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Introducing The Greubel Forsey Différentiel d’Égalité

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Greubel Forsey, makers of some of the most exotic high-end watches on the planet, has for many years had a sort of internal research and development department. The Experimental Watch Technology program's most recent introduction is an evolution of technology first presented in 2009: a form of constant force device that takes the remontoire d'égalité as a point of departure. 

The latest fruit of the Experimental Watch Technology Program is a full-fledged timepiece with a spherical differential constant force system.
Why This Watch Matters
The Différentiel d’Égalité is an example of a certain tradition of horological experimentation: research into a means by which energy which doesn't vary can be delivered to the escapement. The verge, which was the first known escapement, was very sensitive to power variations and was often paired with early constant force devices such as the fusee or the stackfreed. The more sophisticated but much more complex remontoire is another example and variations on this device are among the most interesting mechanisms in horology.  

Initial Thoughts
The mainspring in a watch is just like the spring that powers a windup toy; as the spring unwinds, it delivers less and less power. In a windup toy car, the car goes slower; in a watch, the balance turns through a smaller and smaller arc. Theoretically, the balance and spring are isochronous – that is, the size of the arc shouldn't make a difference – but since no actual watch achieves the perfect performance of a theoretical system, healthy and stable balance amplitude remains a goal of modern watchmaking.


The remontoire addresses this problem by putting a secondary, small mainspring on one of the wheels of the gear train. This secondary spring is much weaker than the mainspring, which rewinds it at regular intervals. The first remontoire in a watch was invented by the famous English chronometer maker John Harrison, who used it in his second experimental sea-clock, H2. The remontoire in his fourth sea-clock, H4 (which is generally acknowledged as the first successful marine chronometer, and which was tested at sea in 1761 with spectacular results) rewinds once every 7.5 seconds.

The remontoire is an ingenious, if delicate and demanding, mechanism but it does have one objection to it, which is that it still doesn't deliver perfectly even torque. This is where the Différentiel d’Égalité comes in. 

The remontoire spring is partially visible just above the running seconds sub-dial, which sits adjacent to the much larger sub-dial for the jumping/deadbeat seconds.
The Différentiel d’Égalité contains a mechanism which is, as Stephen Forsey put it when he described it to me, "of the remontoire class" however it does something a conventional remontoire (if there is such a thing) doesn't do: it provides perfectly even energy to the escapement and balance. To do this, Greubel Forsey developed a differential system that receives one energy input from the gear train, but which has two output paths. The remontoire spring rewinds once per second; one output path goes to a jumping seconds hand, whose movement reflects the one second rewind interval (the F. P. Journe Tourbillon Souverain is another example of a watch with a one second remontoire-dependent jumping, or deadbeat, seconds hand).

The other output path provides energy to the escapement and balance; on this output path is a smaller, conventional seconds display, which advances one increment for every advance of one escape wheel tooth (in other words it works like the seconds hand in a conventional watch). You can see the remontoir spring and differential partly exposed in the center of the dial.


The finish on the Différentiel d’Égalité, both front and back, is every bit as flawless as we've come to expect from Greubel Forsey (well, nothing that exists is flawless but if anything that exists comes within a hairsbreadth it just might be a Greubel Forsey watch, judged from a finish quality perspective). 


I think this is a deeply cool watch. The remontoire is something few companies trouble with, because in general it doesn't bring much to the table from a practical standpoint – modern lever escapement watches with modern alloy balance springs that are well constructed and regulated are light years more advanced in horological terms than watches in the days when the remontoire was invented. However, Greubel Forsey's horological experiments – the 24 second tourbillon, inclined tourbillons, inclined balances (such as the one found in the Différentiel d’Égalité) are all about exploring the most extreme possible refinements to mechanical horology. (Notably, these experiments take place without recourse to silicon.) There is something about the wringing of extremely incremental improvements in performance out of already very high precision systems that gets right in amongst me, and if you are of a similar turn of mind, the Différentiel d’Égalité is a watch you won't want to miss. Hope to get more specific info on how the differential does its thing at some point in the not-too-distant future.

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