Demagnetisation is the removal of magnetism from a part that has become magnetised, so that it returns to a neutral state. In a watch, a magnetised balance spring has coils that cling to one another. The oscillator then beats faster and the watch gains time. Demagnetising does no harm to the movement.
How does a demagnetiser work?
A demagnetiser is a coil, or solenoid, fed with alternating current. This produces a strong magnetic field that keeps changing direction. The part sits in that field as it reverses and fades to nothing over time. Each reversal pushes the magnetic domains in the metal back and forth, a little less each time. In the end they no longer line up, and the part is neutral. With a simple coil-type demagnetiser, the part must be moved well away from the coil before the power is switched off. There is also a pulse type, called a ringing-choke or resonant-coil demagnetiser. It uses the stored charge of a capacitor.
How do you demagnetise a watch?
A watch demagnetiser is a small, simple device. It uses an alternating magnetic field to clear magnetism that has built up in a mechanical movement. The watch is laid face down on the demagnetiser’s pad, or held just above it. One sign that a watch needs this treatment is that it keeps gaining or losing time over a long period. Because the process does no harm to the movement, watchmakers often try it first when they are looking for a fault.
Is demagnetisation still needed?
Non-magnetic alloys and non-magnetic balance springs have changed how often watches need demagnetising. Silicon balance springs in particular have made a difference. The word itself is not limited to watchmaking. It is also used in other fields. Ships’ hulls, for example, were treated against magnetic mines, and that process carries the same name.
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