Inner cases made of soft iron, known as Faraday cages, first protected watches against magnetism in the 1930s. Nowadays, the alloys used to make balance springs ensure that watches are antimagnetic enough to meet internationally accepted standards. This means that the timepiece can withstand a magnetic field of 4,800 amperes per meter (A/m), which is equal to six milliteslas. That’s not a very strong field, especially when one considers that an ordinary household magnet typically generates fields ranging from 30 to 100 milliteslas. This relatively weak protection isn’t sufficient these days, when high-performance subwoofers are installed under the seats of automobiles and powerful magnetic fields are routinely generated by metal detectors that screen passengers at airports and by dozens of other common devices such as cellphones, televisions, computers, tablets and induction ovens. This last can generate magnetic fields as strong as 1.5 teslas, i.e., 15,000 gauss.
There are plenty of reasons to protect watches from the increasing presence of magnetic fields. Faraday cages are sufficient up to 1,000 gauss, i.e., 100 milliteslas, but beyond that level the iron becomes saturated and magnetism can penetrate the protective shield and the watch’s movement. Greater protection can be achieved by modifying the caliber: regulating components can be manufactured from materials that cannot be magnetized. For example, when IWC developed the Ingenieur 500,000 A/m more than 20 years ago, a completely antimagnetic alloy of niobium and zirconium was used for the escapement’s components, thus ensuring protection against magnetic fields stronger than 6,000 gauss. This is the level of magnetism that Omega says its Seamaster Aqua Terra >15,000 Gauss can withstand. Not only does silicon solve watch companies’ centuries-old problem of magnetism, it also offers new possibilities for watch construction. Now that designers can dispense with the opaque soft-iron inner cage, they can give the watch a sapphire caseback. They can also cut windows in the dial to show diverse functions such as the date. Several years ago, when Omega’s engineers began to develop an antimagnetic movement, these were two of the features they wanted the watch to have. They also specified that the watch should have the precision of a chronometer.
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