When a man is young (horologically speaking) he is innocent of watches and barely cognizant of time. The slow crawl of the hour and minute hands is alien to moderns; time is something measured out in homogenous bits by a cell phone, a computer clock, or for that matter, a coffee maker, each second as indistinguishable from the next as grains of sand.
But for those who fall afoul of watches, a gradual obsession with probing deeper into the arcana of their workings gradually may grow, insidious as the encroaching of an alien weed. Gradually, a man finds himself thinking more and more about the principles guiding the homunculus bound to his forearm. The ticking like the beating of a hideous heart more and more is the lodestone of awareness. He hears of gears; rumors of balances, amplitude, positional error, and the need for power right in measure and duration reach his ears. Drawn into the clutching and selfish arms of the horological muse, he soon feels the oscillations of the balance as the throbbing of his own pulse, and each increase in the power of the watch on his wrist is a reprieve from his own inevitable sentence, a winding of his own spring. Like Macbeth, listening in fascination and horror to the dissembling prophecies of the weird sisters, he wants to know the measure of the mechanical lifespan, the renewing of which has become a talisman for a renewing of his own.
In a word, he wants a power reserve indicator.
To those not initiated into the dark arts of horology, the power reserve indicator hardly seems worthy of the title “complication” — after all, what could be more transparently simple? The mind of the enthusiast may be inclined to regard a power reserve indicator as a simple minded device, lacking in romance, lacking in emotional resonance, and failing, to boot, to afford any real horological insight — in essence no more appealing, interesting or complex than the gas gauge in an automobile. But a little inquiry makes all of these objections fall.
The power reserve indicator goes to the heart of fascination with watches. The allure of watches is that of what the dean of cyberpunk fiction, William Gibson, himself a watch enthusiast, once called, “the tamagotchi effect” referencing the keychain sized virtual pets played with as children’s toys (and sometimes, more furtively, by adults as well.) Tamagotchi need to be taken care of — they need virtual food, virtual attention, and virtual love (itself a misnomer — is there anything more virtual?) The power reserve indicator is a window into the life expectancy of the little beast ticking on the arm.
In displaying the reserve of power remaining in the barrel, the power reserve indicator also helps give an idea as to whether or not the part of the mainspring’s energy which gives the best performance is being used. Energy from the mainspring is not like gasoline in the tank of car, which will deliver the same power whether the tank is full, or nearly empty. Mainsprings deliver gradually less and less torque as they begin to reach the end of their power reserve, and a watch running on decreased power may become more susceptible to inconsistencies in rate — in other words, it may keep time less reliably.

Wheels Within Wheels To understand how a power reserve indicator works, it’s first necessary to understand how a watch winds. Modern watches are equipped with a so-called “going barrel” in which, during running, the mainspring barrel rotates (hence the name) to drive the center wheel and thus the power train of the watch, as the mainspring unwinds. However, during winding of the watch the mainspring barrel is stationary, and the crown tightens the mainspring via a crown wheel that turns the ratchet wheel on the top of the barrel.
The problem is obvious: winding the watch causes one wheel to turn (the ratchet wheel) whereas running the watch causes a different wheel to turn (the mainspring barrel itself). Therefore, it’s necessary for a single indicator hand to be able to respond to both the turning of the ratchet wheel and the turning of the mainspring barrel. Several ingenious solutions exist; the epicyclic winding indicator, the differential winding indicator, and the screw differential winding indicator.

The Epicyclic System Epicyclic gears (also known as planetary gears) consist of an outer (planet) gear or gears rotating around a central (sun) gear. In an epicyclically geared power reserve system, the barrel, when unwinding, turns a wheel with a hole in the center through which the axis of the power reserve hand runs. This wheel has, on an offset pivot mounted on its upper surface, a planetary gear that rotates in turn around two sun gears, one of which is connected to the shaft of the power reserve hand. As the planetary gear rolls around the sun gears, they turn, and the sun gear which is attached to the power reserve hand moves, to indicate the running down of the barrel.
The other sun gear is attached to a wheel that’s geared to the ratchet wheel, which turns when the ratchet wheel turns. As the sun gear turns (during hand winding) the planetary gear rotates on its pivot (in other words, remains in place rather than ‘orbiting’ the sun gears) causing the lower sun gear to rotate — moving the power reserve hand in the opposite direction to indicate the accumulation of power.
The planetary or epicyclic differential, in the above form or some approximately similar form, is by far the most commonly used type in modern watches; two notable incarnations are the Lange 1815 and the IWC cal. 50010 used in the IWC Portugieser Automatic. An extremely complex variation on the epicyclic gear system is found in the F. P. Journe Sonnerie Souveraine in which the power reserve hand, depending on whether the watch is set to strike or not, can show the power reserve remaining for either setting — a remarkable system patented by Journe.

