L.Leroy Osmior Chronomètre à tourbillon

— This timepiece features a tourbillon with Duplex escapement and constant force, a fusee-chain and a deadbeat second.

Osmior Chronomètre à tourbillon © L.Leroy

With its Chronomètre à Tourbillon, Leroy is renewing ties with its rich past in the field of chronometry.

It is notably paying tribute to the work of a father and son who were both watchmakers to King Louis XV: Julien (1689-1759) and Pierre Le Roy (1717-1784). Acknowledged as the father of modern chronometry, the latter invented the detached escapement and won a double prize from the French Royal Academy of Science for his work on marine chronometry.

Hand-wound Manufacture Calibre L100, entirely developed and built in the Manufacture in Le Sentier at the heart of the Vallée de Joux, has an escapement with direct impulse on the balance also featuring an impulse wheel and a winding wheel for constant force, as well as a balance-spring with two terminal curves.


The timepiece comes in a 41mm round case in gold (5N red gold, palladium-coated white gold or two-tone) framing a Grand Feu enamel dial, inspired by 19th century Leroy pocket watches.
© L.Leroy

This unprecedented calibre includes 953 parts, almost as many as the iconic historical Leroy 01.

The movement architecture and its pillar-based construction are inspired by the first 18th century marine chronometers. The barrel-bar is enhanced by a now extinct traditional technique involving hand brushing with silver powder to create a grained texture.

The time-setting mechanism, a nod to 18th century French clockmaking, is adorned using the Geneva mast” decorative technique, while the finesse and geometry of the going train are also inspired by the same era.

The calibre comprises an original direct-impulse escapement (one direct impulse and one indirect impulse per oscillation) recalling the “Duplex” escapement developed by Pierre Le Roy. The balance is thus equipped with a system serving to compensate for the undesirable effects caused by the dilatation of the balance-spring, at temperatures extending beyond those normally controlled within a horological mechanism.

The balance with four adjustment screws is equipped with a balance-spring with a double terminal curve such as described in watchmaking theory but never previously used in series-made models and now assembled in the Leroy workshops. There are two curves – an inner curve and an outer curve – that foster isochronism while ensuring perfectly concentric deployment in a vertical position. These two curves also eliminate any potential “unbalance” effect, the disequilibrium referred to in watchmaking literature as the “Grossmann effect” and neutralise the so-called “Caspari effect”.


The back cover is opened by a device on the crown that unlocks the “secret” caseback to reveal the Calibre L100 through a sapphire crystal pane.
© L.Leroy

The calibre oscillates at 18,000 vph with a large-size balance that promotes long-term precision rating.

An integral part of the escapement, the constant-force device serves to ensure stable amplitude throughout the duration of the power reserve.

The fusee-chain device guarantees exemplary linearity of the torque in order to supply the energy required to rewind the constant-force escapement. The chain itself comprises 105 links measuring 1.55 cm each and forming a total length of 16.275 cm!

The Chronomètre à Tourbillon displays the hours, minutes and central deadbeat seconds by means of cut-out steel hands. The power reserve is indicated by a subtle touch representing a nod to Julien Le Roy, inventor, of the à toc watch featuring a discreet indication of this function. The lozenge-shaped marker is painted on a Grand Feu enamel disc applied to the dial and visible beneath a grid. A disc spinning counter-clockwise on a 300° axis between 11 and 1 o’clock indicates the 75-hour power reserve.


The “grid” on the dial is reminiscent of the tapestry motif on the snuffbox-watch made for Marie-Antoinette and dated 1788.
© L.Leroy

The dial is adorned with grand feu enamel and features champlevé Roman numerals.

The round case embodies a subtly sophisticated style with integrated lugs and comes in three versions: 5N red gold, palladium-coated (non-rhodiumed) white gold and a two-tone variation. It is composed of four parts: polished bezel, circular satin-brushed case middle, polished gadroons and lugs, and a guilloché hinged back cover.

The movement wheels are burnished and polished; the pinion leaves are polished using a wooden grinding wheel. The steel parts are specular-polished or adorned using a “Geneva mast” decorative technique. The barrel-bar is decorated with a “silver-powder” grained texture applied by manual brushing. All the screws are blued from the outset to prevent oxidation, while the screw heads are specular-polished and even the profiles of the collets/ridges/convex hole surrounds are also polished.

The timepiece is fitted with a hand-sewn alligator leather strap lined with alcantara and secured by a pin buckle of twin-blade folding clasp.

Each Chronomètre à Tourbillon is individually numbered and delivered with its own chronometer rating certificate issued by Besançon Observatory.