Protected against the effects of magnetic fields, which can affect a movement’s precision by influencing the balance’s rate of oscillation.
One way to protect against magnetism is to make the balance and balance spring of metals that either cannot be magnetized or are resistant to becoming magnetized (e.g., Elinvar or Nivarox hairsprings used in conjunction with brass, nickel or beryllium bronze balances). Other delicate components, specifically the lever, escape wheel and impulse roller, can also be made of nonmagnetic metals.
Another way to protect against magnetism is to enclose the entire movement in a case made from a highly conductive alloy, which prevents magnetic fields from building up inside it. A watch can be described as antimagnetic if, within a magnetic field of 4,800 A/m (amperes per meter), it continues to run with a deviation of no more than 30 seconds per day. One symptom that suggests that a watch has been magnetized is a tendency to run slow.
A magnetized watch can be “cured” (i.e., demagnetized) by a watch repairer using a choking coil. APERTURE A small, often rectangular opening in the dial. Various indications such as date, month, day or hours are displayed through these openings.