Antimagnetic describes a watch built to keep running with only small timing errors when it is exposed to a magnetic field. Such watches are common among electronic engineers and in other jobs that involve strong magnetic fields. Divers’ watches that meet ISO 6425 must be antimagnetic too, as well as water resistant.
What does ISO 764 require?
ISO 764 is the standard that sets out what antimagnetic means. Under it, a watch must withstand a direct-current magnetic field of 4800 A/m. Its rate is measured before the test and again afterwards. To pass, the watch must still run within plus or minus 30 seconds a day of the rate measured before the test. So the standard does not ask for a watch that magnetism cannot touch at all. It sets a limit on how far the timekeeping may drift once the field has been applied.
How is a watch made antimagnetic?
One way is to make the moving parts from alloys that magnetic fields do not affect. Another way is to shield the movement with an extra cover made of soft iron. This cover stops magnetic fields from building up inside the watch, so the movement sits protected within it. In 2013 Omega brought out a mechanical movement made with non-magnetic materials. That movement resists magnetic fields above 15,000 gauss.
Where it comes from
Vacheron Constantin assembled the first antimagnetic pocket watch in 1915. Tissot made the first non-magnetic wristwatch in 1930. In 1954 Rolex first made its Milgauss series of antimagnetic certified chronometers, starting with model 6541. Within four decades, protection against magnetism had moved from the pocket watch to the wrist, and then into a dedicated series of chronometers.
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