The dark art of resonance in watches, how it works, and why it’s so rare

Borna Bošnjak

Complications can go three separate ways. There’s those like a date window or power reserve, showing genuinely useful, everyday information. Then there are all the show-offy ones – complicated chronographs, calendars, and chiming watches – all technically offering some new information, but with limited daily use. And finally, we have complications like a tourbillon, where their actual effectiveness is questionable, and yet watchmakers flock to them due to their inherent complexities. Resonance watches are a bit like tourbillons, in that they’re only relevant to the absolute pursuit of ultimate chronometric performance, but actually even more complicated to get right – you could even argue it’s not a complication. Watches (wrist or pocket) that take advantage of it are so far and few between that they rarely get their time in the limelight, but when they do, the results are pretty spectacular. But what is resonance, and why is its use so limited in watchmaking? By the time you’re finished reading this, you’ll hopefully have some answers.

What is resonance?

Taking watches out of the equation for a quick second, what does the word “resonance” actually describe? This is one of the rare times that I actually get to draw on my engineering background, as the term stems from physics. All oscillating objects vibrate at some natural frequency when not acted upon by an external force. When that external force is applied, and if it matches the system’s natural frequency (or one of its resonant frequencies), resonance will occur, causing the system to oscillate at a higher amplitude.

To explain this more intuitively – think about jumping on a trampoline. If you jump in any random pattern, you’re not likely to bounce that high. However, when you hit the trampoline just right, you’re suddenly flying. That’s because you matched the resonant frequency of the trampoline! Perhaps more relevant to watches – a tuning fork experiment also depicts resonance. If you strike one fork and place it next to another on a table, the non-vibrating fork will start to vibrate at the same frequency. This is actually the phenomenon that gave rise to the term “resonance”, as it was observed in musical instruments.

Why is it used in watches, and why is it so rare?

Since the inception of timekeeping devices, their inventors have been looking for ways to make them more accurate. To answer the question in the subheading succinctly – resonance improves accuracy. How and why that happens will take a few more words than that. Galileo was the first to observe that two pendulums of the same length swinging from a common mounting point will oscillate in phase or 180 degrees out of phase, meaning they either describe their arcs at the same time, or meet in the middle and reach either end at the same time. The earliest mentions of resonance in watchmaking comes from its use in pendulum clocks.

Christiaan Huygens would apply this methodology to clocks in the 1600s, realising that two pendulum clocks (one of his inventions) affixed to the same wall would synchronise their beats, even after a disturbance. Antide Janvier is likely to be the first to put two pendulums into a single clock, beating in opposition to each other. As a result, if one of the pendulum’s experienced an error during any particular swing, the other would cancel out this error.

Timekeeping devices using resonance are rare. Janvier is said to have created three pendulum clocks, with Breguet being the first to adapt and miniaturise the idea so it could work with two balance wheels, and in a pocket watch. Only three Breguet resonance pocket watches are known to exist. No. 2788 (pictured above), No. 2794, and No. 2667, the latter selling at Christie’s in 2012 for more than CHF 4 million.

This miniaturisation into pocket watches naturally came with more significant challenges. It still incorporates two separate movements, but the manufacturing tolerances need to be incredibly tight. The main challenge is ensuring that the two oscillators are close enough in their accuracy for resonance to actually occur. In his notes, Breguet writes that the two balance wheels must be no further apart than 20 seconds per day. Furthermore, the movement’s barrels need to provide a stable amount of torque throughout their power reserve to maintain the same amplitude of the oscillators. For wristwatches, it gets more difficult still, given the smaller balance wheels needing even higher accuracies.

How is the phenomenon implemented?

While the number of watchmakers that have attempted resonance movements in pocket watches and wristwatches can be counted on one hand, the number of solutions to all the technical challenges is impressive. Focusing on modern resonance wristwatches, there is no single way of making one. It can be done with two separate movements housed in a single watch, or a single going train with two escapements. There way the two balances regulate each other is also a differentiating factor, and there are two main schools of thought.

Let’s start with F.P. Journe, who revived the idea of resonance watches, inspired by the works of Breguet and Janvier centuries ago. His Chronomètre à Résonance, first showcased in 1999, is the first and only wristwatch using the Breguet resonance method, where the balance wheels are not directly connected and only affect each other by resonance through sharing a common mainplate. He would iterate on this design in 2020, going to a single-barrel set-up with a differential going to the two going trains and two separate remontoirs. As the balances are not connected, Journe notes that they have to be regulated within 5 seconds of each other’s daily rate.

All other watchmakers use some form of connection between the two balance springs. Armin Strom developed its own “clutch spring”, which is connected to the stud of each individual balance spring. With each beat of one balance wheel, an impulse is sent down the clutch spring, affecting the other balance, and correcting its rate. This means that the deviation between the two escapements can be much greater, allowing for easier regulation.

Beat Haldimann has a history of creating resonance clocks and wristwatches, with his two most impressive displays of the phenomenon being the H101 Resonance Classic pendulum clock and the H2 Flying Resonance watch. The H101 is actually two large and completely independent clocks with pendulums running exactly 180 degrees out of phase, with the only common point of connection is the pendulums affixed to the same bracket.

The H2, however, combines resonance with a central tourbillon. It also uses only a single gear train, leading two the two centrally mounted escapements, one equipped with a remontoir spring. Being mounted on a constantly rotating platform, however, the two balance springs needed to be connected somehow to ensure resonance behaviour. Indeed, they’re connected to each other by a coupling spring, with a hairspring connected to each end.

If Haldimann’s approach was not complex enough, how about upgrading that central tourbillon to a triple-axis one? That’s exactly what Vianney Halter did, while also changing the approach to his resonating balance wheels. As there is no baseplate to carry the vibrations of the balances through, like the Haldimann H2, a different approach was necessary. Here, they’re mounted so they beat opposite to each other, and co-axially, with the two hairsprings connected to one stud holder.

As you can see from the examples above, there is no “right” way to make a resonance watch. Given how far and few between any new discoveries in this space are, I guess it’ll be a long time before there are any major breakthroughs in resonance movement technology – especially if you consider how impractical and expensive it is to produce. And yet, I and many other nerdy enthusiasts alike await the next big thing in resonance with bated breath.