This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A watch demagnetizer that will give excellent satisfaction may be made as follows:
Procure a sheet of 1/16 in. brass, 3 1/4 in. by 7 in. Bend this piece of brass around a piece of hard wood having a rectangular cross section of 2 1/2 in. by 1 in. The joint between the two ends should be made on one side, and the edges should lack about 1/8 in. of touching. Next obtain two pieces of 1/8-in. brass, 3 3/4 in. by 3 in., and cut an opening in each of these, 2% in. by 1 1/8 in., as shown in the sketch. Bend one edge of each of these pieces over at right angles to the main portion of the piece. Solder these two pieces on the ends of the rectangular tube of brass and cut a slot in each of them to correspond to the one in the rectangular tube. Place the rectangular piece of wood back in the tube and you are ready for the winding. Use No. 18 gauge single cotton-covered copper wire and fill the winding space. Several layers of paper should be placed on the brass tube and between the layers of wire, to serve as an insulation. Holes may be drilled in the projecting portions on the ends and the coil can then be mounted on a wooden base. Mount two binding posts on this base and connect the terminals of the winding to them.
Ill: Dimensioned Parts for the Construction of the Core over Which the Insulated Wire is Wound
To use the demagnetizer, connect it to a 110-volt alternating-current circuit with a rheostat in circuit of such a form that the current will not exceed three amperes and that it may be reduced to practically zero in value by increasing the resistance of the rheostat. The magnetic field inside the coil is rapidly changing in direction and will tend to destroy any permanent magnetism that may be possessed by an object placed inside of it. The full current of three amperes should be allowed to pass through the winding for a few minutes after the object to be demagnetized is inserted, and then gradually reduced, and the object removed.