THE FUSÉE-AND-CHAIN MECHANISM.

Lange’s master watchmakers were the first to succeed in fitting a fusée-and-chain transmission into a small wristwatch case. In earlier times, this mechanism was integrated into A. Lange & Söhne’s compli-cated pocket watches and marine chronometers.

In the Lange fusée-and-chain mechanism, the flow of power between the mainspring barrel and the going train is established indirectly by a 636-part chain. One end of the chain is attached to the barrel drum and the other to the fusée. As the mainspring barrel unwinds, the chain wraps around it and reels off the fusée.

When the mainspring is fully wound and thus exerts great force, the chain pulls at the smaller radius of the fusée where the leverage is smaller. As the spring’s tension diminishes, the chain pulls at the larger radius with the higher leverage. Thus, regardless of the state of wind of the mainspring, the same amount of torque is delivered to the escapement, resulting in improved rate accuracy.

Overall, the fusée-and-chain transmission comprises more than 700 parts. It incorporates a planetary gearing which assures that the movement is powered even while the mainspring is being wound. Additionally, it is endowed with a pretensioning mechanism that allows the characteristics of the mainspring and of the fusée to be perfectly harmonised. Finally, the elaborate construction includes a device that blocks the winding mechanism just before the mainspring is fully wound and stops the movement just before it is fully unwound.