F.P. Journe Complications: The Chronomètre À Résonance, How And Why It Works
One of Journe’s most famous complications is also one of his most mysterious.
The problem of creating what F.P. Journe calls a resonance watch, is significant; Journe has said, in a lecture delivered several years ago at the Horological Society Of New York, that if the watch is not constructed and adjusted to a very high degree of precision, the two balances will not resonate with each other. Journe first became interested in the problem in the 1980s and attempted to create a resonance pocket watch in 1984 but he felt that its performance was unsuccessful and it wasn’t until 1994 that Journe began to develop a wristwatch resonance prototype, with the model launching in 1999.
The history of Journe’s resonance watches from a collector’s standpoint has been very well document and described. What is perhaps less clear is what a so-called resonance watch actually is, and why it is – that is, what do we mean by “resonance watch” and why, since they are difficult to produce and expensive to own, would someone go to the trouble of making one? In this article, the first of a technical series on Journe’s complications, we’ll look at both how the complications work, as well as the rationale and principles behind their design and construction.
What Is Resonance?
Perhaps the first thing to talk about is what “resonance” means. There are a number of intuitive examples. One is a wine glass shattering in the presence of a sound wave of the necessary frequency and amplitude; another is a bridge collapsing as it begins to oscillate in time with a driving force like the wind, or marching feet. The simplest definition of resonance, is framed in terms of harmonic oscillators, which are oscillators in which the restoring force is proportional to the displacement from the equilibrium position. In a pendulum clock, the restoring force is gravity; in a watch balance the restoring force is the balance spring and if the restoring force is proportional to the driving force, then the oscillator is isochronous – its frequency is independent of the amplitude of the oscillations.
This means of course that strictly speaking every watch is a resonance watch. The balance oscillates at a large amplitude because the driving force from the escapement has the same period as the natural frequency of the oscillator. A system with two pendulums or two balance wheels might also be called a double oscillator system, or more precisely, a coupled oscillator system.
Coupled Oscillators: Pendulums And Balances
Coupled oscillators can have extremely complex, chaotic modes of motion but what they all end up being is some mixture of just two modes. In one mode, the balances or pendulums swing in the same direction at the same time; both pendulums, for instance swing from right to left together. In the second mode, the oscillators vibrate synchronously, but in opposite directions; one pendulum, for instance, swings right while the other swings left and both arrive at the lowest, or equilibrium position, at the same instant.
How can oscillators be coupled? If we’re talking about mechanical oscillators there are many possibilities. Coupling can be through any medium through which energy from one oscillator can be transferred to the next. As long as there is a physical, mechanical connection between the oscillators which doesn’t lose energy to the environement, the oscillators can be coupled.
The first person to observe this occurring in clocks was Christiaan Huygens, who designed the first pendulum clock in 1657. Somewhat later, while briefly bedridden, he observed two pendulum clocks mounted on the same beam. These were experimental clocks with 100 pound weights hanging from them; they were an attempt to make a pendulum clock practical for a marine chronometer by keeping it vertical while a ship was rocking. Huygens noticed that after the clocks had been set running, they would begin to oscillate in synchrony, but 180 degrees out of phase with each other – the second mode of oscillation described above, which I’ve heard called a “breathing” mode. Moreover, if the clocks were interrupted, they would resynchronize after another half hour. The phenomenon he observed would later be described as ” … an odd kind of sympathy.”
The leap to suspecting that a double pendulum clock might be more precise than a single pendulum clock seems to have been made by Antide Janvier, whose work probably influenced that of Breguet in the development of his resonance, or coupled oscillator, clocks and watches. The thread of Breguet’s work was later picked up by F.P. Journe.
The first question, then, is how do two oscillators couple with each other? The answer is that in a double oscillator system, if there is a mechanical coupling of the oscillators, they can exchange energy with each other. Demonstrations of double pendulum models often show one pendulum coming to a gradual halt, while the other begins to swing wider and wider; if the pendulums keep swinging the situation then reverses itself and the second pendulum gradually slows and stops, while the first begins to swing again.
