Fusée (power equalisation in clocks and watches)

A fusée is a conical part with spiral grooves in a spring-driven clock or watch movement. A cord or fine chain connects the cone to the mainspring barrel. It sits in the power transmission between the mainspring and going train and is intended to even out the spring’s declining driving force.

How does it work?

As a mainspring runs down, the torque it can deliver decreases. The cord or chain progressively acts at a larger radius on the cone. The changing leverage counteracts the difference in spring force. The mechanism therefore concerns power transmission rather than an additional hand or timing function.

The University of Queensland’s Physics Museum describes a particular clock movement with a chain between the barrel and fusée. Its tentative dating and suggested use for that movement cannot date other timepieces. The mechanism also appears in accounts of historical pocket watches and modern wristwatches.

Different from a remontoir

A remontoir d’égalité uses a smaller intermediate energy store that is replenished. A fusée instead changes the effective radius in the power transmission. Both can aim to equalise power delivery, but they describe different constructions.

Examining an old timepiece

Record whether the cord or chain and related parts are present, and whether a watchmaker has actually checked their condition. The construction alone establishes neither maker, age nor present rate accuracy. Practical example: retain the watchmaker’s written observations with photographs of the movement rather than assigning an undocumented production year.

Next step

Browse OBA’s watches and pocket watches. To offer a timepiece for sale, send photographs and known history for assessment of a possible purchase. State clearly whether its operation has been tested.