The Differential System Although the differential system is different from the epicyclic, the basic idea is the same — when the barrel turns (feeding power to the train via the center wheel) a lower wheel geared to it causes the power reserve indicator to move, whereas when the ratchet wheel turns, an upper gear geared to the ratchet wheel turns, again causing a change in the position of the power reserve indicator.
The differential gear is so called because it allows an input to produce different outputs — its most common use is in automobiles, where a differential allows the drive wheels on the right and left sides of the car to turn at different speeds when cornering. The differential power reserve indicator is similar in construction.
When the barrel turns, a wheel geared to the barrel rotates; this wheel has contrate wheel teeth (teeth projecting upwards perpendicular to the axis of rotation) on its upper surface, which engage a contrate wheel (a wheel at right angles to the other gears) that turns on a pivot attached at right angles to a pivot running through the center of the wheel with contrate teeth geared to the barrel. As the contrate wheel rotates, it rolls around the center of the contrate toothed lower wheel, turning the pivot running through its center — and attached to this pivot is a wheel that turns the power reserve hand.
When the ratchet wheel turns (during hand winding) a wheel geared to it, which has downward pointing contrate wheel teeth, turns the contrate wheel, again causing it to roll around the central pivot, but this time in the opposite direction — and the power reserve hand moves to indicate that the mainspring is tightening.
The differential system with contrate wheel is seldom used in modern watches, although one notable example is the mechanism used by Dornblüth & Sohn of Kalbe. Dornblüth & Sohn developed a version of the contrate wheel differential in order to address what in their view is a potential problem with the planetary gear power reserve design — the number of wheels in the planetary gear system can create a certain amount of backlash (the amount of “play” between the teeth of meshing gears) which can accumulate through the power reserve gear train and produce inaccuracies in the power reserve indication. The Dornblüth & Sohn system reduces the number of gears to three, which they claim creates an inherently more accurate indication of the power reserve.
The differential system is also employed, in his two minute tourbillon, by Daniel Roth, the watchmaker who founded the brand that still bears his name and who, under the name Jean Daniel Nicolas, laboriously creates handmade wristwatches from his workshop in Le Sentier at the rate of only a few per year, for a select clientele who can afford and are willing to wait for his brand of slow perfection.

The Differential Screw System The differential screw system is a unique solution which allows the turning of the barrel or the ratchet wheel to be translated into a relatively small movement of the power reserve indicator hand — useful in a watch where the indicator moves through a relatively small angle.
In this system, during winding of the watch, a screw turns on which is mounted a small metal cone. As the screw turns, the cone rises, pushing on a flange attached to the power reserve indicator hand; in rising the cone increases its effective diameter and the power reserve hand moves to a corresponding degree.
During running of the watch, the screw shaft on which the cone is mounted turns in the opposite direction, and as the cone descends the motion of the hand is reversed.
This unusual variation on the power reserve mechanism can be found in some very exotic watches — Audemars Piguet, for example, has deployed it in their Royal Oak Concept watch, which also displays the power reserve in a different way than the usual radial needle; the Concept watch has a linear power reserve indicator instead, and also features a dynamograph indicator which, rather than showing the number of hours of running time remaining, shows the amount of torque actually being delivered by the mainspring to the power train. The cone based differential screw system is also used by some independent watchmakers including Finnish rising star Kari Voutilainen, whose unique complications and exploration of advances in balance construction have earned him an international reputation in an astonishingly short time.
Thus, the pleasure of wearing a watch is augmented by the power reserve mechanism, a window into the vital signs of the mechanical life form on one’s wrist. H

WHAT LIES BENEATH

Bovet 22-Day Tourbillon

Patek 10 Day Tourbillon

IWC power reserve mechanism

Lange 31 power reserve mechanism

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