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Watch 101: Power Reserve Indicators
One of Horology’s Most Useful Complications
This article originally appeared in Our Minutes.
Today, we take a deeper look at one of the most important elements of a mechanical watch: 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% 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.
JAEGER-LECOULTRE 481 CALIBRE
In 1948, only two years after adding an automatic movement to its collection, Jaeger-LeCoultre brought out the first watch in history to combine this function with a power-reserve indicator, the Jaeger-LeCoultre 481 Calibre. When the power gets low, the numerals in the window under the 12 turn red.
First automatic watch with power-reserve indicator. Photo: Jaeger-LeCoultre
PANERAI LUMINOR 1950 8 DAYS GMT
The iconic 233 is a highly 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) is remarkable—and justifies the appearance of that compelling linear power reserve.
Panerai 233. Photo: Martin Wilmsen
A. Lange & Söhne refers to the Lange 31 as an unrivalled masterpiece, and as the first mechanical wristwatch with a power reserve of 31 days, it’s a spot-on description.
LANGE 31. Photo: A. Lange & Söhne
Featuring a patented constant-force escapement, the timepiece offers a high rate of stability and consistent accuracy for a month at a time. Though longer mainsprings offer larger reserves of power, there is also a more noticeable loss of torque as the spring relaxes (which affects the’s watch accuracy). This constant-force escapement re-tensions the spring by 60 degrees every ten seconds to assure that a uniform amount of torque is delivered for an entire month.
Calibre L034.1. Photo: A. Lange & Söhne
Accompanying each Lange 31 is a stainless steel winding key, which generates much more torque than would be possible with a winding crown. Thus, fewer revolutions are needed to fully wind the watch.
Lange 31′s winding key. Photo: A. Lange & Söhne
MB&F LEGACY MACHINE N°1 XIA HANG
By winding a mechanical watch, many would say you’re breathing life into it. In a clever touch, MB&F’s new Legacy Machine N°1 literally adds this metaphor to the watch’s dial. MB&F is known for its wild creativity, and maintains a collection of rebellious art on view and for sale in Geneva, Switzerland at its M.A.D. Gallery. Beijing sculpturist Xia Hang, whose work is on display there, tackled the design of the 45-hour power reserve. One of his so-called “comma men” sits straight up when fully wound. As power diminishes, the comma man bends his back and gradually slumps over.
Legacy Machine N°1 Xia Hang. Photo: MB&F
Xia Hang’s “comma man”
HUBLOT MP-05 LAFERRARI
The 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.
MP-05 LaFerrari. Photo: Hublot
With another exceptionally long power reserve (which would entail an unbearable amount of hand winding), Hublot provides an ingenious winding key, similar to the electric drills one might encounter on a speedway.
MP-05 LaFerrari winding key. Photo: Hublot
MP-05 LaFerrari 637 components. The power reserve appears at center left. Image: Hublot
Katie Wudel is the Managing Editor of Our Minutes. Katie is a timepiece enthusiast and freelance writer contributing to such distinguished literary journals as McSweeney’s, Tin House, Prairie Schooner, and more.