Rotonde de Cartier central chronograph: two celebrated developments for the 9907 MC calibre
With this new 9907 MC calibre, produced, developed and prototyped exclusively in-house in La Chaux-de-Fonds, Cartier is revisiting the chronograph function through this pioneering version of the Rotonde de Cartier model.
A highly original display
The primary distinguishing feature of this new Rotonde de Cartier 9907 MC calibre watch is the isolated chronograph display in the centre of the dial. This ingenious arrangement offers both exceptional legibility of this traditional complication and a novel way of displaying the time. Timekeeping occurs on two levels: the lower level is devoted to displaying the hours and minutes using blued hands which are invisible save for the tips, while the upper level is dedicated to the chronograph function. In the centre, the slim blued seconds hand begins to move when the corresponding push-piece is pressed, while the large counter in the form of an arc records each of its revolutions up to thirty minutes. Thus, the hands indicating the time never obscure those of the chronograph. The central chronograph counter is held as if weightless above the dial by a sapphire ring, whose discreet presence adds to the complexity of this construction, for which a patent request has been registered.
A chronograph displaying excellent craftsmanship
The 9907 MC calibre is equipped with a complex transmission system for the chronograph function. In fact, a column wheel controls the three chronograph functions (start, stop and reset), whereas in the majority of chronographs, this wheel is replaced by a cam system. The design of the 9907 MC calibre with manual winding also involves a smaller number of components than usual. In total, the Cartier watchmakers manufacture and assemble 272 parts. This reduction offers both improved watch maintenance and better long-term endurance. With a diameter of 11 ½ lines, i.e. 25.6 millimetres, and a height limited to 7.10 millimetres, the Cartier 9907 MC calibre has an exceptional capacity to integrate all functional elements in a relatively limited space.
The watchmaking genius of vertical coupling
The 9907 MC calibre is establishing itself as a truly innovative movement thanks to a vertical coupling system. In fact, the Cartier watchmakers decided to substitute the usual horizontal release system mounted on a lateral yoke, which requires a large amount of energy, for a vertical alternative with an extremely modern design. We are thus moving from a gear-train mechanism to one that works with friction. Analogous to the clutch system of a car, this system works by adherence, or in other words, by reciprocal pressure that is controlled very precisely by two small disks. Designed to reduce the energy consumption of the chronograph function, this vertical coupling system conserves exceptional chronometric precision despite the addition of a supplementary function.
Compared to its more conventional equivalent, it thus has three crucial advantages:
• It renders the usual braking and blocking levers, used to keep the various elements of the chronograph display in position after measurement, superfluous.
• It allows the chronograph hand to advance smoothly from the very beginning of time measurement and to move with perfect regularity according to the rhythm dictated by the vibrations of the balance, i.e. in eight little steps per second on the 9907 MC calibre. The precision of the chronometric function is thus improved.
• Activating or deactivating the chronograph only has a very marginal effect on the amplitude of the balance. Such an original function thus enables Cartier to forgo the small seconds. The control is then assured by the chronograph hand, which can remain permanently in motion.