Anti-Magnetic Watches Explained: Faraday Cages vs Silicon and Master Chronometer

The part that suffers is the hairspring, the fine coil that governs how fast the balance wheel swings. If the spring is made of a magnetic alloy and picks up a charge, its coils can cling to each other. That shortens the working length of the spring, and the watch runs fast. Steel parts elsewhere in the movement can pick up magnetism too, which is why a badly affected watch can also stop or run erratically.

Grand Seiko's owner guidance says the same in plain terms: exposure can make a watch gain or lose time, or stop it. It lists the usual culprits, including smartphones, tablets, AC adapters, bags with magnetic clasps, magnetic cooking devices and magnetic health pillows. Most of us carry or sleep near several of those every day, which is why anti-magnetic design has moved from a niche feature to a mainstream one.

The older solution is shielding. A soft iron inner case wraps around the movement and acts as a Faraday cage: the iron draws the magnetic field around the movement instead of through it. The movement inside is conventional, so the protection depends on the shield staying intact and closed.

Rolex's Milgauss is the best known example. Its modern version, reintroduced at Basel in 2007, is rated to resist a 1,000 gauss field, according to WatchReport's coverage, with the movement encased in soft iron. It's a 40 mm watch with the calibre 3131 and a Parachrom Blu hairspring, so it pairs the cage with a hairspring material that isn't affected by magnetism. IWC does something similar with the Ingenieur. The 2023 Ingenieur Automatic 40 has a soft iron inner case, as the 1976 original designed by Gérald Genta did, plus a soft iron dial that adds to the shield, according to SJX.

The trade-off is practical. A shield adds thickness, and a soft iron dial limits what a designer can do with finishing. Beckertime's overview notes that Patek Philippe, IWC, Omega and Blancpain have all made anti-magnetic watches on the same cage principle over the years.

The newer approach skips the shield and removes the vulnerable materials. Omega's description of its programme, as summarized by Monochrome, starts with a silicon balance spring (Si14), launched in 2008. It then adds Nivagauss staffs and pivots, non-magnetic plates in place of the steel ones in Co-Axial movements, and an amorphous material for the shock absorber spring. With no magnetic parts to magnetize, the watch can have a plain steel case back and a normal dial.

Tudor takes a partial version of this route. Its manufacture calibre MT5400 in the Pelagos 39 has a non-magnetic silicon balance spring, according to Europa Star. That's a real advantage over a nickel alloy spring, but Tudor describes the movement as COSC certified, and we couldn't find a gauss rating published for it. Treat it as magnetism resistant, not as a certified 15,000 gauss watch.

Master Chronometer is a certification created by Omega with METAS, the Swiss Federal Institute of Metrology. According to Monochrome's visit to the facility, Omega runs the tests in house under METAS supervision, with a METAS analyst on site, and METAS trademarks the name. The watches are also still COSC certified as chronometers.

The eight tests cover precision over four days in six positions and at two temperatures, with a 15,000 gauss exposure in between. They also check that the movement and the complete watch keep functioning during that exposure, and how much the rate changes afterward. The remaining tests cover water resistance, power reserve, rate deviation between 100 percent and 33 percent power reserve, and rate deviation across positions.

Fifteen thousand gauss is far beyond anything in a home or office, and Monochrome compares it to the field of an MRI. It's also fifteen times the Milgauss figure, though the two ratings aren't tested the same way, so the comparison says more about headroom than about everyday performance. Both are well above what a phone speaker or a laptop clasp will produce.

Gauss and amperes per metre (A/m) both measure magnetic field strength, in different unit systems. Roughly, one gauss corresponds to about 80 A/m. Grand Seiko's own manual gives three tiers in A/m: 4,800 A/m for watches with no special marking, 16,000 A/m for diver's models, and 40,000 A/m for watches marked as such. A fan site tied to the brand puts 4,800 A/m at about 60 gauss.

That puts the typical Grand Seiko rating near the lower end of the scale, and the 40,000 A/m models (roughly 500 gauss) below the Milgauss's 1,000 gauss. Even so, the manual still tells owners to keep the watch at least 5 cm from magnetic products at the base rating, and at least 1 cm at the higher ratings. A rating is a safety margin, not permission to rest the watch on a speaker. IWC's collection pages for historic Ingenieur pocket watches have quoted 40,000 A/m, but we couldn't confirm the current official figure for the 2023 watch, so check the spec for the specific reference you're considering.

The symptom is a watch that suddenly gains a lot of time without any knock, or runs erratically after being near a magnet. Moving it away from the source sometimes isn't enough, because the parts keep their charge until they're demagnetized.

Demagnetizing is done by a watchmaker on a purpose-built device, and the rate is then checked and adjusted if needed. Grand Seiko notes that demagnetizing and readjusting is a charged service even within the warranty period, which is a fair reminder that magnetism damage usually isn't treated as a defect. If you ask for it during a regular service, a good watchmaker will check for magnetism first, since a magnetized movement can mimic worn parts and lead to unnecessary work.

For prevention, keep the watch away from phone and tablet speakers, laptop and bag clasps, and induction hobs. If you wear a watch with a silicon spring or a certified rating, you have more margin, but the cage models and the older alloy-spring watches are the ones that benefit most from the habit.