Fusee Clocks
The story of the fusee—that beautifully spiral pulley found in many of the finest early spring-driven clocks and watches—is one of ingenuity, trial and error, and ultimately, obsolescence. For centuries, the fusee was at the heart of efforts to regulate mechanical timekeeping, compensating for the inescapable laws of physics acting upon coiled mainsprings.
But where did this curious invention come from, and why did it dominate clockmaking for over 300 years?
Ancient Origins: Earlier Than You Think
Despite what many books and articles claim, the fusee didn’t originate in the 1500s. Many credit Jacob Zech of Prague with its invention around 1525, and while he did indeed produce the earliest definitively dated fusee clock, the concept was already in play long before that.
In fact, we have compelling evidence that fusee-like mechanisms were used in the 1400s—not in timepieces, but in weapons. A military manuscript from 1405 includes illustrations of a crossbow windlass using a spiral pulley strikingly similar to a fusee, implying the core idea may have emerged from military engineering rather than horology.
Renaissance geniuses like Filippo Brunelleschi and Leonardo da Vinci also sketched fusee-like mechanisms, and the earliest known surviving fusee clock—the Burgunderuhr or “Burgundy Clock,” dating to about 1430—is thought to have been made for Philip the Good, Duke of Burgundy. That piece is still preserved today in the Germanisches Nationalmuseum.
So while Zech refined and popularised it, the fusee was likely born from a broader culture of mechanical problem-solving during a pivotal age of invention.
Why Fusees Were Needed: The Spring’s Big Flaw
To understand the purpose of a fusee, you need to grasp a core weakness of mainsprings—the tightly coiled springs that power most portable timepieces.
A mainspring doesn’t unwind evenly. It provides maximum torque when tightly wound, but this power tapers off as it unwinds. This inconsistency is disastrous for early mechanical clocks, which were already crude and temperamental. Unlike a suspended weight, which pulls with a consistent force, a spring gradually “tires out,” causing the hands to slow down and the clock to run late.
This phenomenon is known as lack of isochronism, and it plagued clockmakers from the moment spring-powered mechanisms were introduced in the 15th century.
Enter the Fusee: A Mechanical Equaliser
The fusee was designed to flatten the torque curve. How? By using a tapered spiral pulley—wider at the bottom, narrower at the top—connected via a gut cord (later, a chain) to the mainspring barrel.
When the spring is fully wound, its strongest pull is applied to the smallest radius of the fusee, giving it less mechanical advantage. As the spring unwinds and weakens, the chain shifts toward the fusee’s wider base, increasing the mechanical advantage and compensating for the loss in spring tension.
This elegant system made the torque delivered to the clock’s gears almost constant—a huge improvement in timekeeping accuracy.
Interestingly, early fusees weren’t perfect cones. Over time, clockmakers discovered that a true cone wasn’t ideal. The correct profile was more complex—a hyperboloid, carefully calculated or simply arrived at through generations of empirical tweaking.
Stackfreed vs. Fusee: A Battle of Early Solutions
The fusee wasn’t the only attempted fix. Another short-lived solution was the stackfreed—a cam-based compensator that pressed a spring against the mainspring barrel to counteract its uneven force. While ingenious in theory, the stackfreed added friction, drained energy, and was quickly abandoned. The fusee, by contrast, remained the industry standard well into the 19th century.
From Gut to Chain: Improving the Mechanism
The first fusee cords were made from animal gut—flexible but prone to wear and snapping. By the mid-1600s, metal chains became standard. Gruet of Geneva is credited with introducing them in 1664, although some references to chain-driven fusees date back as far as 1540.
You can usually tell which material a fusee was designed for by inspecting the grooves: rounded for gut cords, square-edged for chains.
Keeping Time While Winding: Harrison’s Breakthrough
One of the limitations of early fusee clocks was that winding them temporarily stopped the movement—an issue for critical applications like navigation. In around 1726, John Harrison added a maintaining power mechanism to his marine chronometers. This clever addition used a small spring or gear train to keep the clock running while it was being wound, ensuring uninterrupted operation. This improvement became known as the “going fusee.”
Inside the Fusee: How It All Works
A basic fusee setup consists of:
•A mainspring inside a barrel
•A fusee cone with a chain linking it to the barrel
•A winding arbor to reset the spring
•A gear on the fusee that drives the movement
When fully wound, the chain wraps around the top of the fusee, delivering power to the gear via the small radius. As the spring relaxes, the chain travels downward, keeping torque consistent.
To rewind, a key is turned on the fusee, which pulls the chain back from the barrel while re-tensioning the mainspring.
To prevent over-winding, fusees often included a stop mechanism: as the chain approached the top, it triggered a lever that locked the mechanism, protecting both chain and movement from damage.
Some experimental models even allowed bidirectional winding—called “drunken fusees”—but they were rare and unreliable. John Arnold tried them in marine chronometers but ultimately gave up on the idea.
The Decline: From Masterpiece to Museum Piece
For all its cleverness, the fusee eventually fell out of favour. Here’s why:
•Size: It made pocket watches bulky—an aesthetic problem in the age of slimmer designs.
•Fragility: Broken chains caused damage, and replacing mainsprings required recalibrating the entire fusee.
•Cost: Precision shaping and fitting made fusees expensive to produce and repair.
As clock mechanisms improved—especially with the invention of the pendulum and balance spring—timekeeping became more accurate even without constant torque. New escapements, like the cylinder and lever, were less sensitive to changes in drive force, making fusees unnecessary.
By the late 18th century, fusees had mostly disappeared from new watches and clocks. But they live on in antique collections and restoration projects as a symbol of horological excellence.
Conclusion: The Fusee’s Legacy
The fusee is a testament to the clockmaker’s eternal battle against imprecision. While no longer essential, it marked an important chapter in the pursuit of reliable timekeeping—a mechanical answer to a natural problem.
Whether you’re an antique collector, a horology enthusiast, or just someone fascinated by the beauty of clockwork, the fusee is worth your appreciation. It’s not just a component—it’s a story, a solution, and a shining example of human ingenuity etched in brass and chain.