The Fusee Watch.

A problem apparent when clocks were first built using springs rather than weight for power was the variable time keeping as the spring, which was of consistent thickness and temper throughout its working length, ran down. It was particularly severe with the high friction escapements then available.

The solution was the Fusee, a device that altered the gearing between the spring and the train as the spring wound down.
 

A Verge Fusee movement by Vale & Rotherham 1828

The mainspring is in a barrel and connected to the Fusee with a chain (originally a gut cord on clocks) which is wound onto a spiral grove, as the barrel rotates it pulls the chain off of the Fusee giving a continuously changing gear ration and hopefully maintaining a stead pressure on the centre wheel and train.

Top plate of an 1876 11 Jewel Fusee Lever showing the "Fusee Iron"

To prevent excessive pressure on the chain cause by over-winding Fusee stop work (not to be confused with Geneva or Maltese-Cross stop work) is used, this is a lever (The "Fusee Iron" hinged to the top plate which is lifted by the chain as it moves up the Fusee until the lever hits the cam shaped piece on the top of the Fusee (Fusee "Stop-Piece") which prevents further winding. It also prevents the chain overflowing the Fusee and allows some chain to be left on the barrel keeping the chain on a tangent too it reducing strain on the chain hook and keeping pressure consistent at full wind.

One other refinement, known to people Like Harrison when he designed his chronometer was mechanisms for Maintaining Power, a basic Fusee (or weight driven clock) has the disadvantage that when being wound the power delivered to the train is weakened or stopped – not good for timekeeping! – clocks and later Fusee watches have this addition but many Verge Fusee’s do not – perhaps it was assumed that the time would in any case be adjusted after winding so there was not point.

The inside of Fusse Lever movement by J.W. Benson, 1880 on an  ebauché by John
Wycherley of Prescot one of the English pioneers of the "manufacture of  inter-
changeable machine made movements" for which he took out a patent #880 of 1867.

The Fusee "Stop Piece" is clearly visible on the top of the Fusee.
The spring loaded paw at the bottom is part of the mechanism giving
"Maintaining Power" .

By the late 1670’s watches were being made with hairsprings that had a regulator (Tompion’s being favoured in England), a Fusee and a Verge escapement and from the mid 1700s with Cylinder escapements as an alternate. These (and others) all had a considerable degree of friction within the escapement (in addition to turning friction in the train) which required the Fusee.

The Verge Fusse Movement shown above in a later stage of assembly.
The graduated dial is part of the "Tampion" regulator, the centre
arbor drives a rack supporting the curb pins.

In the 1770s a few watches were being made with low friction lever escapements but it was not until the early 1800s with escapements invented and developed by the likes of Massey, Mudge and Savage that they started to become important. The “classic” English Table Rolled Lever escapement appeared c 1823 and then its close relation the Swiss Lever.

These escapements were much better than the Verge etc. in keeping time as the spring ran down but Isochronism [i] was still not achieved without the use of a Fusee and so the Fusee Lever watch was still made in large quantities in England.

During the 19^th century various improvements were made which gradually made the Fusee redundant and watches did away with them to be known as “going barrel” watches. Probably the most important change was improved mainsprings that by the 20 century were of alloy, of variable thickness, shape and temper throughout its length to the point that the power produced was essential constant for at least 24 hours.

Other improvements included, reduced friction in the train by improved materials, more accurate manufacture, Jewelling, sometimes on American watches including of the mainspring arbor and for a brief period in the late 19^th and early 20^th centuries Geneva stop work. 

The Fusee was not quite dead however, J.W. Benson were still offering a Fusse half-chronometer (The "Rated Watch" and a Fusee Chronometer in the mid to late 1930s.

Benson Catalogue Advert mid to late 1930s.

 

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[i] Isochronism: The ability of an escapement to run at an even rate irrespective of the power supplied to it.