Recently, HODINKEE's Cole Pennington reviewed a couple of new versions of the Ulysse Nardin Freak, one of which is freaky even for a Freak: The Freak S, which has two inclined balances (that is, inclined with respect to the mainplate, to the extent that a Freak can even be said to have a conventional mainplate) connected by a differential. The two balances are mounted on a carrier that functions as the hour hand and which also carries elements of the going train, including two escape wheels. In the comments, a reader asked what the rationale might possibly be for putting two balances in one watch – and thereby hangs a tale.
The wristwatch with two balances may be thought of as a special case of the larger class of timepieces with two oscillators. Fairly early in the development of the pendulum clock, experimenters and horologists noticed that two pendulums could influence each other – for instance, if two pendulum clocks with pendulums of equal length are sitting on the same shelf, they will tend to begin to oscillate in time with each other (usually, one pendulum swinging to the right while the other is swinging to the left; this happens because that's the lowest energy state for the system as a whole). Christiaan Huygens, who is credited with inventing the first practical pendulum clock, observed the phenomenon as reported in correspondence from 1665 but he never constructed a clock around the principle. One of the earliest-known horologist to make a double pendulum clock was Antide Janvier, a French clockmaker, who made several, along with his near-contemporary Breguet.
Breguet also experimented with pocket watches fitted with two balances and took extensive notes on his experiments, writing at one point, of the resonance effect, "This appears to be absurd, but experiment proves it a thousand times over."
The idea behind clocks and watches whose oscillators resonate is that thanks to the exchange of energy between them, any rate errors in one oscillator will tend to be canceled out by the other.
Not all double balance watches are constructed to take advantage of the resonance effect, however. Some double (or quadruple, as the case may be) balance watches have balances linked by a differential. The idea here is that if the balances are positioned and adjusted properly that rate errors will tend to cancel each other out due to an averaging effect, rather than resonance per se.
Probably the best-known example of a modern watch with two balances linked by a differential is the Dufour Duality. The Duality was introduced in 1996 and it is, as far as I'm aware, the first wristwatch to be fitted with two balances. According to Philippe Dufour, the balances are adjusted so that one runs slightly fast and the other slightly slow, the idea being that the rate differences averaged together should yield a closer rate than either taken alone. Certainly, it's a very beautiful watch to see in action and very rare – and on the very off chance you see one come up at auction, expect there to be a seven-figure price tag; one hammered at Phillips in 2017 for $915,000.
You can, of course, have more than two balances – in 2013, which seems to be much longer ago than a paltry nine years, Roger Dubuis introduced the rather operatic (or maybe histrionic is a better word) Quatuor, which, in one of the most striking contrasts between a writer's brand image and that of a watch, was covered for HODINKEE by none other than Jason Heaton. The Quatuor has four balances, all linked by differentials, and the balances are inclined and distributed around the case 90º apart from each other. The inclination of the balances as well as their physical separation is, once again, intended to produce an averaging effect in which the most extreme rate variations tend to cancel each other out.
If you have two balances, inclining them at angles opposed to each other buys you additional insurance against rate errors, but what'll really give you an edge – at least theoretically – is if you put them in a tourbillon, as well. Greubel Forsey is a good example. The company has a number of inclined balance designs, including this year's Double Balancier Convexe, but they also produce watches like the Quadruple Tourbillon GMT, which has two balances gyrating inside two double tourbillon cages.
And then there's the watch that kicked off the question that inspired this story: The Ulysse Nardin Freak S. This is a double balance watch, but it's also a double balance tourbillon watch – like all Freaks, the oscillators are mounted on a carrier that orbits the dial once per hour, acting as the minute hand.
One other variation on the theme worth mentioning is the Audemars Piguet Royal Oak Double Balance Wheel. Here there are two balances, however, they're on the same balance staff and cannot oscillate independently. Each balance has its own balance spring, with inner and outer pinning points opposed to each other by 180º – the idea is that the two springs will oscillate perfectly symmetrically and once again, tend to cancel out rate variations.
Keeping The Beat: Resonance Watches
Resonance watches have two balances but operate on a different principle. The idea with resonance watches is to have two identical oscillator systems – that is, two identical balances and balance springs – but to adjust the balances so their rate is close enough to each other that a resonance effect causes them to oscillate with exactly the same period. Here the theoretical advantage isn't due to averaging the rates of two balances, but rather to the fact that two oscillators in resonance should have a more stable rate than one alone.
For this to happen, several conditions have to be met. First, the two oscillator systems both need to have the same natural frequency. Secondly, they need to be, somehow, mechanically coupled to each other. And finally, they need to be adjusted so their daily rate is fairly close. Breguet found that in his pocket watches the balances had to have less than a 20-second daily rate difference for the resonance effect to occur.
The best-known and earliest maker of resonance wristwatches was and is F. P. Journe, whose Chronomètre à Résonance launched in 2000, originally as a subscription piece. I was very skeptical of the watch at first but realized after reading The Art Of Breguet, which goes into quite a lot of granular technical detail about Breguet's resonance watches, that Journe had followed exactly the same principles.
It had occurred to Breguet that the mechanical coupling of the balances might be due to aerodynamic turbulence, but, thorough experimenter that he was, he checked for this by placing a thin steel blade between the balances, and also by testing his resonance watches in a vacuum chamber. Neither prevented the resonance effect from occurring and Breguet concluded that as minute as the forces involved were, the balances were being coupled to each other by the lateral force exerted on the movement plate by the balance springs.
Journe found that he had to adjust his balances to a closer rate than Breguet – a maximum difference of five seconds per day – thanks to the smaller size of his movements and the smaller amount of energy involved.
Now, there are other ways to mechanically couple two oscillators and one option is to mechanically couple the balance springs – in which case they essentially function as a single balance spring for two oscillators. The best-known modern watches using this system are made by Armin Strom, in its Mirrored Force Resonance Watches.
One of the very few other examples of this approach I'm aware of is from Beat Haldimann – the H2 Resonance Flying Tourbillon.
There are many other types of watches with two balances, but in general, these are some variation on high-frequency chronographs, in which there is one balance for the going train, and another, much higher frequency balance for the chronograph train (usually such chronographs have a fairly short running time for the chrono train, which generally has its own, separate mainspring barrel). Resonance and dual-balance differential watches are after a different game and represent efforts to test the potential of unconventional solutions for high precision timekeeping – and provide a lot of intellectual entertainment and flat-out entertainment, as well.
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