« The Heartbeat of a Watch – How the Mechanics of an Automatic Watch Work »
Mechanical wristwatches that supply themselves with the energy they need, and therefore no longer have to be wound, unite true watchmaking artistry and precision. Englishman John Harwood is regarded as the inventor of the first automatic watch. Inspired by children on a seesaw, he developed a self-winding mechanism, which he had patented in Switzerland. Yet it was only with Rolex's development of the Perpetual rotor in 1931 that the automatic watch became truly wearable. It harnesses even the smallest movements to wind the Rolex Oyster Perpetual. The exceptional accuracy of these wristwatches became legendary, and today it is part of the repertoire of virtually every maker of luxury timepieces.
How Does the Heartbeat of an Automatic Watch Work?
The term "automatic watch" refers to mechanical wristwatches that wind themselves through the wearer's movements, thereby supplying the movement with the energy it needs. In an automatic watch, a built-in rotor exploits the physical laws of gravity and inertia and applies torque to the winding mechanism. There are systems in which the rotor can store kinetic energy even under conditions of weightlessness. Although the technical inner workings of an automatic watch are rather complicated for a layperson, the watch itself is easy to wear, care for, and handle, as the long history of the Rolex Oyster Perpetual exemplifies. Today, not only mechanical watches but also quartz watches are powered by an automatic movement. 
In either case, the kinetic energy of the wrist drives the winding rotor. When a quartz movement is to be supplied with electrical energy, that energy is produced by a small generator that charges the rechargeable battery or capacitor. The first automatic watches were still very expensive compared with other mechanical wristwatches. After the Rolex Oyster Perpetual, watches from Junghans and other manufacturers also came onto the market. An automatic watch is driven either by a central rotor, which rotates above the movement and makes use of the full size of the case, or by a micro-rotor integrated into the movement itself.
The Drive Variants of an Automatic Watch
Drive via Central Rotor
The ball-bearing-mounted oscillating weight moves at the center of the movement and is usually mounted beneath it on the caseback side. This allows the full size of the case to be used for generating energy. The oscillating weight is usually a semicircular mass made of a heavy metal such as platinum or gold. It rotates parallel to the movement and ensures that the mainspring is always evenly tensioned. When the watch moves with the wrist, the oscillating weight is set in motion. The energy thus generated by the rotor is transferred to the mainspring, which winds the movement. Tiny ball bearings ensure that the rotor runs smoothly and with low friction. Winding works either in one direction or in both directions of rotation. This bidirectional reversing system is more complex to engineer and therefore more expensive. As a rule, the rotor is installed beneath the movement, where it turns freely around its central axis. To achieve one turn of the barrel arbor, the rotor must complete about 150 revolutions. However, fitting a central rotor means the case can no longer be made quite as thin.
The Central Rotor with Bidirectional Winding
Here, each direction of rotation supplies the movement with energy. The first caliber with a reverser was built by Felsa as early as 1942. Depending on the central rotor's direction of rotation, a rocker passed the energy on to the barrel, using one or two gears for this transmission. Longines also developed a central rotor with bidirectional winding, in which an eccentric with pawls and click transferred the kinetic energy of the wrist to the barrel. Shortly afterward, an Eterna engineer developed a tiny, low-friction ball bearing that was patented and used by many other manufacturers. The Pellaton winding system developed by IWC also drew energy from both directions of rotation: here, an ingenious system of pawls interacts precisely with a cam. Unlike a unidirectional central rotor, a central rotor with bidirectional winding can harness and convert all energy to drive the movement. Gear reversers as well as pawl and eccentric reversers are still found in automatic wristwatches today.
Drive via Micro-Rotor
When people in the 1950s favored particularly thin and elegant wristwatches, automatic watches driven by a central rotor had a hard time. Most of these watches had a case height of about 5.5 millimeters, which had to accommodate not only the central rotor but also the centrally mounted hands. In 1954, Büren Watch became the first manufacture to succeed not only in shrinking the rotor but in integrating it into the movement itself. Automatic watches could now be made thinner. Although the oscillating weight of the micro-rotor is considerably smaller than that of the central rotor, it is correspondingly thicker.
What Is the Difference Between an Automatic Watch and a Hand-Wound Watch?
A watch's delicate gear train is always driven by the mainspring in the barrel. In a hand-wound watch, turning the crown winds this mainspring back around the barrel arbor so that the watch keeps running. Here, the spring is attached both to the inner wall of the barrel and to the arbor. When the spring is fully wound, the wearer can feel it at the crown. As the spring unwinds, it drives the gear train with extremely slow rotations. No matter how sophisticated the spring's design: its torque is uneven. The less tension the spring has, 
the more force it loses, the amplitude of the balance decreases – and the watch loses a little time. The initial torque is also somewhat lower. In an automatic watch, the rotor ensures that energy is constantly fed back to the spring. As long as the watch is worn on the wrist, the spring torque remains in the optimal range. Only when it sits unworn for a longer period does the mainspring's power run down. Only then does the torque become as uneven as in a hand-wound watch. An automatic watch that is worn regularly – or at least before its power reserve runs out – therefore keeps more accurate time. If you want to rotate between several automatic wristwatches, you can use a so-called watch winder, which keeps the automatic movement running at all times.
The Rolex Oyster Perpetual: Certified Excellence
Rolex watches deliver on what the manufacturer promises. Many tests have proven it. For example, a waterproof Rolex Oyster on the wrist of swimmer Mercedes Gleitze spent ten hours in the English Channel and proved that it is indeed waterproof. Rolex has continually perfected the technical performance of the Rolex Oyster Perpetual and has it tested and certified by the Swiss chronometer testing institute. Like every automatic watch, the Perpetual movement harnesses the momentum of even the smallest movements of its wearer and winds itself. Because the mainspring of the Rolex Oyster Perpetual is always evenly tensioned, the watch runs precisely and with absolute consistency. Even with vigorous movements, the spring cannot be overwound: 
this is guaranteed by the slipping clutch, also known as the bridle. When the watch is taken off overnight, the power reserve ensures that the movement does not stop. Depending on the automatic watch, this power reserve is usually between 12 and 36 hours; the Rolex Oyster Perpetual has a power reserve of at least 42 hours. The "Oyster Perpetual" principle is used in many of the brand's calibers and is found not only in the Oyster Perpetual collection itself. On the dials of the Air-King, Submariner, Yacht-Master, and many other collections, "Oyster Perpetual" usually appears directly below the Rolex lettering.
