France · c. 1795 (described in Breguet's own papers; predates modern patent numbering)
Breguet Overcoil (Balance Spring Terminal Curve)
Historical attributionOne of the most durable individual improvements in the history of the mechanical watch; still manufactured essentially unchanged in premium calibres today.
Breguet raised the terminal coil of the flat balance spring out of plane and curved it back over the spring, so the spring could breathe more concentrically as it expands and contracts.
The overcoil markedly reduces positional error compared with a flat spring, and the geometry (a Breguet overcoil, later refined mathematically by Moritz Grossmann and others) remains standard in high-grade mechanical watches.
How to verify this yourself
France did not have a modern patent-number system in 1795 in the sense used today; Breguet's own workshop records and correspondence, held partly by the Breguet company archive and the Musée des Arts et Métiers, are the primary sources. No patent number should be cited.
Manufacturers known to have used it
- Breguet
- most high-grade Swiss manufacturers using a classical balance (as opposed to modern flat/Phillips-terminal-curve or silicon springs)
Sources & references
- Daniels, G., The Art of Breguet
- Musée des Arts et Métiers, Breguet archival holdings (existence noted, not directly consulted here)
Where sources disagree, the disagreement is stated rather than resolved silently.