A Faraday cage is a soft iron shell inside a watch case that surrounds the movement and protects it from magnetism. IWC used one in its Ingenieur of 1955, and Rolex used the same kind of enclosure in the Milgauss.
How does a Faraday cage protect a watch?
The cage has three parts: the dial, a movement ring and the base. Together they enclose the movement. The metal is soft iron, which here means very pure iron with nothing alloyed into it. This iron is highly permeable, so a magnetic field travels through it easily. The field lines run through the metal of the shell instead of passing through the movement inside. Watches built this way usually have a screw-down crown. They also usually have no date display.
Which watches use a Faraday cage?
IWC first fitted the cage to its military pilots’ watches. It later used it in the civilian Ingenieur, launched in 1955. The first Ingenieur was protected against magnetic fields of up to 1,000 gauss. Rolex came after IWC with the Milgauss, which used the same kind of soft iron enclosure. Today Sinn, Damasko and Bremont are among the brands that use the cage. In its EZM series, Sinn has combined soft iron cages with titanium cases.
Is “Faraday cage” the right name?
The device is named after the scientist Michael Faraday, who first used it in 1836. In physics, a true Faraday cage is a mesh or solid box made of a material that conducts electricity very well. One writer argues that the watch term is technically wrong, because, in that writer’s view, a real Faraday cage does not block static or low-frequency magnetic fields. A magnetic shield works because the field lines travel through its highly permeable metal, not through what it encloses. When watchmakers speak of a soft iron inner shell, they mean this same part.
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