An octagonal case, a tourbillon movement with fusee-and-chain transmission and a pillar-based construction: the Chronomètre FB 1 has played a major role in defining the identity of Chronométrie Ferdinand Berthoud. Calibre FB-T.FC powering this model is now preparing to take a last bow, in the form of a final 38-piece edition presented for the first time with a round case: the Chronomètre FB 2T.
Equipped with a constant-force device known as fusee-and-chain transmission, Calibre FB-T.FC features a unique modus operandi, as winding is not done via the barrel, but by means of a differential placed in the centre of the fusee. This mechanism serves to reduce the force required for winding. Seventy turns of the crown are thus required to fully arm the mainspring, smoothly and without the slightest effort. Its 9 mm diameter makes the crown easy to grip. It is also equipped with a safety device to protect the movement from excessive winding.
Located at the top of the barrel, a Maltese Cross stopwork device mechanically limits the operating range of the mainspring where its torque is most constant. Although the barrel can potentially perform eight full revolutions itself, the Maltese Cross allows only six, marked by the six teeth on its profile. The last two indentations on the Maltese Cross prevent the mainspring from delivering too much energy to the gear train (just after complete winding) or too little (at the end of the power reserve).
During the entire 53-hour power reserve, this constant energy is delivered to the escapement, a tourbillon with central seconds, for which a patent has been filed. Positioned at 6 o’clock, the tourbillon carriage engages directly with the seconds (fourth) wheel thanks to a wheel located on its axis whose diameter and properties are identical to those of the seconds. The two wheels thus make a complete rotation in 60 seconds: the seconds (fourth) wheel in a clockwise direction and the tourbillon wheel counter-clockwise.
This particular construction optimises the flow of energy transmission by keeping the required number of gears to a minimum and likewise reducing the risk of irregularities due to friction. A long central wire-shaped hand thus displays the seconds accurately and without any stuttering of the hand—very much like the Marine Clocks produced by the Master Watchmaker who dedicated his life to the quest for precision.
The power-reserve display of calibre FB-T.FC is based on a patented process that is as judicious as it is elegant. Directly linked to the barrel by a large-diameter driving wheel, the mechanism uses the movement of arming and disarming the mainspring to move a cone up and down a threaded central arbor attached to the mainplate.
A feeler tipped with a ruby roller end measures the upward and downward movement of the cone. When the movement is fully armed, the cone is at the top of the threaded arbor and the feeler arm therefore rests at the base of the cone (its widest diameter). As the movement is disarmed, the cone rotates and moves down the threaded arbor.
The diameter of the cone at the feeler decreases and the feeler moves closer to the axis of the threaded arbor. The feeler arm rotates by about 15°, as does the rack below it, which is on the same axis. It is the latter that transmits the movement’s power-reserve level to the hand on the dial side.
Three versions will be offered by the Maison: FB 2T.2 in 18-carat rose gold with a black satin-brushed dial; FB 2T.2-1 in 18-carat rose gold with a ruthenium satin-brushed dial; and FB 2T.1 in 18-carat white gold with a blue satin-brushed dial.
These three references are sources of inspiration for collectors who will have the opportunity, for this “Final Edition”, to create their own customised timepiece by choosing the case material, the colour and the finishing of the dial and the leather strap. There is however a limit with regard to the case shape: this FB 2T collection is offered exclusively with the round case presented in 2020 with the FB 2RE Remontoir d’Egalité collection.