|A Verge Fusee signed by Thos Gilbert of Hythe, 1833.|
It is a particular problem with escapements with high friction such as the Verge Escapement invented in the 13th Century and still being used in pocket watches well into the 19th century.
The problem was recognised and addressed by various means over the years, the first successful approach was the Fusee, described in detail my previous post, which provided variable gearing between the spring barrel and the train.Various other measures contributed to improvements but one of the most significant developments in the late 19th and 20th centuries was the improvements in metallurgy (including a move from steel to alloy construction), design and manufacture of mainsprings which by giving them variable thickness along the length of the spring and other features, allowed them to provide a relatively steady “pull” from shortly after fully wound for at least 24 hours.
Now for the conservator we have a problem. The Fusee compensates for variable power from the mainspring but 20th and 21st century mainsprings are designed for “Going barrel” watches to give reasonably constant power so when the mainspring is replaced the Fusee is over compensating.
The result is that Fusee watches will typically keep time with varying accuracy over time, most particularly those with Verge escapements.
The Verge Fusee:
This is the first proper timekeeping check (a few runs of 1 - 2 hours first got it to this state) on a jewelled Verge Fusee movement by Vale & Rotherham from 1828 with a new mainspring.
To mitigate the problem I will, when practical, shorten a much longer spring than would fit in the barrel, by removing the first third or so of the spring the rest will usually be more like an old spring but the best that can realistically be done is to set the regulation so that the watch keep acceptable time for 12 - 18 hours, in the example above setting it to start off running a little fast would see it keep time within about 90 seconds throughout a working day. So don’t expect too much from a Verge Fusee watch.
The Fusee Lever:
The same problem applies to Fusee lever movements but because of other improvements, the low friction lever escapement and some improvements in mainsprings by the mid to late 19th century the Fusee was far less aggressive and a modern mainspring will therefor have significantly less impact on the lever watch than on the Fusee.
 Isochronism : The ability of an escapement to run at an even rate irrespective of the power supplied to it.
 In a going barrel movement the barrel containing the mainspring meshes directly to the centre wheel (or sometimes in English movements, notably by the Lancashire Watch Co, through a dummy Fusee – just an extra gear in the train - to give anticlockwise winding) rather than via a Fusee.