Why it happens
A mechanical watch keeps time with a balance wheel and a hairspring finer than a hair. Close to a strong magnet, the steel hairspring becomes magnetized and its coils stick together, which shortens it and makes the watch run fast, sometimes by several minutes a day. Phones, laptop and tablet magnets, speakers, magnetic bag clasps and MagSafe chargers are the usual sources. Quartz watches are affected only while they're in the field and recover when moved away.
How to check
Put a compass, or a compass app on your phone, flat on a table and pass the watch slowly over it. If the needle swings toward the watch, it's magnetized. A watch that gains suddenly but doesn't move the needle may need regulating or a service instead.
How to demagnetize it
- Get a demagnetizer: a small blue or black box sold for watch use, usually under $30. Any watch repair shop will also do it, often free.
- Take the watch off and place it flat on the demagnetizer.
- Press and hold the button, then slowly lift the watch straight up and away, at least 30 cm, before letting go of the button.
- Check again with the compass and repeat if needed, trying the watch on its side as well.
Your watch should go back to its normal rate straight away. If not, have a watchmaker look at it.
Anti-magnetic watches
ISO 764 calls a watch anti-magnetic if it resists 4,800 A/m, about 60 gauss. Some go much further: the Rolex Milgauss was built for 1,000 gauss, and Omega's Master Chronometers resist 15,000 gauss thanks to silicon and non-ferrous parts. Many modern movements now use silicon hairsprings, which magnetism doesn't affect.