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.
Quite often the attendants or a visitor to an electric-light plant discovers after a few days that his watch is losing a half hour or more a day by having become magnetized by the dynamos. In stations where the old types of machines are still in use there is a great deal more danger from what is called "stray" magnetic fields than in those where modern machines are installed.
The jeweler demagnetizes a watch in the following way: He has a piece of soft iron with an opening cut in its center of such shape and size as to receive the watch, and with a fine wire wound about it. After the watch has been placed in position, an alternating current, that is, one whose direction is changing at regular intervals, is sent through the winding, and thus a magnetic field is produced that also changes in direction as the current reverses. The current is gradually reduced in value and the magnetism originally possessed by the watch is removed. When an alternating current is not available, a direct current may be used, its direction being rapidly reversed by what is known as a "polarity changer."
Anyone can demagnetize his own watch, however, with very little trouble and no expense by a much simpler method. Procure a piece of heavy linen thread about 3 ft. long, attach one end of it to the ring of the watch, hold the other end and turn the watch around until the thread is twisted at least one hundred times. Now allow the thread to unwind, and as the watch revolves, pass it back and forth near a powerful electromagnet. The field magnet of a good-sized generator or motor will answer. The machine should be in operation, or at least there should be a current in the windings about the fields, when you attempt to demagnetize the watch. While the thread is unwinding, and the watch moved in the magnetic field, gradually withdraw from the magnet so that when the watch ceases to revolve, it is just outside of the field.
Always be sure to keep the watch revolving while it is in the magnetic field, otherwise the results will be very unsatisfactory, and more harm than good may result.