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The Encyclopedia of Horology

France · c. 1795 (described in Breguet's own papers; predates modern patent numbering)

Breguet Overcoil (Balance Spring Terminal Curve)

Historical attribution

One 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.

Background

A balance spring is designed to be isochronous so that the period of oscillation remains independent of the amplitude of oscillation. This property is essential because no mechanical drive train, such as a mainspring that provides a diminishing force as it unwinds, can provide a constant force. The Breguet overcoil serves as a common method for achieving this necessary isochronism in timepieces.

The invention functions by placing a portion of the outermost turn of the hairspring into a different plane than the remainder of the spring. This geometric adjustment allows the coil of the hairspring to expand and contract with more symmetry and evenness as the balance wheel rotates.

Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.

How it works

  • The gradual overcoil is constructed by applying two gradual twists to the hairspring to reach a second plane over half the circumference.
  • The Z-Bend type of overcoil uses two complementary 45 degree kinks to rise to a second plane within three spring section heights.
  • Modern watches may utilize a less effective dogleg consisting of sharp in-plane bends to provide clearance for the outer coil.

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)

Photographs

Insight: The Overcoil Hairspring, From Breguet to Phillips | SJX Watches
Insight: The Overcoil Hairspring, From Breguet to Phillips | SJX Watches — watchesbysjx.com, Rights unverified — web image search result, review pending. Web page (watchesbysjx.com)

Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.

Sources & references

  1. Balance spring, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
  2. Daniels, G., The Art of Breguet
  3. 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.