I built a demagnetizer about 10 years ago. This is just a flat coil of wire and a light bulb. The light bulb is what determines the power rating of the magnetic field. If you want a lot of power us a 100 watt light bulb but if you only need a small amount of power use a 25 watt light bulb. Connect your power cord, light bulb, and coil all in series. You can use the light bulb as the switch just screw it in for ON then unscrew for OFF. If you don't want to burn your fingers in a hot 100 watt bulb then use a light switch to turn it ON/OFF or unplug the cord. This is how it works, you need to over come the magnetic field you are trying to remove so you only need a little power for things that are slightly magnetizer and you need more power for things that have more magnetism. A 100 watt light bulb produces a large magnet field you need to pull the part about 4 ft away from the demagnetizer to remove the magnet field. A 40 watt light bulb produces a smaller magnetic field so you need to pull the part away about 2.5 ft. See the photo at the top.
If you want the Rolls Royce version add a Variac in series with everything you no longer need the ON/OFF switch. Turn the power UP then turn the power DOWN very slowly. No need to pull the part off by hand and pull it way back away from the demagnetizer. You can use a 100 watt bulb all the time so if you only need a little power then turn the variac up about 50% for half power then very slowly turn the power down. See the second photo.
I made 2 of these coils and I built one real nice demagnetizer in a 1/4" plywood box. It worked great then I got the idea maybe it would sell on ebay and it did for $95. I have been planning to build the other demagnetizer but I keep putting it off. Sorry but I never took a photo of my finished demagnetizer. I am getting older and my projects are a lot slower getting finished.
The coil is made from #24 enamel coated copper wire. It is 1/4" thick and about 5" diameter probably about 1000 turns of wire. The center of the coil is a piece of 1" diameter PVC pipe 1/4" thick. I clamped 2 pieces of plexaglass over the PVC piece and turned it in the lathe to wind the coil. You need to paint the coil as it winds up with Polyester Resin or 30 minute Epoxy so the whole thing gets hard and holds together as one solid piece. Put a good amount of epoxy on one side of the finished coil and glue it to the under side of your 1/4" plywood. I put my light bulb on the right side and the light switch on the left side and build it all in a nice little wood box about 2.5" tall. If you use the variac your box will have to be larger. I experemented with the variac but never built it onto a box. I had the variac setting off to one side and it works great.