A fusee and chain works together to deliver constant force to the tourbillon. But beyond the lofty chronometric objectives there are more mundane but equally important operational concerns – how does one protect the chain whilst winding the watch, how does the calibre continue to run while winding the watch? Previous iterations of this complication have appeared in more classical interpretations like the Zenith Academy Georges Favre-Jacot, and while the new Zenith Defy El Primero Fusée Tourbillon takes the 15th century high complication and gives it a modernist, technical countenance, the underlying mechanical principles are still the same.

In physics terms, a fusee essentially helps maintain even or “constant” delivery of energy from the spring on the gear thanks to the changing radius of the spiral architecture. Hence, as the mainspring unwinds, the more of the chain wraps around the barrel, detaching from the wider conical at the base of the fusee. This greater turning movement compensates of the weakened mainspring thus delivering constant torque thus maintaining the amplitude of the escapement be it a standard regulating organ or something more outlandish like a tourbillon. These operating principles are fine but other practical concerns are necessary as well.

Zenith Defy Fusee Tourbillon assumes mantle from the Academy Georges Favre-Jacot

Bruno Devevey, Movement Constructor or in French, Designer, for Zenith explains it best – the barrel holds the mainspring, the chain pulls the fusee while the barrel unwinds. Fully wound, its like a biycle deraileur, it doesn’t need much effort so the fusee unfurls from the conical peak with the smallest diameter or “high gear”. As the barrel unwinds, it continues pulling on the fusee, following the conical slope, when the mainspring is almost completely unwound, the fusee is pulled from the larger base or “low gear” – much like when you’re tired and downshift gears using the larger ones to make for easier cycling uphill.

When one winds the crown, the fusee pulls the barrel instead, when the barrel is full, the chain pushes on a ratchet and protects the calibre from overwinding. Alternatively, an independent gear is in the fusee, also stopped by a ratchet, to either prevent discharge of energy from the mainspring while you’re winding the watch but there’s a hidden spring which provides 30 minutes of power reserve to the gear train to keep the Zenith Defy El Primero Fusée Tourbillon running while you’re “charging” the watch.

Fusee and chain tourbillons are high complications practised by very few manufactures – among them, A. Lange & Sohne, Ferdinand Berthoud and Breguet. But Zenith’s approach to the complication is a more “Lego” approach, rather than from scratch, the Zenith standard tourbillon is adapted to work with the fusee and chain mechanism in the manual-wind Zenith in-house caliber El Primero 4805 SK, incidentally, a similar calibre used in the Academy Georges Favre-Jacot introduced in 2015 during the brand’s 150th birthday.

Carbon Defy El Primero Fusée Tourbillon – Limited Edition of 50 Pieces

Featuring an openworked or skeletonised carbon dial, transferred hours markers. and black faceted ruthenium-plated hands coated with SuperLuminova, the 44mm Limited Edition Carbon Defy El Primero Fusée Tourbillon enjoys a bicolour signature aesthetic on its plates and bridges. Constant force blued fusée-chain transmission connected to the barrel stands out but not overtly so. It is matched with a carbon crown and buckle, and paired with a black rubber strap with blue stitching, it can also be swapped out for a dressier look – black rubber core coated with black alligator leather strap with “cordura effect” with titanium double folding clasp with black carbon head.

Platinum Defy El Primero Fusée Tourbillon – Limited Edition of 10 Pieces

Sharing the domed AR-coated sapphire crystal, sapphire display back, and 100 metre  water resistance with its carbon brother is the Limited Edition 44mm  Platinum Defy El Primero Fusée Tourbillon. The exclusive 10 piece production comes on a black rubber core coated with black alligator leather strap with “cordura effect”. Titanium and 18kt white gold double folding clasp.

Limited Edition Defy El Primero Fusée Tourbillon Price and Specs

Movement Manual winding calibre El Primero 4805 SK with 50 hours power reserve
Case 44mm carbon or platinum with 100 metres water resistance
Strap rubberised leather or rubber
Price Platinum at SGD159k. Carbon at SGD123,400