Magnetism (resistance to)

Accidental magnetization remains, even today, the primary cause of precision loss in a mechanical watch. You don't need an industrial environment: it's enough to rest your wrist on a laptop, stand near a speaker, or pass by a wallet with a magnetic clasp. ISO 764 certifies resistance up to 4,800 A/m (60 gauss), a threshold that everyday objects routinely exceed — a laptop can generate fields up to 16,000 A/m. A watch meeting the base standard, in other words, is not protected from the real environment in which it lives.

Historically, the problem has been addressed through shielding: a soft iron cage — the so-called Faraday shield — wraps around the movement and deflects the field lines before they reach the balance wheel and hairspring. Soft iron works because it retains no residual magnetism. The Rolex Milgauss and the RAF Mark XI are canonical examples. The structural limitation of this approach is clear: the shield adds thickness, demands an opaque caseback, and if the field is intense enough to saturate the iron, the protection fails.

The alternative solution works at the root: replacing ferromagnetic components with materials that are intrinsically non-magnetic. A silicon hairspring cannot be magnetized, eliminating the need for shielding and allowing sapphire crystal casebacks. ARCAP alloys — iron-free — are used by URWERK for plates and bridges for precisely this reason. The Glucydur balance wheel, in beryllium copper, is non-magnetic and corrosion-resistant. Phynox — a cobalt-chromium alloy — is employed for steel components adjacent to the balance wheel in movements where magnetic stability is critical for chronometric performance. Titanium and its alloys, including Timascus, are naturally non-ferromagnetic.

Seiko's Spring Drive occupies a category apart: it uses the mainspring as an energy source, but the regulator is a magnetic oscillator that brakes the glide wheel through eddy currents. There is no traditional balance wheel to protect, but the principle remains magnetic in its regulatory essence.

The extreme case is represented by the Ocean Bund ref. 3519 AMAG: designed for divers tasked with mine removal, it had to be completely free of residual magnetic field — not to protect the movement, but to avoid triggering the mines themselves. Resistance to magnetism here was a matter of operator survival, not watch precision.

At my bench, I still encounter movements from the Sixties and Seventies that have been magnetized and lose ten minutes a day. Demagnetization with a proper device solves the problem in thirty seconds, but the customer usually believes the movement is broken. The first thing I check, before opening any case, is always this.