Accidental magnetization is, even today, the leading 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, get close to a speaker, or pass near a wallet with a magnetic clasp. ISO 764 certifies resistance up to 4,800 A/m (60 gauss), a threshold that everyday objects easily exceed — a laptop can generate fields up to 16,000 A/m. A watch compliant with the basic standard, in other words, is not protected from the real environment it lives in.
Historically, the problem has been tackled with shielding: a cage made of soft iron — the so-called Faraday cage — wraps around the movement and deflects field lines before they reach the balance wheel and hairspring. Soft iron works because it doesn't retain residual magnetism. Rolex's Milgauss and the RAF's Mark XI are canonical examples. The structural limitation of this approach is clear: the shield adds thickness, requires an opaque caseback, and if the field is intense enough to saturate the iron, protection ceases.
The alternative solution works at the root: replace ferromagnetic components with intrinsically non-magnetic materials. Silicon hairspring doesn't magnetize, eliminates the need for shielding, and allows sapphire crystal casebacks. ARCAP alloys — free of iron — are used by URWERK for main plates and bridges precisely for 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 non-ferromagnetic by nature.
Seiko's Spring Drive occupies a separate category: it uses the mainspring as an energy source, but the regulator is a magnetic oscillator that brakes the glide wheel via 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 the operator's survival, not the watch's precision.
At the bench, I still encounter movements from the 1960s-70s that are magnetized and lose ten minutes a day. Demagnetization with a specialized device solves it in thirty seconds, but the customer usually thinks the movement is broken. The first thing I check, before opening any case, is always that.