I have an old fleamarket growler for checking electric motor armatures. It warns not to be turned on without armature in V. My physics has faded over the years, what will happen with an incomplete magnetic circuit? Can this be modified to use as a de-magnetiser or is it a piece of junk just worth the copper?
If the armature is not there then the inductance will be very low, and thus the current will be very high, so the coil will likely burn out unless it has been designed to handle the power dissipation.
I think you could limit the current through the coil (for example, with a series electric heater element) and use it as a demagnetizer (but it might not create a strong enough magnetic field to demagnetize a strongly magnetized object). You could also try a series degaussing PTC thermistor out of a CRT monitor or TV.
You'll "use up" the field in the thing, instead of the thing you want to demagnetize. This could result in your thing still attracting things, which would be a bad thing. Something would have to be done, before things got out of hand.
When you slap anything ferrous into that 'V', you complete the magnetic circuit, and provide an easy path for the flux. Take it out, and it takes a lot of ampere-turns to drive the flux through the gap -- this is why the current goes up when you don't load the 'V'.
Now if you put _two_ things in the gap, then the flux will get split up between them -- so your "thing" in the gap will steal the flux from whatever thing you want to demagnetize. So it's best to just fling one thing into the gap while demagnetizing.
I have used the Sun growler at work a number of times to demagnetize screwdrivers and it still works OK. I put the screwdriver on the magnet and turn it on and quickly remove the screwdriver straight up and turn it off, all in about 1 second.
I use an old TV Degaussing coil, similar to this one:
It's great, I've had it for probably 35 years and it works wonderful on hand tools, and is big enough to do the vise.
I hate magnetized tools... unless I purposely magnetize a screwdriver or whatever for something special. Magnetized vise jaw inserts are much nicer when you can quickly degauss your vise jaws once done with them.
Turn it on an arm's length away from the object/s to be demagnetized, bring it and the object together and wave it around, then pull it back away before releasing the switch. The whole operation only needs like 5 or 7 seconds tops. (I don't know the duty cycle of these things, but I bet it's not long.)
In my opinion, the one in the link above is pricy. I bet if you snooped around a serviceable used one could be had for a few bucks...
I try to keep my watch, wallet, cell phone, VOM's and the like away during use, but have forgotten a few times with no apparent ill effects.
Once for snorts and giggles, I tried erasing a VHS tape with it... it didn't work worth a damn.