Anti-magnetic (A/m) unaffected by magnetism. If the parts most affected by a magnetic field (balance, balance spring, and escapement) are made of non-magnetic materials or the materials are blocked from magnetism by a soft metal cage, the watch is called anti-magnetic. Typically this type of watch has a soft iron cage around the movement to prevent magnetic fields from affecting the timekeeping, however, Omega has a movement where all the critical parts are anti-magnetic so no soft-iron cage is needed.