"It’s not enough to have a watch be able to run longer," says Ruediger Albers of Wempe Exquisite Timepieces & Jewelry in New York. "They have to be able to run accurately during that time period, and that is not so easy to engineer." Albers, a highly experienced watch retailer, is addressing a question that now confronts many watch collectors who are choosing between watches offering longer and longer power reserves. A watch’s power reserve measures the number of hours the watch can run between windings; long power reserves are convenient not only for pieces that have to be wound manually, but for automatic watches, which still require resetting when they wind down. Many owners of perpetual-calendar wristwatches, which can require an exhaustive process to reset, will attest to the value of a long power reserve.

But as Albers indicates, creating a quality watch with a long power reserve is not a simple issue. Mechanical watches, as many people know, are powered by coiled springs. The cylindrical housing containing the spring, known as a barrel in watch parlance, rotates as the spring unwinds, thus turning the rest of the gears. The spring, unfortunately, generates different amounts of power it unwinds, which can affect the watch’s ability to keep time. "Without careful engineering, the watch will run at a slower rate as the spring unwinds and the energy it produces drops," explains Marc A. Hayek, president and CEO of Blancpain (one of the companies that specialize in long-power-reserve watches). "The trick becomes attaining not only a long running period, but a flat force curve for that period, so that the watch’s rate will remain little changed as the mainspring unwinds."

Blancpain’s solution to the problem has been to use multiple barrels in sequence inside the watch. As one winds down, the next takes over, keeping a more constant level of energy during the entire period. Blancpain’s new L-Evolution models are built with the company’s latest generation of movements that offer a sequence of three barrels. Together, they give the watches a power reserve of eight days, which compares quite favorably with average mechanical watches that can only promise some 40 to 50 hours.

While Blancpain’s multiple-barrel design offers a more consistent supply of power than a more powerful single barrel, it too experiences some change in energy over the running time of the watch. For other companies, the solution is to keep the wearer informed not only of the power reserve left in the watch, but of the state of that energy. Several years ago, Audemars Piguet experimented with a device it called a dynamograph, which measured the tension in the barrels and indicated when they were offering the best level of power for accurate timekeeping. Today you can find a similar system in certain very complicated watches like the Bell & Ross Instrument BR 01 Tourbillon, which carries a so-called Trust Index (an indicator of optimal movement function) along with a power-reserve indicator.

In 2006 the watchmaking world was stunned by the appearance of a mechanical timepiece from American jeweler Jacob & Co. that boasted a power reserve of a full month between windings. This massive tourbillon, called the Quenttin (engineered by the same Swiss specialty company that makes Bell & Ross’s Trust Index), accomplished that feat of endurance with seven barrels arranged sequentially. Not long afterward, the highly regarded German maker A. Lange & Söhne announced its own marathon contender, the Lange 31, which can also run a full month without being rewound. But unlike most of the other long-power-reserve watches made today, the Lange 31 uses just two stacked mainspring barrels to contain its power. Although one would be well justified in assuming that the Lange 31 design would be hampered by accuracy variations as its two springs uncoil over a month, a small device cleverly does away with this possibility. Lange’s engineers equipped the watch with a mechanism once found in pocket watches, called a remontoir, which is essentially a small intermediate spring that feeds the watch power in discreet increments. The mainspring power that energizes the remontoir may vary over time, but the regular discharge from the remontoir remains stable, as does the beat of the watch.

Timepiece aficionados remember A. Lange & Söhne for its revival of an even more antiquated system for power compensation, the fusee and chain. In this design, first seen in the Tourbillon Pour le Mérite and now used in the Richard Lange Pour le Mérite, the mainspring is coupled to the rest of the power train not by a toothed gear wheel, but by a tiny, finely wrought chain that is wound around a stepped, cone-shaped cam called a fusee. At full power, the chain pulls against the small upper part of the fusee; as the mainspring—and the chain—unwind, they pull against the wider lower portion, where greater leverage is exerted. The Tradition Tourbillon from Breguet is probably the watch that displays the fusee and chain to best effect today. It is undoubtedly a clever system for keeping your watch running accurately over the course of its power reserve, but more importantly, it is just the kind of slightly too-elaborate construction that will keep you absorbed by the mechanics rather than the time itself.