Anti-magnetic refers to a watch's ability to resist the effects of magnetic fields, ensuring accurate timekeeping by protecting its movement from magnetic interference.
Anti-magnetic watches resist the effects of magnetic fields on their movements. Traditional approaches use a soft-iron inner cage (Faraday cage) surrounding the movement to deflect magnetic fields. Modern approaches use non-ferrous materials like silicon, Glucydur, or Nivarox alloys for key components (hairspring, escapement parts) that are inherently non-magnetic and immune to magnetization.
A magnetized watch typically runs significantly fast—sometimes 5-30+ minutes per day—because the hairspring coils attract each other, effectively shortening the active spring length and increasing frequency. You can test with a compass: hold it near the watch and see if the needle deflects. A watchmaker can demagnetize the movement quickly and inexpensively using a demagnetizer.
ISO 764 defines the minimum anti-magnetic standard: a watch must resist a direct magnetic field of 4,800 A/m (60 gauss) without losing more than ±30 seconds per day accuracy. Modern electronics and bag closures commonly exceed this. IWC's Ingenieur and Rolex Milgauss (rated to 1,000 gauss) significantly exceed this standard, while Omega's Master Co-Axial movements resist up to 15,000 gauss through silicon components.