echnological innovation is not always visible to the person wearing the watch. It is often concealed inside complex mechanisms, set in alloys, or transforming production methods. But in the case of the variable-length hand of the Breguet Reine de Naples Coeur 9825, it is there before our very eyes, graceful and astonishing.

The oval shape has undoubtedly always been a minority choice because of the geometrical impossibility of getting the minute hand to espouse the contours of the case. To achieve this, the key obviously lies in extensible hands capable of varying their length to adapt their course to the watch’s ovoid shape.

Solutions do exist. We know of a few, rare examples, such as the Ovale Pantographe by Parmigiani Fleurier, a solution inspired by an oval pocket watch by Vardon and Stedmann that was restored at the brand. Its extensible hands are based on the principle of the pantograph: two telescopic structures in the form of a parallelogram which, as they move, follow the elliptical shape of the dial. The architecture is a delicate display of mechanics inspired by the work of a certain Mr. Eiffel.

The solution developed by Breguet for its beautiful oval-shaped Reine de Naples is totally different. Its extensibility is a matter of suppleness and elasticity, rather than the articulation of rigid parts.

The oval shape has undoubtedly always been a minority choice because of the geometrical impossibility of getting the minute hand to espouse the contours of the case. But solutions do exist.

The Queen’s heart

Beyond the scientific and technical aspects of the minute hand of the Reine de Naples, you cannot help admiring the beauty and finesse this flexible appendage lends to the ovoid timepiece, which is inspired by what is believed to be the first watch ever designed to be worn on the wrist: that of the queen of Naples. Showing the passing minutes by means of a tiny heart that swells and contracts is a beautiful, poetic achievement, as well as a horological feat.

But behind the heart is a whole team of “surgeons” who combined concepts, calculations, materials research, tests and prototypes to allow the minute hand to breathe while exactly tracing the watch’s oval shape.

Flexure bearings

Behind the theory we find the famous flexure bearings (theorised back in 2001 by Simon Henein, among others, who today heads up Instant-Lab at EPFL), development of which has spawned numerous recent innovations, notably in watch regulation. These flexure bearings have added a whole new field to watchmaking mechanics.

Applied here to a hand of variable length, they supply an aesthetic, visible solution to a technical problem.

The hand is made using the LIGA process from an alloy of nickel and phosphorous, a stable and hard-wearing material chosen for its suppleness and high yield strength. It is made up of a heart-shaped tip attached to two blades, each of which has a flexible section, a rigid section and an hour-wheel. The whole hand is extremely fine, the flexible parts no thicker than half a hair’s breadth.

To connect the hand to the movement, the two hour-wheels are placed one on top of the other on the same axis and are driven by two coaxial cannon pinions. The hand does not turn. It “simply” changes shape and length as the two arms rotate in identical fashion, but in opposite directions. This causes the hand to expand from a minimum length of 7.7mm to a maximum length of 16.8mm. This variable hand principle is now patent-protected.

The hand is made up of a heart-shaped tip attached to two blades, each of which has a flexible section, a rigid section and an hour-wheel.

Setting the hand in motion

So the hand is flexible, but now you have to set it in motion. And that is where the most complex research comes in. For the hand to advance and change shape and length, each arm has to be able to move separately. The watch movement, via the cannon pinion, drives the hand activation system by rotation through a defined angle. The mechanism of an additional plate converts the angle of entry into one rotation by the hour-wheel of the right arm and another rotation by the hour-wheel of the left arm.

We will spare you the complex calculations needed to display the time accurately. They depend both on the deformation properties of the hand itself and the desired variation in length, which depends on the curve of the ovoid contours of the dial. In addition to calculating this equation, you also have to evaluate the non-constant speed of rotation of the hour-wheels.

This is how the minute hand (shown by the heart) moves from 60 minutes to 20 minutes. The paths taken by the right arm and the left arm are different (green and red arrows).

The activation mechanism

Breguet’s engineers and watchmakers studied several possible types of activation mechanisms for the hand. One avenue of inquiry they followed up right to the tested and functioning prototype stage was a double, non-circular gear train. But although this prototype met the specifications, offering a simple, robust and precise method of functioning, the idea (for which a patent is pending) was shelved owing to the intrinsic limitations of non-circular gears: a “modulation of the angular displacements of the arm of the hand” subjected to over-abrupt variations would necessitate gears of which the “non-circularity” would be too extreme.

Another problem was that the architecture of such a non-circular gear train would not be easy to modify or adapt. To drive it, “the system is set on a rotating support of which the cannon pinion is an integral part” (editor’s note: this is the pinion that controls the motion works). At each rotation, this support drives two cannon pinions which are a fixed part of the hand’s two arms. They interact with a satellite that carries a sensor. This sensor interacts with the cam, the only fixed element of the assembly.

The hand does not turn. It “simply” changes shape and length as the two arms rotate in identical fashion, but in opposite directions. This causes the hand to expand from a minimum length of 7.7mm to a maximum length of 16.8mm.

Dial of the prototype with activation based on non-circular gears. It can be seen that the hand completes a non-circular course but does not espouse the contours of the dial. On the basis of these lessons learned, the method chosen to activate the variable-length hand was by a cam, which would allow it to exactly follow the dial’s ovoid periphery. To achieve this, the hand had to be larger, and the ratio between the lengths of the two arms during their course nearly 1:2.2, which “exceeds the technical possibilities of non-circular gears”, the upper limit of which is a ratio of 1:1.6.

It would be too lengthy here to explain in detail the precise operation of the mechanism, which has to juggle the respective angles of the two arms of the variable-length flexible hand, calculations of the angle of rotation, the pivoting of the satellite and interactions with the cam. The hand is permanently under tension and holds the sensor against the cam. (For more detail, see Actes de la Journée d’Étude 2021 by the Société Suisse de Chronométrie, SSC)

Simple and robust, made up of few parts, allowing design modifications by means of simple changes to the geometry of the cam, and “relatively easy” to assemble, this system passed all the homologation tests for final approval. Two patents are pending for it. For the research, Breguet worked with Nivarox, Asulab and ETA, all Swatch Group companies.

But ultimately, it is the delicate grace of this unique hand that arouses admiration and even emotion. Which just goes to show that technological innovation can produce poetry, too.