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A brief explanation of a major complication #5: The Power Reserve

Anna Wu-Chauvineau
Le 20 April 2017

There are watch complications that are simply beautiful, even hypnotising at times, but whose function isn’t always that useful for real life. There are others, however, that we can use every single day and add true value to a watch. The display of the power reserve, like the chronograph function that we talked about last, is such a useful added bonus. A real cherry on top of the cake!

Jaeger Lecoultre Master Control power reserve

All who have a mechanical watch, whether it’s automatic or manuel, know that it stops after a certain time, if it hasn’t been wound or worn, it will need to be restarted and adjusted, this is what we would take a pass on…especially when it comes to the moonphase or the complete calendar!

Fundamentals of the power reserve

In all mechanical watches, we find a system (and sometimes several) that stores energy. This system, namely the cylinder, uses a spiral spring that contracts itself during winding and then delivers progressively stored energy to make the watch work. Just think of it as a sort of gasoline tank of a watch.

Baume & Mercier - Clifton 1830 - 8 day power reserve

The power reserve is actually the time the cylinder needs to deliver all the energy it can store. In other words, it’s the time that passes between the moment a watch is wound to its maximum and the moment that it stops (without being wound again in between time of course).

In a nutshell:
“The power reserve of a mechanical watch is like the running life of a car
when it has just filled up its tank completely.”

Generally equipped with 40-something hours on most watches, the power reserve can also reach much longer durations on certain models: 3 days, 8 days and believe it or not 30 days! This becomes an independent complication in their own right because we are dealing with a movement capable of storing a lot of energy or of consuming very little…or both at the same time.

Let’s get deeper…

The other complication connected to the power reserve is its display on the dial, or on the case back. Even if it isn’t considered as a major complication in its own right since it “only” converts the remaining amount of energy into time, the display of the power reserve proves to be incredibly useful in the day-to-day.

For a hand-wound watch, the power reserve display allows to wind the watch neither too early, when it has sufficient running time left, nor too late, when it stops. For an self-winding piece, the usefulness of the complication remains questionable since it winds itself with the movement of the wrist… unless it isn’t worn all the time. This is the case of most dressy watches that we leave to rest on weekends.

Regardless of the winding mechanism:
“The power reserve display allows you to follow a watch’s running life to avoid it stopping…very much like a car’s gauge that keeps it from gasoline shortage!”

Small precision: if an automatic watch stops, or almost, it’s often necessary to manually wind it before wearing the piece. This is even more true when we’re talking about vintage pieces with shorter running life, and the oscillating mass is much less heavy and smaller.

If we’re just going to restart it by shaking it the first few hours, the winding will be insufficient to have a watch that functions in a optimum and precise way. We can say, then, that in most cases, the self-winding mechanism serves less to wind the watch, and more to maintaining it at an optimum level of winding that we would otherwise do manually.

Power Reserve Display

Enough technical talk, let’s get down to what interests us, and you, the most: beautiful watches equipped with the power reserve display. How does it appear on our dials? More often than not the power reserve display is on the dial, but it can also be placed on the case back. The choice is purely technical, depending on the movement’s architectural design, or simply aesthetic in order to lighten the front side of the watch. In front as in the back, the display modes remain quite similar.

The hand:

The most common way to display the power reserve is to have a hand that indicates the level at a portion of the circle. This is very close to the gas tank level display in a car. The portion of the circle is graduated in hours or days, or it simply indicates a level that goes from “full” to “empty.”

An elegant power reserve of A. Lange & Söhne’s Datograph: a model of simplicity and functionality that perfectly integrates itself to the dial.

The graduated segment:

The display can also be linear with a cursor that moves along a graduated segment, as we can see on certain Panerai Officine models like the 10-day Radiomir 1940.

Radiomir 1940 - 10 Day GMT

The window:

Less common, but nonetheless convenient, there is a type of display that consists of showing a portion of colour, more or less important, via a display window and with a disc that turns behind the dial. When the window is “full,” the power reserve is at its maximum. When it is “empty,” the watch stops. Please note that technically speaking, regarding the movement, the function is very similar to that of the display by hand…

But it doesn’t stop here. The display of this complication is limited only by the creativity and imagination of designers and master watchmakers. What will the power reserve of tomorrow be? A digital display that flashes? Hopefully not…