Magnetism: the most common cause of a fast watch

If a mechanical watch that used to keep decent time suddenly starts gaining, and gaining a lot, magnetism is the first thing to suspect. Not wear, not a service, not a fault in the movement. Somebody put it down next to a speaker, or slipped it into a bag with a magnetic clasp, or left it on a tablet cover overnight.

It is also the cheapest problem in watchmaking to fix. A watchmaker holds the watch over a demagnetizer, presses a button, draws it away, and the fault is gone in about ten seconds. The difficulty is recognizing it, because a magnetized watch looks and feels exactly like a watch that has gone mechanically wrong.

Why magnetism makes a watch gain time

The rate of a mechanical watch is set by its balance and hairspring, a fine spiral spring a few hundredths of a millimeter thick whose coils breathe in and out several times a second. The frequency of that oscillation depends on the stiffness of the spring and, crucially, on its effective length.

Traditional hairsprings are made from iron-nickel alloys of the Elinvar family, sold under trade names such as Nivarox, chosen because their elasticity barely changes with temperature. The price of that chemistry is that they contain iron and can be magnetized.

Put such a spring in a magnetic field and its adjacent coils become weak magnets facing each other. They attract. Coils that should be separated by a hair's width cling together, so the part of the spring that is free to flex gets shorter. A shorter spring is a stiffer spring, a stiffer spring oscillates faster, and the watch gains. The stronger the magnetization, the more coils stick and the faster it runs. In bad cases the whole spring bunches up and the watch gains hours a day or stops making sense entirely.

Other steel parts can hold magnetism too, including the balance staff, the pallet fork and the escape wheel, and a magnetized escapement can add drag rather than speed. That is why a small minority of magnetized watches run slow or stop instead. But the hairspring effect dominates, and gaining time is by far the usual symptom.

What is strong enough to do it

Magnetic fields fall away very steeply with distance, roughly with the cube of it for a small magnet. This is the single most useful fact for an owner: a magnet that will wreck your rate on contact may be harmless a hand's width away. Almost every magnetization incident involves the watch touching, or nearly touching, the source.

Airport security is the hazard people worry about and the one that matters least. Walk-through detectors and X-ray baggage scanners are not designed to produce strong static fields, and watches pass through them constantly without incident.

What "antimagnetic" on a dial actually promises

There is a standard behind the word: ISO 764, the international specification for magnetic resistant watches. To claim it, a watch must keep running when exposed to a direct field of 4,800 A/m, which is roughly 60 gauss in air, and afterwards its rate must not have shifted by more than about 30 seconds a day compared with before the exposure.

Read that twice, because the number is modest. Sixty gauss is less than many fridge magnets produce at their surface. ISO 764 is a floor, a guarantee that a watch is not pathetically vulnerable, and it is nothing like immunity. A watch can be fully ISO 764 compliant and still be magnetized by a tablet cover.

Watches that genuinely resist magnetism quote much larger numbers, and there are two quite different ways of getting them.

The old answer: a soft-iron inner case

The traditional solution is to wrap the movement in a container of soft iron: an inner case, a dial plate and a solid inner caseback made of a highly permeable alloy. Field lines take the path of least resistance, so they run through the iron shell rather than through the movement inside it. This is usually described as a Faraday cage, though strictly a Faraday cage shields electric fields; a soft-iron shell works by shunting magnetic flux around the contents.

The classics of this school all date from the same decade: the IWC Ingenieur of 1955, the Rolex Milgauss of 1956, named for the thousand gauss it was built to withstand, and the Omega Railmaster of 1957. All three were aimed at people who worked near generators, laboratory magnets and electrical plant.

It works extremely well, and it costs you three things.

• Thickness and weight. A full inner cage adds several millimeters and a good deal of mass.
• The view. The shell must close completely, so an exhibition caseback is impossible. Every soft-iron antimagnetic watch has a solid back.
• Design freedom. Openings for date windows and pushers are holes in the shield, and each one has to be engineered around.

The modern answer: materials that cannot be magnetized

The alternative is to stop shielding the movement and start building it from things that magnetism ignores. If the hairspring and escapement parts are not ferromagnetic, there is nothing to magnetize and nothing to shield.

Three families of material do the work. Silicon hairsprings, etched from wafers rather than drawn from wire, are entirely non-magnetic, very light and made to tolerances metal cannot match. Niobium-zirconium alloys, such as the Parachrom hairspring Rolex introduced in the early 2000s, are paramagnetic rather than ferromagnetic. Titanium-based alloys, including the Nivachron spring the Swatch Group announced in 2018, take a similar route with more conventional manufacturing.

This is why a METAS Master Chronometer can be certified to keep running and keep its rate in a field of 15,000 gauss while still having a sapphire caseback and a normal-thickness case. There is no cage. There is simply nothing inside that cares. Our guide to chronometer standards sets out what that certification tests and in what order.

The trade-offs are real but different. Silicon cannot be bent back into shape the way a watchmaker manipulates a metal hairspring, so a damaged one is replaced rather than repaired, and that means depending on the manufacturer's parts supply for the life of the watch.

How to tell whether your watch is magnetized

Two home tests, neither of which requires buying anything expensive.

The compass test. Put a compass on a table, well away from steel, and let the needle settle. Bring the watch slowly toward it from about 15 cm (6 inches) and rotate the watch as you go. A magnetized watch pulls the needle around and keeps pulling as you turn it. A slight, brief wobble as a steel case passes by is normal; a firm, sustained deflection is not.

A magnetometer app. Most phones have a magnetometer for their compass function, and free apps display its reading, usually in microtesla. The Earth's field is roughly 25 to 65 microtesla depending on where you are, so note the ambient reading first, then move the watch to within a centimeter or two (half an inch or so) of the phone's sensor. A magnetized watch produces a reading several times the background. These apps are crude and the sensor's location varies by handset, so treat a big jump as evidence and a small one as inconclusive.

Demagnetizing, and what it will not fix

A demagnetizer is a coil that produces a strong alternating field. Switching it on and then slowly withdrawing the watch, or ramping the field down, cycles the steel through smaller and smaller magnetic loops until almost no net magnetization is left. It takes seconds, it does not open the case, and it does no harm to a healthy mechanical movement.

Any watchmaker has one on the bench and will usually treat a watch in moments. Bench demagnetizers of the simple mains-powered kind are also sold for roughly the price of a decent strap, which makes owning one reasonable if you have several watches. Whichever you use, withdraw the watch slowly and to a distance of 30 cm (about a foot) or so before switching off, because snatching it away in a live field can leave it as magnetized as it started.

Then test again properly. Wear the watch for a week, compare it each morning with an internet time source, and work out the mean daily rate, a method our guide to timing your own watch sets out step by step. Demagnetizing a watch tells you nothing until you have measured what it does afterwards.

And be clear about what it cannot do. Demagnetizing does not fix a watch that runs fast because it is badly regulated, because a shock has distorted the hairspring so that coils physically touch, or because the balance has been damaged. If the watch is still fast after treatment, or the rate is wildly inconsistent between positions, the next step is a service rather than another pass over the coil.