The Constant Force Tourbillon is part of the Royal Collection and, like every other model from Arnold & Son, features an in-house mechanical movement. John Arnold (1736-1793) was constantly searching for higher precision in his timepieces, as the more accurate the chronometer, the more precise the calculation of longitudinal (east/west) position at sea. Arnold and his son were the first to develop and produce technically superlative (but reasonably priced) marine chronometers in significant quantities.
Constant force is an essential element in the quest for higher precision because it is difficult regulating a consistent output, e.g. isochronal time when the power input from the mainspring is constantly changing. Imagine trying to drive a car at a consistent speed with an engine that continually varies its power output.
To maximize power consistency in the Constant Force Tourbillon, Arnold & Son began at the beginning, i.e. the mainspring barrel, as this is the source of the movement’s power. Instead of using just one mainspring barrel, which would produce significantly different amounts of torque between fully wound and nearly empty, the Constant Force Tourbillon has two symmetrical barrels in series, visible dial side at 10:30 and 1:30. The first mainspring barrel alone powers the gear train, while the second barrel tops up the first whenever its torque output drops below optimal. This ensures that the power to the regulator flows as constantly as possible.
The Constant Force Tourbillon features a patented constant force mechanism. Instead of power from the mainspring feeding directly to the escapement/tourbillon, it charges a small hairspring which in turn releases a consistent amount of power to the escapement/tourbillon once each second. The device also drives the true-beat seconds hand (also known as jumping seconds or dead seconds), a highly cherished Arnold & Son complication. When the power from the mainspring drops below that required by the constant force mechanism, the movement stops rather than running at lower precision. The constant force device rotates once per minute in increments of one second, visually mirroring the rotation of the constantly rotating tourbillon cage. The bridges supporting the constant force regulator and tourbillon are also symmetrical, both horizontally with each other and vertically with the barrel bridges.
While the constant force mechanisms optimize precision with the movement in stable positions, the 60-second tourbillon averages out gravitational errors on the escapement by constantly rotating it through 360°. The difference between the rotating tourbillon and rotating constant force device is that the former turns continually while the latter steps in increments of one second.
The Constant Force Tourbillon is a limited edition of 28 timepieces, and is available in a 46 mm 18-carat red gold case with anti-reflective sapphire crystal and sapphire display back for viewing the superbly hand-finished movement.