Many years ago, a much respected independent watchmaker named Francois Paul Journe ­came out with a most intriguing watch. That watch was the Resonance Chronometer – to give it its proper name, the “Chronomètre à Résonance.” It debuted to much fanfare in the year 2000, and was hailed as the hottest thing since Huygens started playing with spiral springs. The Chronomètre à Résonance is a watch that has two balances, and the inspiration behind the watch is a phenomenon called resonance.

Resonance, in Brief

In physics, resonance means the tendency for some physical system to oscillate with greater amplitude at certain frequencies than at others. A related phenomenon is that if you have two oscillators with the same resonance frequency, and they’re mechanically coupled in some suitable fashion, they will begin to oscillate in phase.

For more background on resonance, please refer to our Watch Word article.

Stirring Controversy: Does it Work?

Over the years since the Chronomètre à Résonance watch appeared, there have been several objections raised by sceptics and I’ve been myself guilty of being fairly vocal in airing my own doubts.

The two objections are that firstly, the rotational movement of a balance seems insufficient to induce a resonance effect and, as a corollary, the energy in a balance is too small to propagate through the plate to the other balance; it seemed unlikely that two balances could be mechanically coupled through a watch plate in the same way that two pendulums can be coupled through their suspension.

The second objection was that, given the construction of the Chronomètre à Résonance, in which the balances are set very close together, the two balances if they influenced each other at all did so by aerodynamic effects rather than mechanical resonance through the plate. Such an aerodynamic coupling would be affected, one presumes, by changes in barometric pressure and temperature and not produce a satisfactory mutual compensation of the two balances for any error in either.

This second objection led to the observation that a sure way to test the Chronomètre à Résonance would be to put it in a vacuum chamber and see if a resonance effect could be observed, and over the years, on various internet discussion forums someone would occasionally say they remembered hearing from someone who had heard from someone that someone had tried to put a Chronomètre à Résonance in a vacuum chamber and it hadn’t worked. Or it had. But nobody, it seemed, could ever remember exactly who it was who’d tried it.

Resolution: A Voice from the Past

A few weeks ago, in pursuit of information on something else, I happened to open a book I’ve looked in on countless occasions: The Art Of Breguet, by Dr. George Daniels, published by Sotheby’s. As luck would have it, this time the book happened to fall open onto pages 76 and 77, and I happened to read this sentence:

“I thought that the air would have a very great influence on their facility to work together… I was very surprised to find that it influenced the mechanism far less than the effort accord each other by the impulsion of their mutual movements.”

I was thunderstruck, and read the rest of the section closely, followed by  the related section on Breguet’s experiments with resonance pendulum clocks. As it turns out – and I find it hard to believe that someone else hasn’t noticed this – the person who first tested a dual-train resonance watch with a double balance system in a vacuum was none other than Breguet!

In the same (undated) document on his experiments with resonance, he notes, “The first of these double watches (no. 2788) was three months in the hands of M. M. Bouvard and Arago without the seconds hands having parted by the smallest part of a second; it was put twice in a vacuum and maintained in ‘absolute void’ for 24 hours, as well as worn, laid flat, and hanging from a chain without ceasing to keep to the second.” Breguet apparently fitted no. 2788 with a thin steel barrier between the balances to rule out the effects of turbulence on the balances as well, but as the text by Daniels notes, “…as he was now satisfied with his conclusion he did not include it in no. 2794.″ (his next resonance watch.)

In Sync, Sans Regulator

So if it’s not air friction coupling the balances, what is it? Daniels remarks (p. 76) that “His experiments with clocks led him to conclude that the whole of the matter composing the frame was in continuous microscopic motion with the vibration of the pendulum. He realized that the same phenomena must occur in a balance wheel system where the motion would be transmitted to the plate of the watch by the couple of the balance and the spring at the limit of the arc of vibration.”

Later Daniels also notes that, “…excepting in some of his later tourbillon watches it is rare for Breguet to use a spiral spring without a regulator, but in this case it was essential if the effect of the vibration was to be fully transmitted to the cock.” This is apparently a key point —a conventional regulator, with its two pins, reduces the mechanical coupling of the balance spring to the cock and mainplate at the limits of its “breathing,” to such an extent that a resonance effect cannot be achieved.

Recipe for Resonance

P. Journe’s Chronomètre à Résonance is constructed along exactly these principles. The balances are free-sprung (with no regulator) and regulation is through the use of weights to vary the inertia of the balances. As with Breguet’s resonance pocket watches, the adjustable masses used for regulation are inside the rim of the balance (in the case of Journe’s watches, on the arms.)

Breguet’s notes on pendulum clocks are detailed and he notes that the two oscillators have to be closely regulated to each other for a resonance effect to occur although this is dependent on the stiffness of the common mount. Breguet apparently experimented with different attachment points for the balance cocks in his resonance watches, as they vary from one watch to the other.

The technical notes on the F.P. Journe website are extremely detailed and there one can read that the frequency of the balances is 21,600 vph (3 hertz). I don’t know how long Mr. Journe experimented with these systems or how many variations were tried but I suspect there is probably a sweet spot of some sort involving balance inertia, frequency, and construction of the cocks and plate that gives optimum results; Mr. Journe did mention, in an interview from 2003, that the balances needed to be regulated to run within 5 sec/day for a resonance effect to occur (Breguet’s pendulum clocks, depending on the stiffness of the suspension, might be able to be as far as 20 sec/day apart in rate but still begin to resonate) and in the same interview he also alludes to some of his early experiments with a pocket watch prototype.

Engineering Challenges

To achieve a true resonance effect as Journe has done in a wristwatch this small (38mm and 40mm, and only 9mm thick) is remarkable to say the least, and the problem of adjusting the two balances so that they both keep time accurately and run to a variation in rate of no more than 5 sec/day and do so with no more than a 5 sec/day rate variation between the two of them is… well, let’s just say I wouldn’t want to do it.

In any case, 14 years after the Chronomètre à Résonance first saw the light of day (and the arguments began over whether and how it could work), it’s enormously satisfying to finally understand the principles behind the watch, and just as humbling.

My sincerest thanks to the staff at the F. P. Journe boutique, Madison Ave. New York, for their kind permission and assistance in photographing this platinum Resonance Chronometer. The watch remains in the current collection in this version, with a digital display on the left and conventional analog display on the right, in either platinum or rose gold. The movement, Journe calibre 1499.3, is executed in 18k rose gold. For more information on the Resonance Chronometer, explore the comprehensive entry on this timepiece at www.fpjourne.com.

Adapted from an article by Jack Forster, managing editor of Hodinkee, in REVOLUTION’s archives, available in full here.