Feature: How Can A Watch Work With No Battery?
The battery in your phone will last you about a day before you need to charge it, same with your smart watch. That energy is depleted through use, as you would expect, and when it runs out, you need to add some more. With a mechanical watch, however, there’s not a volt or amp of electricity to be seen. How is that possible?
“Aha!” you’re thinking, “that’s easy. A mechanical watch uses a spring.” That’s correct, in the simplest terms; it does use a spring. But think about that a little harder and the idea no longer makes any sense. Think what happens when you compress a spring—it will only remain compressed the whole while there’s force being applied to it. As soon as that restriction is removed, the spring expands, usually in an explosive and uncontrolled manner. If you were to coil a spring and put a watch hand on the end of it, it would spin faster than the eye could see until the energy had been depleted in a matter of seconds.
“Aha!” you say again, “but a mechanical watch has gears. Just a like a bicycle, the speed of the unwinding mainspring can be geared down to the appropriate speed.” Again, that’s correct, but only when viewed in the isolation of theory. In reality that would require a gear ratio of some 17,000:1, and there’s simply just not enough room.
And that’s on the assumption that the mainspring winds down in a completely uniform manner, which it won’t. Not only will the torque decrease as time goes by, but any variations in friction that cause intermittent acceleration or deceleration will be exaggerated by 17,000 times. It’s quite simply not an option.
What we need to give some control to the unwinding spring is a mechanism that releases small, even amounts of energy from the mainspring at fixed intervals. Enter the escapement. Sounds pretty straightforward when you say it like that, but it’s a problem that’s plagued some of the best minds—including genius inventor Leonardo Da Vinci—for millennia.
Greek engineer Philo of Byzantium described the very first known example of an escapement, a counterweighted spoon suspended over a basin that was being fed by a continuous flow of water. When the spoon filled, its weight would exceed that of the counterweight, tipping it, emptying the water and returning it to its starting position once more. This process was used to release a piece of stone, which could be counted as a measure of time passed.
Mass was depended upon in one form or another to provide uniform time for a millennium and a half, be it the flow of water, the passing of sand through a restricted opening, or the descent of weights on a rope. These methods were dependable, but being reliant on the stationary effect of gravity, they were not portable. With the boom in maritime exploration in the 1400s, an escapement was needed that could be transported at ease without sacrificing accuracy.
Now, springs had been used by humans since the Bronze Age, from the simple elasticity of a string on a bow to the mechanical dexterity of a pair of tweezers, but combining the ability to store this elastic energy with a controlled method of releasing it was proving tricky. The answer? That would take half a millennium to figure out. Indeed, the mechanical escapement, whose origins were found in the early clocks of the 13th century, would be the solution to this problem of the ages. Coincidentally, the solution bears parallels with the modern capacitor, a device that stores electrical energy to discharge when full.
The answer to the problem can be laid out as such: if the energy from the unwinding mainspring could be taken from it, held in a capacitor-like device for a moment and then returned, it would be possible to draw out the power of the spring slowly enough to measure time with. If that sounds complex, that’s because it is, and it took almost five centuries to perfect.
Of some three hundred different types of mechanical escapement that have come and gone over the years, it’s the lever escapement that has dominated. British clockmaker Thomas Mudge takes credit for its invention in 1755, refining a multi-step process that is bafflingly complicated into just a few small parts—a few small parts that I am now going to attempt to explain to you.
It may surprise you to learn that the lever escapement does not sit between the mainspring and all the gears the transpose the time on the dial; it’s actually the last link in the chain. After the mainspring housing, or barrel, which is the first wheel in the chain, we have the second, or centre wheel. The centre wheel rotates once per hour and is directly connected to the minute hand, with a separate 12:1 gear feeding off to the hour hand. The centre wheel also gears down to the third wheel, which gears down to the fourth wheel, which turns once every minute and can be directly connected to the second hand, if there is one.
It’s only now that we reach the escapement, the first part of which, the escape wheel, is fed by the fourth wheel. You’ll notice that the escape wheel is not like the other gears, its teeth asymmetrical and barbed. They are carefully and mathematically calculated, because the job they do is one of utmost precision. Remember how the escapement is like a capacitor, temporarily storing energy from the mainspring and then returning it? It’s these teeth that do the job of transferring and receiving this energy in an exchange that measures its tolerances in microns.
As the escape wheel turns, the flattened tip of a single tooth pushes one of the angled jewels on the next component, the pallet fork. This does two things: firstly, the pallet fork pivots such that its opposing jewel drops behind a tooth on the escape wheel, locking it. Secondly, the pallet fork pushes the impulse jewel, a ruby tooth that stands proud on the balance wheel.
The balance wheel is the heart of the escapement, the capacitor we’ve been speaking about, and it stores the energy of the locked mainspring with—surprise—another spring. Once pushed by the pallet fork, the balance wheel spins, coiling the balance spring tight. From here, it’s everything in reverse. The balance wheel spins back again as the balance spring unfurls, the impulse jewel knocking the pallet fork, releasing the locked escape wheel. As the escape wheel tooth is freed, its flattened tip pushes the pallet fork, starting the process all over again.
The mechanical movement, and in particular the lever escapement, is an incredibly intricate and impressive mechanism, even more so for being over two-and-a-half centuries old and not requiring a single drop of electricity to run. Is it perfect? No. In 1676, scientist Robert Hooke discovered that the force a spring exerts is proportional to its extension, which means that as the mainspring winds down, its torque decreases and timing slows. This requires a different solution, one called “constant force”—but that’s another story for another day.
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