Mechanical watches are powered by a coiled spring known as a mainspring. As this spring uncoils, the amount of time that the watch can run diminishes. This remaining amount of time is referred to as the power reserve, winding indication, or up/down indication. An indication turning through an angle or a linear indicator appears on the dial to display the power reserve, similar to a car’s fuel gauge.
A simple form of the power reserve complication uses a differential screw. The differential screw is a wheel that is screwed onto a threaded shaft parallel to the barrel. The top of the differential screw has a tapered section that will direct the power reserve indicator hand as it moves up and down the threaded shaft. This is accomplished through the use of linkage with a finger resting on the tapered section.
When the watch is running, the barrel will turn the differential screw, causing it to descend on the threaded shaft. The angle of the taper combined with the number of turns of the differential screw causes the power reserve indicator hand to move across at a certain angle. Barrels usually have thick teeth to accommodate increased torque load, but care must be taken to ensure the differential screw’s teeth are always properly meshing with the barrel’s teeth.
When the watch is being wound, the winding works wind a wheel that is attached to the threaded shaft. When the threaded shaft is turned, the differential screw ascends and directs the power reserve indicator hand to an increase in available power. Other versions of the power reserve complication include an epicyclic and differential system. All of these systems can be adapted to indicate linearly rather than through an angle. With the increasing popularity of skeletonized watches, some manufacturers advertise a “natural” power reserve indication where the barrel and barrel cover are cut away so that the mainspring and its state of unwinding is clearly visible.