The case of the Chronomètre FB 2RE picks up the shape of these Marine Clocks and this modular construction. The movement is installed in a cylindrical container made of 18-carat gold and water-resistant to 30 metres. Short, tapering all-of-piece lugs are solidly secured to the case by stylised bolts and ensure the Chronomètre FB 2RE smoothly moulds the contours of the wrist. In order to facilitate reading off the hours and minutes during sea voyages, Ferdinand Berthoud had designed a two-tiered dial with two types of graduations for his Marine Clock No. 6: in the centre, the hours indications in Roman numerals stand out from the minutes circle in Arabic numerals appearing on the circumference.The FB-RE.FC calibre is one of the rare examples to feature fusee-and-chain transmission, serving to ensure constant force for the escapement. It acts like an infinitely variable automatic reduction gearbox.Exceptionally, the FB-RE.FC calibre, in addition to the constant-force mechanism, is equipped with a one-second remontoire visible on the back of the movement. It is one of the most complex torque regulation systems in the history of time measurement.
The escape-wheel pinion is connected to the stop wheel and marks off the seconds. The entire assembly is connected to the central seconds hand and enables the Chronomètre FB 2RE to display true seconds, or deadbeat seconds, like marine chronometers or regulator-type longcase clocks. To enhance the accuracy of the display, the connection between the escapement pinion and the deadbeat seconds hand is achieved by means of a special deadbeat seconds mechanism.
The remontoire is traditionally very energy-intensive, particularly because of the friction it generates. Chronométrie Ferdinand Berthoud has reduced this friction by installing the remontoire where the torque is lowest. The escape wheel is the one that receives the least force from the entire gearing and is therefore the best candidate for the position. In addition, the use of the ruby cam and an extremely light seconds hand (0.01 grams), thanks to the use of titanium and an extremely tapered shape, reduces the weight driven by the remontoire.
All these decisions together enable the FB-RE.FC calibre to offer a power reserve of 50 hours. This is indicated on the back of the movement by means of a blued arrow-shaped hand placed on a sector engraved on the mainplate. The barrel stores energy for several more hours, but the stopwork system blocks it outside the guaranteed precision timekeeping (chronometry) range.