Magnetism and Its Effects

A watch's hairspring is normally made from a steel or steel-based alloy, and steel alloys used for hairsprings, while chosen for elastic properties rather than magnetic ones, can still become magnetised if exposed to a sufficiently strong magnetic field — a phenomenon that has become considerably more common with the spread of magnetic clasps, phone cases, laptop lids, speaker magnets and induction chargers in modern daily life, alongside older sources such as magnetic tool trays and loudspeakers.

Once magnetised, adjacent coils of the hairspring can attract or repel one another along their length, effectively shortening the spring's active length or causing it to cling together at some point in its swing. Because a hairspring's effective stiffness and the resulting balance rate depend directly on its geometry, an unpredictable partial clinging of the coils typically produces a sudden, sometimes dramatic gain in rate — a watch that was previously keeping reasonable time and suddenly starts gaining many minutes a day is one of the single most characteristic symptoms of magnetism in a mechanical watch, sometimes accompanied by intermittent stopping if the coils cling badly enough.

Magnetism is straightforward to test for without opening the case: a watchmaker holds a small compass near the watch and observes whether the needle deflects, and can confirm suspected magnetism, along with its approximate degree, on a dedicated magnetism-detecting instrument before deciding on treatment. This makes magnetism one of the few faults that can be confidently identified from outside the case, which is why it is often checked early when a watch presents with a sudden, large, otherwise unexplained rate change.

Demagnetising a watch means passing it through a rapidly alternating and then diminishing magnetic field, produced by a demagnetiser coil, which cancels out the residual magnetism in the steel by repeatedly reversing and reducing the field's strength until none remains; done correctly on a movement's steel components, this generally restores normal timekeeping immediately without any disassembly at all, which is why professional demagnetisation is often a very quick and comparatively inexpensive fix for what initially looks like a serious rate problem.

Despite how simple the fix usually is, demagnetisation should still be carried out by a professional with the correct equipment and technique rather than attempted with an improvised magnet or coil, since an incorrectly applied field can in principle worsen the existing magnetisation rather than remove it, and a watchmaker's assessment also confirms that magnetism, rather than some other coincidental fault, was actually the cause before concluding the watch is fixed.