1A magnet comes close
A strong magnet, such as the one in a phone speaker, tablet cover, handbag clasp or headphone case, is brought near the watch. Its field is invisible but fills the space around it and passes straight through the stainless steel case and the glass, because those do not block a magnetic field.
2The steel parts pick up the field
Several parts inside contain iron: the hairspring alloy, the pallet fork, the escape wheel and screws. In a strong field, the tiny magnetic regions in the metal (domains) line up with it. Many stay lined up after the magnet is gone, so each part becomes a weak magnet itself. The orange glow marks the magnetised parts.
3The hairspring coils stick together
The hairspring’s coils are only about 0.25 mm apart. Once they are magnetised, neighbouring coils attract each other and touch. Touching coils can no longer flex freely, so a run of coils is stuck together as one rigid piece.
4A shorter spring beats faster
Only the free coils still flex, so the spring is effectively shorter and stiffer. A stiffer spring swings the balance faster (the beat rate rises roughly as one over the square root of the free length). The watch gains minutes or hours a day. The sticking coils also drag on each other, so the swing gets smaller. Moving the magnet away does not fix it, because the parts keep their magnetism.
5How it is caught and prevented
- Detecting it: a watch that suddenly gains minutes a day is the classic sign. A watchmaker’s timing machine (timegrapher) shows the beat rate high and the amplitude low, and a compass held near the watch twitches if the parts are magnetised.
- Fixing it: a demagnetiser slowly cycles a decaying alternating field through the watch; the domains scramble and the coils spring apart again. It takes about a minute.
- Preventing it: hairsprings in silicon, or in niobium-zirconium or other non-iron alloys, do not magnetise, and non-ferrous pallets and wheels help.
- Shielding: a soft-iron inner cage around the movement diverts the field; watches such as the Rolex Milgauss (1,000 gauss) use this, and some Omega Master Chronometer models with silicon and non-magnetic parts resist 15,000 gauss.
- Everyday habit: keep watches away from phones, speakers, magnetic clasps and laptop covers (a few centimetres is enough).