It is every bit as mind-blowing as it looks.

A FUSÉE-AND-WHAT?
It is exactly what it says it is. There are two components here: a chain (well, we all know what that is), and a fusée, which is less commonly seen. A fusée is a cone-shaped barrel to which one end of the chain is attached (see below, 'A'). In a watch, the other end of the chain is attached to the mainspring barrel ('B'). A mainspring barrel is the component that stores up the energy when a watch is wound.

WHAT DOES IT DO?
It helps a mechanical timepiece tick with undulating precision. In watchmaking terms, it is known as 'constant torque'. Like how a toy car has more power when it is fully wound, and less power towards the end, a mechanical watch's performance and precision declines at the end of its winding cycle. The fusée-and-chain, which is based on a 16th century invention, is among a variety of mechanisms that watchmakers have devised to help ensure consistently optimum energy transmission in a watch, whether at the start or the end of its winding cycle.

HOW DOES IT WORK?
The mechanism is based on a theory of compensation. When the mainspring is fully wound and loaded with energy, the chain tugs and wraps around the smaller radius of the fusée. The reverse happens when the mainspring is almost unwound - the chain then pulls at the larger radius of the fusée.

SOUNDS LIKE A LOT GOING ON.
It is. And not many brands have the technical smarts and skills to do this. Being an archaic mechanism previously used on clocks and pocket watches, the fusée-and-chain had to be miniaturised to insane proportions to fit into a wristwatch. In A. Lange & Söhne's version, each chain comprises 636 parts. While it looks delicate - measuring 15cm long, 0.5mm wide, and weighing just 0.12g - the chain can haul up to 2kg of load.

Size of a chain link in comparison to a one-cent Euro coin

LANGE'S WATCHMAKERS HAVE THEIR WORK CUT OUT THEN.
They do, and that is why you find the fusée-and-chain mechanism only in the best A. Lange & Söhne watches. The brand even has a name for this class of timepieces - calling them the 'Pour le Mérite' collection.

A. Lange & Söhne's 'Pour le Mérite' collection from 1994 to 2017

Although each 'Pour le Mérite' creation bears its own stamp of mechanical supremacy, ranging from grand complications (Tourbograph Perpetual 'Pour le Mérite', second from left) to ultra-precise, pocket watch-inspired chronometers (Richard Lange Tourbillon 'Pour le Mérite', second from right), they are all distinguished by having the fusée-and-chain mechanism in their movements.