Magnetic resistance

A magnetic field doesn't damage a mechanical watch the way a physical shock would: it silently alters it, deforming the hairspring and causing the watch to gain or lose minutes per day until the movement is demagnetized. I've seen magnetized balance wheels that were off by over three minutes in twenty-four hours, with the customer convinced there was a regulation problem. This is why magnetic resistance is a technical specification, not an optional feature.

The traditional method is the soft iron inner case: a structure that wraps around the movement and redirects magnetic field lines away from the caliber, without ever becoming permanently magnetized itself. IWC Ingenieur has used it since the 1950s, Rolex Milgauss used it—where the honeycomb metal dial of the ref. 6543 added supplementary protection—and many pilot watches with original military specifications still use it. The limitation of this solution is the added weight and thickness, plus the need to open the case from only one side to avoid compromising the shielding.

The alternative approach is to act on the materials of critical components. The hairspring is the most vulnerable point: steel magnetizes easily. Paramagnetic alloys—traditional Nivarox, then Rolex's Parachrom, then Nivachron developed by Nivarox for ETA and Hamilton—drastically reduce this sensitivity. Silicon, used by Longines in the L888.5 caliber and others, is practically immune. When the hairspring and balance wheel are replaced with non-magnetic materials, the internal cage becomes less necessary: the new Dynapulse escapement in Rolex's 7135 caliber was developed precisely to eliminate the internal shield without losing precision.

Units of measurement create confusion. The gauss is the unit of magnetic field in the CGS system: the Milgauss resists 1,000 gauss, and METAS certification for Omega and Tudor movements reaches 15,000 gauss. In the ISO standard, amperes per meter are used: 4,800 A/m corresponds to roughly 60 gauss, the minimum threshold of the ISO 764 norm. These are different scales, not directly comparable.

Ressence brought magnetism inside the movement, using it as a transmission system: micro-magnets of 1 × 0.5 mm connect the dry chamber of the caliber to the oily chamber of the dial without a mechanical axis. In that case, the shielding cage exists to neutralize the effects of internal magnetism on the balance wheel, not to protect it from the outside—an inverted logic compared to the classic solution.

From the workbench: the quick test with a compass still works. If the needle deflects when you bring the watch closer, the caliber is magnetized. Demagnetization with an alternating current demagnetizer takes ten seconds and costs nothing; delaying it means leaving the customer with a watch that runs poorly without knowing why.