The Clocks Site

I thought we'd have a look at Fusee Clocks, or more accurately, the Fusee. The name comes directly from the French, and from late Latin, 'fusata,' meaning a spindle full of thread.

Fusees were not originally used in clocks. There's an example of a crossbow windlass in a manuscript dated to 1405 which distinctly shows a fusee to assist with the winding.

The fusee is a cone into which helical grooves have been cut. The whole point of it is to equalize the power of the mainspring. When this is fully wound, there's obviously more power in it than when it's only partly wound.

The fusee is coupled to a barrel in which is a powerful mainspring. A chain, or length of gut is attached from the fusee to the barrel, and it's the fusee which is wound, not the barrel. That's stationary and held in place by a very robust 'click,' or ratchet. The barrel moves around its arbor in order for the mainspring to be wound.

When the fusee is wound, it carries the chain or gut from the thickest part of its cone down to the smallest. An 'iron' then comes across to prevent the chain from flicking off the end. Then the unit begins to unwind. Remember now that the mainspring is fully wound, with all its power to dissipate.

The fusee itself may be looked upon as a continuously expanding pulley. The mainspring starts to unwind the fusee at the latter's smaller end. Effectively, large pulley driving small pulley, so lots of velocity ratio.

As the mainspring keeps unwinding, though, it encounters less and less tension on the fusee as this is continually increasing in diameter until we reach the end of the run, when the mainspring has nearly unwound and in effect a large pulley is driving another large pulley, so that there's a lot more mechanical advantage.

There shouldn't be any difference, therefore, in the power of the mainspring from start to finish. It should be uniform and indeed in a well made unit, it is.

No-one knows who invented the fusee. I've mentioned the drawing of a windlass, but the earliest existing fusee clock is the Burgunderuhr, or Burgundy Clock. It was made for Philip the Good, duke of Burgundy in about 1430.

Jacob Zech of Prague made the first definitely dated clock in 1525. In fact, he was credited at one time with inventing the unit, but of course he missed out by about 120 years!

The fusee isn't actually a cone, but a hyperboloid, the shape most recognizable today in the cooling towers of power stations. I'm sure our ancestors would be most interested to know what shape they'd actually developed!

A problem common to all clocks until 1726 was that when they were wound, the power was taken off the mainspring. They seldom had Intermediate Wheels which would have allowed the mainspring to continue its force, so of course over a period, with each winding, time was lost.

None other than our good friend John Harrison came up with the answer. He invented maintaining power, which is a wheel with ratchet teeth, set flat against the Great Wheel, but below the level of the teeth of that wheel.

The Maintaining Wheel is set firmly against the Great Wheel and under the control of a powerful leaf spring, set in a groove cut in the face of the Great Wheel. A lever set on the Great Wheel meshes with the ratchet teeth of the Maintaining Wheel so that when the clock is wound, it pushes hard against the Maintaining Wheel, keeping the Great Wheel going in the right direction.

England was the only country that continued to make fusee watches until about 1900. The Marine Chronometer always used fusees until the timepieces were phased out around 1970.

My personal belief is that the well and properly made fusee clock and watch are still the best timekeepers, although complicated and time consuming to make