Magnetism is the bane of any mechanical watch. A magnetic field wreaks havoc on a movement’s accuracy by affecting the swing of the balance wheel, and continues to do so even after it’s gone should the movement become magnetised. From obvious sources like MRIS, to insidious ones like a handbag’s magnetic clasp, this invisible force permeates our daily life. Naturally, the perfect sports chronograph must guard against it.
There are two ways to render magnetism moot. The first is to shield the movement using a soft iron inner case, like what IWC does with some of its pilot’s watches. Such an inner case protects the movement by redirecting the magnetic field through itself, while remaining non-magnetised due to its soft iron construction. The advantage of this method is its simplicity and low cost – crafting an inner case with this common material is easy. In a sufficiently strong magnetic field, however, the soft iron inner case will be magnetically saturated, and any “residual” magnetic field will still pass through it to affect the movement. In addition, this principle requires a specific design – a sealed inner case that encases the movement – to work well. The dial and case back must thus have no cut outs lest the magnetic field affects the movement through these holes.
The alternative to shielding a movement is making its regulating organs amagnetic. The hairspring, pallet fork, and escape wheel can all be made in silicon, which is nonmagnetic, thanks to improved production techniques like DRIE (Deep Reactive Ion Etching). As a silicon hairspring is already cut specifically to promote concentric breathing, the balance assembly is free sprung and not regulated. This necessitates a variable inertia balance wheel with weighted screws on its rim for regulation, so the balance wheel is not rendered in silicon.
|| THE VERDICT ||
In most environments, a soft iron inner cage is more than sufficient protection for a watch movement; the design’s longevity attests to its effectiveness. Why stop there, though? Silicon parts aren’t just impervious to magnetism, but also require little to no lubrication while weighing less than their traditional counterparts. The no holds barred option will have to be silicon.
The IWC Pilot’s Watch Chronograph has a soft iron inner cage
Left: Rolex’s Syloxi hairspring Right: Silicon pallet and escape wheel visible through the dial cut-out