Timepiece Chronicle

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Unlimited Power: A Look at the Power Reserve Complication

Unlimited Power: A Look at the Power Reserve Complication

Angelus tourbillon power reserve U10.png

The power reserve indicator complication is often forgotten about. It's not as glamorous as a worldtimer nor is it noticeable as an alarm, yet the power reserve indicator remains one of the most important complications for one reason, accuracy.

As the mainspring of a mechanical watch unwinds, energy is transferred through the gear train to the escapement which in turn regulates the time displayed on the hands. When the mainspring is full or near full, it generates the most torque and provides an even distribution of power to the escapement. When the mainspring is near empty, there is less torque generated and the energy transferred through the gear train to the escapement is less regular.

If you've ever looked at a hand-wound watch towards the end of its wind, you'll see that the timekeeping isn't as accurate. This might irritating during your commute home, but in the past accurate timepieces were paramount to safety.

When sailors were first navigating the world's oceans, their ability to know their latitude and longitude was linked to the accuracy of their chronometers. An inaccurate timepiece would put their position hundreds of miles off course so clockmakers began putting power reserve indicators onto chronometers as way of reminding the crew to wind them.

The first prototype wristwatch with a power reserve indicator complication was created by Breguet (the brand, not Abraham-Louis) in 1933. Jaeger-LeCoultre would make the first production wristwatch with power reserve indicator in the 1950s

Jaeger-LeCoultre Futurematic with 'porthole' dial.

Jaeger-LeCoultre Futurematic with 'porthole' dial.

The LeCoultre Futurematic was released in 1951 and was the first production wristwatch to have some form of power reserve indicator. The lack of crown meant that the mainspring could only be wound by the bumper movement, as the crown on the caseback was only able to set the hands. What is even more interesting about the watch is that it would always have at least six hours of charge held in the mainspring. When the watch was removed from the wrist, the movement would stop. Placing the watch back on your wrist would press the crown in which would activate the movement. As charging the mainspring could only be accomplishing by the rotation of the rotor, the power reserve indicator was crucial to maintain optimal accuracy.

Released a few years later, the LeCoultre Powermatic advanced on the Futurematic concept by focusing solely on the power reserve indicator complication. An aperture at 12 o'clock revealed a colored disc which displayed the remaining power left in the mainspring. The LeCoultre Caliber 481 was used inside both the Futurematic and the Powermatic.

Unfortunately for watchmakers, a power reserve complication is not as easy as sticking a hand onto the mainspring and having it stick out onto the dial.

The process is a little more complicated than this and there are endless design variations but the basics are as follows. For the sake of this example, the power indicator is a hand on the dial.

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A series of gears linked to the ratchet wheel (the gear that sits atop the mainspring barrel) connects to the power reserve hand. As the crown winds the mainspring, the ratchet wheel turns and begins moving the gear train connected to the indicator hand. A catch disengages that allows the indicator hand to move and then re engages when the ratchet wheel stops turning (either because the mainspring is full or the wearer stopped winding). As the mainspring unwinds over the next few hours, the ratchet wheel slowly turns and activates the gear train linked to indicator. This way, through a series of differential gears, the movement of the indicator is always linked to the amount of power in the mainspring.

What about Power Reserves on Automatic Watches?

This is one of the age old arguments that take place in forums among the watch community. What is the point of a complication that alerts the wearer to the amount of power stored in the mainspring when the watch is self-winding?

It's worth remembering that the best practice for maintaining daily accuracy for a mechanical watch is to fully wind it daily. This is true for both automatic and hand-wound watches. The power reserve indicator is valuable for automatic watches as sometimes there is no resistance given by the crown when winding. It's impossible to overwind many modern mechanical watches, but the presence of a visual indicator on the dial is a nice touch.

Zenith Captain Elite Power Reserve #zenith #zenithwatches

A photo posted by @senmurv on

Also let's remember that people are forgetful! I have a modest three watch collection (Ok, two of those actually belong to my wife but sssh, don't tell her) and I've forgotten to wind my automatic Zenith. Sometimes I've reached the afternoon without realizing how inaccurate the watch is due to a half empty mainspring. Infact, there is a reference of my watch that excludes the GMT complication in favor for a power reserve indicator, or as its called in French reserve de marche.

Watchmakers have continued to innovate the design of indicator

Photo Courtesy of Oris

The most traditional way is using an extra hand on the dial, typically off to the side rather than connected to the central pinion with the main hands. The exact design will vary from brand to brand to suit their particular tastes and the theme of the watch. The new Oris Artelier Caliber 113 has the indicator to the right of the hands with grey rectangles marking the 10 days of reserve power with a tapering red triangle to show the final day and a half.

Photo courtesy of Seiko.

Photo courtesy of Seiko.

The complication isn't only reserved for dress watches though, as the Grand Seiko SBGA029 has a indicator on it as well. As the watch is powered by Seiko's proprietary 'Spring Drive' movements, the inclusion of the power reserve is surprisingly prescient. ISO 6425, the one that sets out the requirements for a professional dive watch, state that there must be an end-of-life indicator for the battery. Could Seiko have gotten away with a second hand that jumps every 5 seconds? Yes, but that's not how Seiko likes to treat its Grand Seiko Line.

The Panerai Special Edition Radiomir 1940 Equation of Time 8 Days Acciaio. Photo courtesy of Panerai.

The Panerai Special Edition Radiomir 1940 Equation of Time 8 Days Acciaio. Photo courtesy of Panerai.

Panerai has put a twist on the simple hand indicator by choosing to put the indicator on the movement itself. The Panerai Special Edition Radiomir 1940 Equation of Time 8 Days Acciaio has a small indicator that roughly shows the remaining power reserve. I say roughly because out of the 8 days available, there are only markings for full, half and empty with very little space between.

The linear power reserve of the Angelus U10 Tourbillon Lumiere. Photo courtesy of Angelus.

The linear power reserve of the Angelus U10 Tourbillon Lumiere. Photo courtesy of Angelus.

This isn't the end of brands putting the indicator in weird places as the Angelus U10 Tourbillon Lumiere has the indicator on the side of the case. The linear design is very similar to that of a fuel gauge, especially with the F & E symbols donating full and empty. The design of this indicator is supposedly inspired by the multi-dialed travel clocks that the original Angelus brand were making in the 1930s.

The Blancpain L-Evolution Tourbillon Grande Date Reserve de Marche sur Mas Oscilante. Photo courtesy of Blancpain.

The Blancpain L-Evolution Tourbillon Grande Date Reserve de Marche sur Mas Oscilante. Photo courtesy of Blancpain.

Blancpain took things to a whole other level with the L-Evolution Tourbillon Grande Date Reserve de Marche Sur Mas Oscilante. The location of the indicator? On the rotor itself! Is this an amazing example of haute horology? Yes. Is it practical in anyway? No.

These extreme examples aside, the power reserve indicator is a useful complication. It might not be as charming as a sonniere, but its a hell of a lot more affordable and way more useful.

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