Secrets Behind a Watch's Power Reserve
An in-depth look at the mechanical watch's energy source
By The Editors of Tourneau's Our Minutes and A. Lange & Söhne
One of the most important elements of a mechanical watch is its reserve of power. The mainspring (a metal coil) is what powers a watch movement: when it's tightly coiled, a watch has reached peak power and, in fact, can't be wound any further. Whether they're hand-wound or automatic, watch movements run most accurately when fully wound, losing precision over time as the mainspring's tension is released (or the coil "loosens"). In order to run at a regular rate, a watch should retain approximately 25-30 percent of its mainspring tension.
A variety of factors can impact how long it takes for a mainspring to unwind, slow down, and eventually stop a timepiece from running. The longer a mainspring is, the longer it will take to loosen and thus, the longer its power reserve. A standard mechanical watch will maintain power for about two days, or 40-50 hours. Many watches include something called a power reserve indicator, akin to a car's gas gauge.
In general, it's good practice to wind one's hand-wound watch once a day, whether or not the timepiece has a power reserve indicator. Some might say that a power reserve indicator is more helpful on an automatic watch, though—in theory—it won't stop running as long as it makes a frequent appearance on the wrist. However, we live in an era in which our most vigorous daily activity might involve typing out a heated email. This type of wrist movement isn't necessarily enough to keep an automatic watch at optimal power. An indication of lower power reminds the wearer to get up from his or her desk and take a literal power walk.
Though power reserve indicators are among the most pragmatic watch complications, they're also often very creatively executed. Below, we take a look at a few noteworthy examples.
The Lange 31 (seen above) is the first mechanical wristwatch with a power reserve of 31 days. Featuring a patented constant-force escapement, the timepiece offers a high rate of stability and consistent accuracy for a month at a time.
Photograph by Martin Wilmsen
The iconic Panerai 233 (seen above) is a coveted model for Panerai enthusiasts. Its in-house, hand-wound movement offers a variety of impressive features, including day/night and GMT indicators. Its ability to store power for eight days (or, for those keeping track, quadruple the standard) justifies the appearance of that compelling linear power reserve.
Photograph courtesy of Hublot
The Hublot MP-05 LaFerrari takes the idea of the power reserve as gas gauge to an entirely new level. Its unprecedented 50-day (read it again: that's days, not hours) power reserve appears on the left side of the dial.
For more watch news, reviews and advice, visit Our Minutes from Tourneau.
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