Antimagnetic Watch - ISO 764 Magnetic Resistant Watches Standard

ISO 764 Magnetic Resistant Watches Standard

The international standard ISO 764 Horology—Magnetic resistant watches defines the resistance of watches to magnetic fields. According to ISO 764 or its equivalent DIN 8309 (Deutsche Industrie Norm - German Industry Norm) a watch must resist exposition to a direct current magnetic field of 4 800 A/m. The watch must keep its accuracy to ± 30 seconds/day as measured before the test in order to be acknowledged as a magnetic resistant watch. Annex A of ISO 764 deals with watches designated as magnetic resistant with an additional indication of intensity of a magnetic field exceeding 4 800 A/m.

There are two ways of building an anti-magnetic watch:

• The first way consists in using different alloys, capable of withstanding magnetic fields. These alloys include Invar (iron - nickel - carbon - chromium alloy), Glucydur (beryllium - bronze alloy), Nivarox (iron - nickel - chromium - titanium - beryllium alloy) and Elinvar - an alloy similar to Invar, though less resistant to magnetism and more resistant to thermal influence. These alloys are preferred by different watchmakers due to their differing properties. Since the 1950s, Nivarox and Glucydur were extensively used by watchmakers. In the 1960s, almost all Swiss watches had Glucydur balance and Nivarox hairsprings. The anchors, escape wheels and other watch mechanisms were also made of non-magnetic metals or alloys.

• Another way of making a watch non-magnetic is to house the entire movement into a case made of a highly conductive (permeable) material. The movement is covered by an additional soft-iron clasp to prevent the forming of magnetic fields inside the watch itself. (See Permeability (electromagnetism).)