Can magnets damage my watch?

Yes, in the sense that magnets will stop a mechanical watch keeping time. No, in the sense that they usually break nothing. A magnetized watch is a watch running fast, sometimes wildly fast, with every component still intact and functioning. Once the magnetism is removed the rate returns to what it was.

That makes it one of the very few watch faults that costs nothing to put right, and the first thing to rule out when a watch that used to keep decent time suddenly does not.

What a magnet actually does inside the watch

The rate of a mechanical watch is set by the balance wheel swinging against a hairspring, a fine spiral 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 on its effective length.

Traditional hairsprings are made from iron-nickel alloys of the Elinvar family, sold under names such as Nivarox, chosen because their elasticity barely changes with temperature. The price of that chemistry is iron content, and iron can be magnetized. Put the spring in a magnetic field and adjacent coils become weak magnets facing each other. They attract, and coils that should be separated by a hair's width cling together. The part of the spring free to flex gets shorter, a shorter spring is a stiffer spring, a stiffer spring oscillates faster, and the watch gains.

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

What in a normal house is strong enough

Magnetic field strength falls away steeply with distance, roughly with the cube of it for a small magnet. That single fact explains nearly every real incident: a magnet that wrecks your rate on contact is harmless a hand's width away, so magnetization almost always involves the watch touching, or nearly touching, the source.

What "antimagnetic" on a dial promises

There is a standard behind the word: ISO 764. 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 exposure.

Read that number twice. 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 fully compliant watch can still be magnetized by a tablet cover.

Watches with real resistance claim much larger figures and get there two ways: a soft-iron inner case that shunts field lines around the movement, which is why such watches have solid casebacks, or non-ferrous components, meaning silicon, niobium-zirconium or titanium-alloy hairsprings that cannot be magnetized at all. The second approach is why a METAS Master Chronometer can be certified at 15,000 gauss while keeping a sapphire caseback. Our guide to magnetism and watches covers both in detail.

Testing and fixing it

1. Check the rate first. If the watch is suddenly gaining more than about 20 seconds a day and used to be fine, magnetism is the leading explanation. Our accuracy calculator turns two time checks into a daily figure.
2. Try a compass. Set one on a table well away from steel, let the needle settle, then bring the watch slowly toward it from about 15 cm (six inches), rotating the watch as you go. A firm, sustained deflection means magnetized. A brief wobble as a steel case passes is normal.
3. Or use a magnetometer app. Most phones have the sensor. Note the ambient reading first, then hold the watch within a couple of centimeters of the sensor. A reading several times the background is evidence; a small change is inconclusive.
4. Have it demagnetized. Any watchmaker has a demagnetizer on the bench and will usually do it in moments, often for free or for a nominal charge. A simple mains-powered bench unit costs roughly $20 to $40 if you own several watches.
5. Withdraw the watch slowly. Draw it 30 cm (a foot) or so away while the field is still on, then switch off. Snatching it out of a live field can leave it as magnetized as it started.
6. Measure again over a week. Compare against a network-synchronized time source each morning. Demagnetizing tells you nothing until you have measured what the watch does afterward.

What demagnetizing will not fix

It will not correct 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 is damaged. Those look similar from outside and need a bench.

So if the watch is still fast after treatment, or the rate swings wildly between positions, the next step is a service rather than another pass over the coil. And if the watch is running well again, leave it alone: there is nothing to gain from demagnetizing a healthy watch on a schedule.