Technical: The Fusee and Chain
Chances are if something isn't working quite right in today's world, all it needs is a bit of software tinkering. The speed of technological development has rocketed since the birth of the personal computer in the 1980s, making the software industry worth over $300 billion. Rewind back over half a millennium, however, and problem solving required a much more hands on approach.
The sticking point in question surfaced in the 15th century with the introduction of the spring driven clock. Traditionally, clocks used hanging weights as a source of constant power, but despite the spring being a lot more compact, it presented a whole new problem of its own; lack of isochronism. Isochronism describes the regularity of repeating events, such as a waveform, a pendulum or in this case, a main spring.
The problem is straightforward, even if the solution is not; as a fully coiled mainspring unwinds, the torque it produces decreases. This effect can be seen clearly with a clockwork toy car. Fully wound, it scoots away, but instead of maintaining the same speed until the spring has completely unwound, it gradually slows, the spring's torque decreasing exponentially.
A fix had already been penned by two great Italian engineers, Filippo Brunelleschi and Leonardo da Vinci, albeit in a basic form. Sourcing its name from the Latin 'fusata,' meaning 'spindle full of thread,' their fusee design was one of two solutions presented—the other being the stackfreed, a mechanism of levers and cams—and the two technologies did battle in much the same way Betamax and VHS did. The only difference was that the battle lasted just over a century (the duration between the first powered flight and the iPhone), by which point there was a clear winner—the fusee.
Everything about the fusee is complicated, from the geometry, to the manufacture, to the ancillaries. The idea goes something like this; rather than letting the mainspring drive the mechanism directly, a vaguely conical tube with a spiralled groove (the fusee) is placed in between. A chain, wrapped around the fusee, joins it to the mainspring. As the mainspring unwinds, it pulls the chain from the fusee, turning it as it unwraps.
The clever part comes from the shape of fusee itself. It isn't a simple conical shape, having a profile more like the bell of a trumpet, which is calculated specifically for the torque characteristics of the mainspring. Think of it like the gears of a bicycle; the smaller the driven cog, the harder it is to turn. This is because the leverage on the axle is less, and so more torque is required. The fusee is effectively a continuously variable gear, its diameter increasing as the spiralled chain unwinds, requiring less torque to turn it and therefore compensating for the torque loss in the mainspring.
The fusee presented its own problems however, including bulk and complexity, particularly in the manufacture of the chain. The introduction of both the pendulum and balance spring in the 17th century—which added their own isochronous 'beat' to the movement—made the fusee and chain obsolete in all but the most accurate marine chronometers. Today it has its place as one of the most impressive watchmaking feats possible, a demonstration of the skill of the handful of watchmakers that can still make it—even if they do use a bit of software now and then to help them.
