BREGUET - Tourbillon Fusée with silicon balance-spring

Key to a movement’s accuracy, the balance-spring is now being produced in silicon by Breguet, as featured in the new Tourbillon Fusée model.

Revue FH - February 11, 2010

Tourbillon fusée. © Breguet

The balance-spring, a very delicate mechanism wound in the shape of a spiral and normally made of metal, can withstand disturbances from shocks and magnetism but also terrestrial gravity, which pushes it out of shape. The alloy traditionally used for its manufacture has the special property of rigidifying when the temperature rises, thereby offsetting the increased inertia of the balance-spring as a result of its thermal expansion.

Considered since time immemorial as one of the keys to a movement’s accuracy, the balance-spring has been the subject of much research. In 1795, Abraham- Louis Breguet invented the «Breguet overcoil» which remains today the benchmark in terms of balance-springs, intuitively transforming the outer coil of the spring: it is raised and bent slightly to improve its isochronism. To overcome the problem of sensitivity to magnetic fields, it was another Breguet (Louis- Clément, the grandson of Abraham- Louis) who experimented in 1830 with glass balance-springs as a replacement for metal... And in 2006, Breguet presented the first watches with a balance-spring and escapement made of silicon, a material offering many advantages: it is light and totally insensitive to magnetic fields, offers good shock-resistance and is highly resistant to corrosion.

Tourbillon fusée © Breguet

The convincing results obtained over a period of nearly four years by the flat silicon balance-springs used by Breguet in different movements led the manufacturer to develop a silicon balance-spring with the famous «Breguet» outer coil. Today the job is done. Producing a curve that gives a third dimension to this piece of silicon is a real feat. Indeed, ordinary pieces of silicon are cut from a plate and therefore remain uniformly level. Silicon is not malleable like metal and to make a curved outer coil rising up from the spring it was necessary to reinvent the production process in its entirety. The Breguet manufactory rose brilliantly to this new technical challenge. Thanks to the «silicon Breguet overcoil», it will now be possible to apply general improvements already provided by the silicon balance-spring to all Breguet calibres, regardless of their basic construction.

Today, this new «silicon Breguet overcoil » receives its baptism of fire in the Tradition 7047 model with tourbillon and fusee-chain transmission. This piece, reminiscent of the architecture of the first tourbillon pocket watches invented by Breguet, is presented in a platinum version in which the movement features a special alloy of anthracite coloured metals. The surface finish required is a new and improved electroplating technique which uses an alloy of precious metals from the platinum family, darker in appearance than ruthenium.

The fusee-chain attached to the barrel guarantees constant power throughout the working of the watch. The large tourbillon placed at 1 o’clock has been the subject of several patent applications, including one relating to the titanium balance and three to the silicon Breguet overcoils. Another patent was issued for the indication of the power reserve placed directly over the barrel. Displaying a high level of technical viruosity, the new «silicon Breguet overcoil» is at the cutting edge of research and development. It gives the manufactory in L’Abbaye extremely exciting development prospects.