it to disconnect the outer coil when degaussing 7" tapes or smaller, just to reduce the chances of popping a fuse or circuit breaker in the apartment.

I later got a couple of tape degaussers designed for thicker tapes (this was for 1/4" wide tape). Those had two "C" shaped forms, one above and one below the tape, with a motor which rotated the tape and slowly drove the drawer to pull it out, and switch it off at full extraction. That one had both coils in series, with a capacitor between them to tune to near resonance when a tape was loaded between the pole pieces. This resulted in a much stronger field, needed for 1/2" tapes, or even more so for 1" and 2" tapes, as were used in instrumentation recorders and multi-track studio recorders.

None of these had a mechanism for reducing the field -- just for making it more distant. Though when the tape was not between the pole pieces the series LR circuit was not as close to resonance, so the current was somewhat lower -- but it still had to switch off when the tape reached the end of travel in the carrier drawer.

Not exactly what you wanted, but what you could do is set up a motorized Variac to slowly reduce the field and switch off when it reached zero. Start with something like the first one above.

Or -- you could take another electromagnetic chuck made for the surface grinder and drive it from the AC line instead of DC. I know that my Sanford surface grinder came with a DC Mag chuck, and a switch position to feed it unrectified AC to at least partially demagnetize the workpiece.

Enjoy, DoN.

The demagnetizing table I used seems the have had one. The parts didn't have to be moved away. Switch it on, push a button and then slide the part around on the table. The internal setup must have looked they way you and Lloyd described it. The top of the table was out of SS and had a gap (sealed with plastics, of course) along the middle of the table. I guess it separated the poles of the E or C core. Should have been a C-core, an E wouldn't help that much. To be effective, the part had to be more or less centered on the gap, for obvious reasons.

Close enough. The idea with the motor is quite mechanical. :-)

Nick