What magnetism does to a hairspring
A steel hairspring is ferromagnetic: exposed to a sufficient field, it becomes a weak permanent magnet itself, and its adjacent coils begin to attract one another. Coils that cling together stop contributing to the spring's restoring force, which shortens the spring's effective length — and a shorter spring oscillates faster. The result is the classic symptom: a watch that suddenly runs dramatically fast. Not seconds — minutes, sometimes tens of minutes per day. Owners almost never guess the cause; the error is so large it reads as mechanical catastrophe, when in fact nothing is worn, nothing is broken, and nothing needs replacing.
The fields required are not exotic. Smartphone and laptop speakers, magnetic tablet covers and bag clasps, headphone drivers, induction hobs, magnetic tool holders, the clasp magnets in eyeglass cases — all can carry fields strong enough to magnetise a conventional spring at close range. This is why magnetisation has become more common over the past twenty years, not less: the modern desk is a minefield that the 1960s never imagined. Lesser effects are possible too — magnetised steel parts elsewhere in the train can add drag and subtler rate instability — but the stuck-coils-running-fast signature accounts for most real-world cases.
The historical responses: cages of soft iron
The first systematic defence was shielding. A liner of soft iron — magnetically permeable but retaining no magnetism of its own — surrounding the movement gives field lines an easy path around the calibre instead of through it, a Faraday-cage logic applied to magnetism. The format was perfected for professionals: IWC's Mark 11 navigation watch, built for the RAF from 1948, put its movement inside a soft-iron inner case; IWC's Ingenieur (1955) civilianised the idea; and Rolex's Milgauss of 1956 — named for the thousand gauss it withstood — was aimed squarely at the engineers and physicists at places like CERN whose workplaces hummed with field sources. Omega's Railmaster of 1957 served the same constituency. The approach works well against everyday static fields, and these references remain a beloved collecting category, but the shield has limits: strong fields partially saturate the iron, the solid inner case forbids a display back and date window, and the protection is bulk the modern dress watch cannot carry.
The material solution
The deeper fix was to make the vulnerable component out of something fields cannot grip. Nivarox-family springs (standard since mid-century) are already far less susceptible than carbon steel, but not immune. Rolex's Parachrom alloy of 2005 — niobium-zirconium, drawn in-house — is paramagnetic and indifferent to everyday fields. And silicon, adopted across the industry from the mid-2000s (Ulysse Nardin pioneered silicon escapement parts in 2001; Patek Philippe's Spiromax arrived in 2006; Rolex's Syloxi and Omega's Si14 followed), is effectively non-magnetic altogether: expose a silicon hairspring to a field that would wreck a steel one and it returns to its exact prior rate the moment the field is gone, because nothing about it has changed. Pair a silicon spring with silicon escape wheel and lever — as in Patek's Oscillomax/Pulsomax ensemble or Omega's Master Chronometer calibres, which also swap steel staffs and springs for amagnetic alloys — and the entire regulating organ becomes functionally immune.
In 2013 Omega demonstrated a Seamaster Aqua Terra that shrugged off 15,000 gauss — MRI territory, and far beyond any soft-iron shield — by making the movement itself amagnetic rather than hiding it. The METAS Master Chronometer certification, introduced with Omega in 2015 and since adopted by Tudor, formalised the achievement: cased watches are tested during and after full 15,000-gauss exposure and must hold chronometer rate (0/+5 s/day) throughout. It is the first certification in the industry's history to treat magnetic immunity as a tested criterion rather than a marketing adjective — and it quietly retired a problem that had stalked precision watchmaking for a century.
Demagnetisation: the easy fix
For the millions of existing watches with conventional springs, the cure is almost comically simple. A handheld demagnetiser — a mains-powered coil costing under twenty dollars — produces a rapidly alternating field that scrambles the aligned magnetic domains in the spring; pass the watch slowly through and away from it for a few seconds and the magnetism is gone. The fix is complete, not partial: the spring returns to its exact original mechanical behaviour, because magnetisation never deformed it in the first place. Any watchmaker will do this in the time it takes to say hello, usually free.
The diagnostic is nearly as easy. If a mechanical watch abruptly starts gaining minutes per day, suspect magnetism before anything else. A traditional compass — or a phone compass app, used as a rough screen — held near the watch will twitch if the watch carries a field. Demagnetise first; only if the problem persists does the conversation move to service. The number of healthy movements that have been opened, "serviced," and billed for what a ten-second demagnetisation would have cured is one of the trade's quiet embarrassments.
Magnetism is invisible, common, and usually fixable — which makes it the most misunderstood problem in watch ownership. Not because it is complex, but because its symptom, a watch running wildly fast, gets attributed to mechanical failure when a ten-dollar demagnetiser would end it in ten seconds.