The mainspring is the sole energy source in a mechanical watch: everything else in the movement merely manages it, distributes it, or restrains its release. It is a strip of tempered steel, wound in a spiral inside a cylinder called the barrel, that accumulates energy under tension and releases it progressively to the gear train. Understanding how it works means understanding nearly everything else about mechanical watchmaking.
The fundamental problem with the mainspring is that it does not deliver constant torque: when fully wound it pushes hard, when nearly discharged it pushes weakly. Over a standard spring of 25-30 cm this decay curve is manageable, but not negligible. This is why an entire family of technical solutions exists to straighten it. The fusée-and-chain, present already in eighteenth-century pocket watches and still mounted on certain contemporary calibres, compensates for the decline with a variable-diameter pulley connected to the barrel via a chain. The remontoir d'égalité breaks the gear train into two sections and periodically recharges a small intermediate reservoir, isolating the balance from torque variations. Modern constant-force mechanisms, such as those found in A. Lange & Söhne's L034.1 calibre or in Girard-Perregaux's constant escapement, intervene even further downstream, stabilizing the push right at the escapement entrance. The traditional lever escapement, for its part, dissipates up to 30% of available energy: it is the system's main bottleneck.
The length of the mainspring directly determines the power reserve. A standard spring gives 40-50 hours. A 3-metre spring, like that of the Haute-Rive Honoris housed in a chamber carved directly from the plate, reaches approximately 1,000 hours. The Panerai Jupiterium mounts eight barrels for a collective spring of 32 metres. The most common solution to extend the reserve without exaggerating dimensions is to place two barrels side by side in synchronous configuration, as in the Louis Moinet LM135 which reaches 96 hours with a regular discharge curve. Miyota, on the 82 calibre, cuts steel strips to 1 mm width with 0.01 mm tolerance and subjects them to heat treatment along their entire length: even at this industrial level, spring consistency matters.
From the bench I add something that magazines rarely say: every calibre has an optimal torque window, and many watches keep better accuracy with the spring half-wound than with it fully wound or nearly discharged. It is worth observing your watch's behaviour in the first hours after winding compared to the last, before drawing conclusions about the movement's accuracy.