This will go on as long as there is kinetic energy to exchange. (Clockmakers noticed that in weight driven pendulum clocks, once the weight had descended to the level of the pendulum bob, the same thing could happen and the pendulum would stop while the weight oscillated wildly on its chain). Under specific conditions, however, the two pendulums – or balances – will enter a stable breathing mode. This happens because the two pendulums share the total energy of the system between them and share the same natural frequency.
The second question is, why? More specifically, what possible advantage can a double oscillator system offer over a single oscillator system?
Precision Of Double, Coupled Oscillators
The answer is that if one oscillator is disturbed, its error will tend to be distributed through the entire system and so one oscillator, or balance, will tend to correct any error in rate of the other. Breguet, in an undated note quoted by George Daniels, described the problem of a swinging weight causing a pendulum to stop and then continued, “These facts having been established there are still other factors that influence the rate of clocks, even though they are constructed with the greatest care. Some influences are found only in certain places and are almost impossible to overcome. Vibrations transmitted by passing traffic, the number of people in the room, … movements in the building which throw the clock out of beat and the effects of gravitational changes are some of the causes of variation in rate which resist the best endeavors of the makers. It occurred to me that if I could make use of the effect of induced movement by using two clocks constructed so that their pendulums could influence each other they would, by their compensating effect, produce a better rate than has been previously achieved … “
“I set up a very heavy frame and fitted to it two, separate clocks and two pendulums hanging one behind the other from a very strong bracket. Each of the clocks was separately regulated and then the two set in motion together with their pendulums oscillating in opposite directions. They ran perfectly together without any difference and always crossed exactly at the center line.”
“If there is any error in the wheels, compensation, or escapement of one of the clocks the other will influence the change in its rate however small the error [through energy exchange through the coupling medium] but not without, I presume, a very small reciprocal error. I have not yet been able to experiment sufficiently to pass judgement on this, but am quite sure that external influences do not affect the rate, because they cannot influence on clock without a compensating effect on the other.”
The Journe Chronomètre à Résonance
Obviously if you take Breguet’s experimental system, substitute balances and balance springs for pendulums and gravity, and shrink the whole thing down to wristwatch size, what you get is the Journe Chronomètre à Résonance.
Daniels would go on to conclude, “Breguet was not prepared to pass judgement on the effect on one pendulum of an error in the other, and does not seem to have noted this in his earlier experiments. Since, however, the two pendulums remained exactly together with one regulated to run fast, then half the error must have been transmitted to the other. If one were regulated fast or slow by, say, ten seconds the other would assume half the error and the two would run five seconds in error … any external influence on the clock causing disturbance of the case, and consequent change of rate would, as Breguet says, be cancelled out by its reciprocal effect on the two pendulums.”
In practical terms, this means that two balances closely regulated, will tend to return to a stable rate more rapidly if one is disturbed and that the effect of any difference in rate on rate stability will also be reduced. This advantage is somewhat offset by the need to construct each going train, escapement, balance spring, and balance very precisely and adjust them very closely; F.P. Journe has said that the daily variation in rate between the two balances must be five seconds or less.
It is often asked whether or not a double balance resonance chronometer is better than a single balance watch and the reality is, there is no absolute answer. A well made, adjusted, and regulated modern watch is capable of keeping a rate of only a few seconds’ variation per day to begin with so any contest between a Chronomètre à Résonance and a high grade modern single oscillator wristwatch would necessarily be a close one. However, there seems to be a clear principle behind the behavior and construction of Journe’s resonance chronometers and their ingenuity, combined with their connection to the history of chronometry, gives them a unique interest.
Finally, it has sometimes been speculated that the two balances are coupled by air friction. Journe himself has said that he has tested his resonance watches by placing a barrier between the two balances, as did Breguet (who also ran his resonance watches in a vacuum) and they still worked. The two balances according to both Journe and Breguet are not coupled by air resistance, but rather by the microscopic vibrations transmitted through the mainplate by the balance springs at the extremity of their oscillations.
As minute as this force seems to be, remember that Huygens’ clocks had enormous mass relative to the mass of the pendulums and it was under those circumstances that a stable, “breathing” mode synchronization was observed. As Breguet himself wrote, “This appears to be absurd, but experiment proves it a thousand times over.”