A constant-force escapement — known in classical horological literature as a remontoir d'égalité, or simply a remontoir — is a mechanism designed to isolate the timekeeping element of a watch or clock from the variable driving force of the mainspring. By interposing a small auxiliary spring between the primary power source and the escapement, the device ensures that each impulse delivered to the balance wheel or pendulum is identical in magnitude, regardless of how fully or partially the mainspring is wound. The result is a marked improvement in isochronism — the property whereby the oscillator completes each swing in precisely equal time — and, consequently, in the overall rate stability of the timepiece. Constant-force escapements rank among the most technically demanding complications in precision horology, and examples by makers such as A. Lange & Söhne and Girard-Perregaux are regarded as benchmarks of the watchmaker's art.

The Problem of Variable Torque

A conventional mainspring delivers its highest torque when fully wound and progressively less as it uncoils. Even in well-regulated going-trains equipped with a fusée or barrel-and-click system, this variation is not entirely eliminated. The escapement — the last wheel in the gear train before the oscillator — therefore receives slightly different impulse forces at different points in the power reserve. Because the amplitude of a balance wheel's oscillation is sensitive to the energy it receives, a stronger impulse produces a wider arc and a weaker one a narrower arc. Since the period of oscillation is not perfectly independent of amplitude in a real balance-and-hairspring system, these fluctuations translate directly into rate errors: the watch runs slightly fast when freshly wound and slightly slow as the mainspring approaches exhaustion. For a pocket watch or wristwatch worn casually, this variation may be acceptable; for a precision regulator or a competition-grade timepiece, it is not.

Principle of Operation

The constant-force escapement addresses this problem by inserting a secondary spring — the remontoir spring — into the power path between the going-train and the escapement wheel. This auxiliary spring is deliberately small and is designed to operate over a very narrow range of deflection, so that the force it exerts on the escapement remains essentially constant throughout that range. The mainspring, through the gear train, periodically rewinds the remontoir spring at fixed intervals — typically once per beat, once per second, or once per several seconds depending on the design. The escapement therefore never draws directly on the mainspring; it draws only on the remontoir spring, which has just been recharged to a known tension. The variation in the mainspring's torque is absorbed upstream, and the escapement sees only the uniform output of the remontoir.

The recharging interval is critical to the design. If the remontoir is rewound once per beat, the mechanism is described as acting at the escapement itself and is sometimes called a true constant-force escapement. If it is rewound at a lower frequency — say, every four or eight beats — it is more properly a remontoir in the broader sense, and a small residual variation in the remontoir spring's tension between rewindings remains, though this is far smaller than the variation in the mainspring alone.

Historical Development

The concept predates the modern wristwatch by several centuries. English clockmaker Richard Towneley is credited with an early remontoir design in the seventeenth century, and Thomas Mudge, John Harrison, and Abraham-Louis Breguet all explored variants of the principle in the eighteenth and early nineteenth centuries. Harrison's marine timekeepers incorporated a form of remontoir to achieve the extraordinary rate stability required for longitude determination at sea. Breguet applied the concept to both pocket watches and precision regulators, and his influence shaped subsequent French and Swiss practice throughout the nineteenth century.

In the era of precision observatory regulators — instruments used to rate other timepieces and to determine time for navigational and scientific purposes — the constant-force escapement became a standard feature of the finest examples. Makers including Dent, Kullberg, and Riefler produced regulators with remontoir mechanisms whose daily rate errors were measured in fractions of a second.

Notable Modern Implementations

The late twentieth and early twenty-first centuries saw a revival of the constant-force escapement as a prestige complication in high-end watchmaking. Several implementations have attracted particular attention:

• A. Lange & Söhne — the remontoir in the Lange 1 Tourbillon and Zeitwerk family: The Saxon manufacture reintroduced a remontoir mechanism wound every ten seconds into several of its tourbillon references. The device is visible through the dial, and its periodic release — marked by a small jumping action — has become a signature of the brand's technical identity.
• Girard-Perregaux — Constant Escapement L.M.: Rather than a conventional remontoir spring, Girard-Perregaux's engineers developed a mechanism using a thin silicon blade that stores and releases energy through controlled buckling. The blade snaps between two stable states at each beat, delivering a consistent impulse. This approach, introduced publicly around 2013, represents a departure from classical spring-based remontoirs and exploits the elastic properties of silicon at a micro-mechanical scale.
• Greubel Forsey and independent makers: Several independent ateliers have incorporated constant-force mechanisms into grand complication pieces, often combining the remontoir with a tourbillon to address both the torque-variation problem and the effects of gravity on the oscillator.

Relationship to Isochronism

The constant-force escapement is one of several strategies watchmakers employ in pursuit of isochronism. Others include careful profiling of the hairspring (the Breguet overcoil being the classical solution), the use of free-sprung balance wheels with adjustable weights, and the selection of low-hysteresis materials for the oscillator. A constant-force escapement does not, by itself, make a balance wheel isochronous — it cannot correct for the intrinsic non-isochronism of the balance-and-hairspring couple — but it removes one of the principal external disturbances that would otherwise mask or compound that non-isochronism. In this sense, it is complementary to rather than a substitute for careful oscillator design.

In precision pendulum clocks, the constant-force escapement serves an analogous function: the pendulum's period is sensitive to the amplitude of its swing, and any variation in the impulse it receives from the escapement will alter that amplitude and therefore the rate. The remontoir eliminates this source of disturbance, allowing the pendulum to swing with consistent amplitude over long periods.

The Jewelled Connection

As with all high-grade escapement work, the constant-force mechanism depends on jewelled bearings — typically synthetic ruby or sapphire — at its critical pivot points. The remontoir spring's arbor, the locking detent or click that holds the rewound spring, and the escapement wheel's own pivots are all conventionally jewelled to minimise friction and wear. In the finest examples, the jewel count of a constant-force escapement complication may add a dozen or more stones to the movement's total. These jewels are not decorative; they are functional components whose hardness, low friction coefficient, and dimensional stability are essential to the mechanism's long-term performance.

Collector and Market Context

Among collectors of complicated timepieces, the constant-force escapement occupies a position of considerable respect. It is regarded as a horological complication rather than a display complication — it does not add a new indication to the dial but instead improves the fundamental function of the watch. This distinction is prized by technically minded collectors who value what the movement does over what it shows. Auction results for pieces incorporating well-executed remontoir mechanisms — particularly those by Lange, Breguet, and the independent ateliers — consistently reflect a premium over comparable pieces without the feature. The complication's relative rarity, the skill required to execute it correctly, and its direct lineage to the precision-timekeeping tradition of the eighteenth and nineteenth centuries all contribute to its standing in the market for serious horological objects.

Further Reading

• GIA — Horology and Watches overview