In Jaeger-LeCoultre’s Calibre 177, the three-dimensional rotations of the balance-wheel are at the very heart of an extraordinary horological invention. The ultra-light external carriage makes a complete turn about its axis every sixty seconds. The internal carriage which supports the balance-wheel, balance-spring and escapement rotates far more rapidly – every 24 seconds to be exact, hence completing 2.5 turns per minute. To be sure, Jaeger-LeCoultre’s master-watchmakers did not settle on this unusual rotation speed by chance. In fact, it means that the gold balance-wheel appears once every minute in all its glory before the eyes of the observer, before continuing its progress, as though weightless, but following a carefully pre-planned course through the microcosm of the spherical tourbillon, which is of infinite lightness – the total weight of the 90 parts composing this complex rotating body is only a third of a gram. It would have been impossible to achieve this weight, worthy of all the superlatives, without recourse to aluminium for the spherical external carriage and to titanium and aluminium for the elements of the internal carriage. Thanks to the exceptional lightness of aluminium, the external body weighs just 0.035 grams. In steel the same part would weigh 0.11 grams. Aluminium is used in aeronautics and in high-performance engines, and, because of its exceptional density/resistance to corrosion ratio, is perfectly suited to use in a tourbillon. In any case, this is not the first time the master-watchmakers of Jaeger-LeCoultre have used this metal. In their ongoing pursuit of innovation, they had already experimented in the 1940s with movement prototypes made entirely of aluminium. Only in the case of the balance-wheel have Jaeger-LeCoultre’s technicians not skimped on weight. Along with its setting screws, the balance-wheel is made of 14-carat gold in order to obtain a particularly high force of inertia, which is useful in such unusual oscillation mechanisms. The spherical tourbillon of Jaeger-LeCoultre Calibre 174, of the Reverso Gyrotourbillon 2, embodies new groundbreaking progress in this extremely complex domain. A mechanical marvel in its right, this spherical tourbillon and the hundred or so parts composing it exercise an irresistible fascination by the complexity, speed and beauty of its three-dimensional motion. Its frequency has been increased to 4Hz (compared with 3Hz of the Jaeger-LeCoultre Calibre 177 of the Gyrotourbillon 1). It has been fitted with a cylinder-shaped balance-spring, such as is found in marine chronometers. Contrary to flat balance-springs, this cylinder-shaped balance-spring is fitted with two end curves, one on each end, thus ensuring a perfectly concentric beat. These twin characteristics – a high frequency and a concentric balance-spring – considerable improved precision. Its inner, faster carriage rotates at the speed of 3,2 complete turns per minute, meaning one revolution every 18.75 seconds, and its ultra-light aluminium-made outer carriage performs one turn per minute. Other specific technical characteristic of this model lies in the barrel, which is equipped with a torque-restricting device. This means that the regularity of rating is perfectly guaranteed for a full 50 hours: when the torque is too low, the restrictor comes into action and stops the watch running. This same restrictor also locks manual winding if there is a risk of supplying excess energy. Eager to enliven still further the visual enchantment of this horological mechanism, the designers of the Manufacture enhanced its appeal by delicately open-working the spokes of the balance-wheel, an operation that creates an elegantly slender effect and also implies a considerable reduction in weight.