What is a Fusee?

What Powers a Mechanical Clock?

Let’s take a quick look at some of the terms mentioned in recent posts by starting with the basics: how mechanical clocks are powered.

The clocks featured here were built long before the general use of electricity.

Steam engines would normally be far too large and inefficient to power small household timepieces — not to mention the inconvenience of continually stoking a fire just to keep a clock running overnight. So what powers these clocks?

We’ve discussed examples of the very earliest clocks that operated on water power. But the tried-and-true method for running an accurate timepiece is to harness the power of gravity.

This is the original green power. Gravity is constant, renewable, and reliable. As long as we have a planet, we have gravity.  

Weight-driven clocks also offer remarkable consistency. In any given location, the force of gravity remains constant, so the clock’s power source does not weaken or fluctuate during its run.

Of course, weights can be cumbersome. What if you want a smaller clock to sit neatly on a shelf? There simply isn’t room for long descending weights. Early American clockmakers addressed this by building tall shelf clocks with weights concealed inside the case sides.

By the 1400s, an even more compact solution emerged: the coiled metal spring. Springs eliminated the need for hanging weights, allowing clocks to become smaller and more portable. However, springs introduce a new problem — their power diminishes as they unwind. This loss of torque would cause a clock to slow slightly over time.

Enter the fusee.

The fusee is a cone-shaped pulley connected by chain (or cord) to the spring barrel. When the spring is fully wound and strongest, the chain pulls from the narrow top of the cone. As the spring weakens, the chain shifts to the wider lower portion of the cone. The changing leverage compensates for the decreasing force of the spring, maintaining a more consistent output of power.

Cutting and shaping a fusee with precision requires extraordinary craftsmanship. For this reason, fusees were generally reserved for high-quality clocks and watches made by skilled artisans.

Over time, however, advances in escapement design and spring technology reduced the need for fusees. These mechanisms were delicate and complex, and eventually they became largely obsolete. Today, fusees remain a fascinating and admired feature among horologists — a testament to mechanical ingenuity in the pursuit of accuracy.