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The Encyclopedia of Horology

Component record · Power

Fusee

Verified

A tapered, spirally grooved cone that equalises the torque delivered by a mainspring as it unwinds.

What it does

Compensates for the mainspring's uneven torque by pulling on a progressively larger radius as the spring weakens, producing more even power to the going train.

How it works

When fully wound, the mainspring's strongest pull acts on the fusee's narrowest radius (nearest the base of the cone), giving low mechanical advantage. As the spring runs down and weakens, the chain has paid out to act on progressively larger radii near the top of the cone, giving proportionally greater mechanical advantage — the two effects roughly cancel, keeping torque delivered to the great wheel relatively constant.

A ratchet and click stop-work (maintaining or limiting the number of turns) is commonly fitted to a fusee to prevent overwinding, since running the chain off either end of the cone can damage the mechanism.

The fusee was the dominant solution to spring non-linearity before detached escapements reduced the need for constant torque, and it required a taper profile empirically cut to match a given spring's torque curve, making fusee movements comparatively difficult and costly to produce.

Evolution

Standard in verge and early lever watches; retained longest in English watchmaking and in marine chronometers (where isochronism was paramount) well after Continental makers had shifted to going barrels.

Known variations

  • Chain fusee (steel chain, most common in fine watches)
  • Gut-line fusee (used in some clocks and earlier or cheaper watches)
  • Fusee with stop-work versus without

Identification for collectors

  • Visible tapered cone and chain running between barrel and fusee when the movement is examined from the dial or back
  • Common in verge watches and English fusee lever watches; strongly associated with pre-1900 English production

Common failure modes

  • Broken or stretched chain
  • Worn fusee groove causing chain to jump
  • Stop-work failure allowing overwinding and chain damage

Photographs

Lindquist Fusee Pocket Watch Movement
Lindquist Fusee Pocket Watch Movement — Gervald Frykman User:G N Frykman, Public domain. Wikimedia Commons file page
Spindeluhr fusee verge watch fabris 02
Spindeluhr fusee verge watch fabris 02 — Sergio Fabris, CC BY-SA 4.0. Wikimedia Commons file page

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

Sources & references

  1. Britten, F. J., Old Clocks and Watches and Their Makers
  2. Daniels, G., Watchmaking

Where sources disagree, the disagreement is stated rather than resolved silently.