Wristwatch Essentials: What is an Automatic Movement?

Automatic is a word we often see printed on dials, written on casebacks or thrown around on the websites of watch brands. But what actually is an automatic movement and what makes it different from its predecessor the manual wind movement?

If you’ve had a chance to review our article on manual wind movements you will know that the difference between an automatic and manual wind movement can simply be explained as the way in which they provide power to the mainspring. As the name suggests an automatic movement provides power to the mainspring automatically. No prizes for working that one out. In practice this means there is (almost) no need for the wearer to have to manually wind their watch in order to keep it powered.

The invention of the automatic movement can be traced back to Abraham-Louis Perrelet who in 1777 first introduced a automatic calibre in a pocket watch. The problem was a pocket watch remained stationary in a pocket so there wasn’t enough kinetic energy to power the watch. Hence it didn’t really catch on. Monsieur Perrelet was one of those unfortunate individuals who was just ahead of his time. Fast forward to 1922 and British watchmaker John Harwood would create the first automatic movement as we know it today.

The genius behind an automatic movement is the inclusion of a weighted rotor. The automatic rotor is a small, often semi-circular disk that is pivoted at the centre. Harwood’s inspiration came from a seesaw in a children’s playground and he created what are known as ‘bumper movements’. In a bumper movement the rotor only rotated approx. 270° where it would then hit a spring, or bumper, which would propel it back in the opposite direction. By 1931 Rolex had perfected the automatic movement which now rotated a full 360°. Although, Rolex kept their rotors hidden until recently with the 2023 Daytona being the first exhibition caseback to reveal the 360° gold rotor which sits on top of the movement.

This creation of the automatic rotor removed the need for the movement to be manually wound via the crown. As your wrist moves, the rotor swings back and forth winding the mainspring. In other words simply wear it and it will power itself. However, once you take your watch off and the power reserve starts to run down and should it run out completely, the watch will grind to a halt.

One of the great things about automatic movements is that they can still be manually wound. In fact, should your watch run out of power completely, it’s usually recommended that you restart it by manually winding with the crown. This saves you wildly shaking the watch to get the rotor spinning.

The length of time the mainspring can provide power to the watch without additional winding is referred to as the power reserve. There are many elements that can impact the power reserve of a watch from the length of the mainspring, to the frequency of the movement and number of complications. In the watch community 72-hours is often regarded as the magic number for a power reserve as it makes a watch ‘weekend proof’. Put it down on Friday and pick it up again on Monday morning and you’re good to go.

In some cases even 72-hours of power reserve might not do the trick. If you have a collection of multiple watches and regularly switch from one to the other you can find yourself constantly resetting watches you haven’t worn for a little while.

This problem led to the creation of watch winders that keep the watch moving when not being worn. There is a lot of debate around the impact of watch winders and how beneficial they are for your watch. On the one hand they keep your watch from running out of power so you won’t need to reset or wind it whenever you pick it up. For complicated perpetual calendar watches this is a lifesaver as often these watches won’t have a quick set function. Instead it needs to be wound forward to the correct time, date and year. There are many horror stories of perpetual calendars that have been left idle for years and the only way to reset them is to wind the watch forward through the years until you reach the current date.

On the other hand, some argue that keeping a watch continually moving so the mainspring is fully under tension isn’t good for the long term health of a watch. Think of it like keeping the engine on in your car when you’re not using it. That said, for older vintage watches, watch winders can help keep everything functioning smoothly. If left idle for too long the lubricants and oils in an older movement could start to gunk up and not perform as they should. Sticking with the car analogy using a winder for an older watch is like taking a vintage car for a spin every now and then to keep it ticking over. You can see how these contrasts mean the jury is still split on watch winders. In order to find the right balance more sophisticated watch winders have a ‘turns per day’ or TPD function that allows you to choose winding and rest periods so it’s not constantly in motion.

An automatic rotor does have a clever engineering piece up its sleeve. If you’ve ever looked through an exhibition case back and tried to get the rotor to spin you’ll notice that the rotor can rotate both clockwise and anti-clockwise. The impressive engineering part is ensuring that in whichever direction the bidirectional rotor spins it charges the mainspring. Without this you can imagine how incredibly frustrating it would be to power the watch only when the rotor spins clockwise. The bidirectional rotor was first created in 1942 when watch movement manufacturer Felsa created their Bidynator calibre.

The operation of bidirectional winding is one of the big differences with automatic movements. In a manual wind movement the mainspring can only be wound when the crown is turned in a single direction. There are several different methods that allow for an automatic rotor to wind in both directions, most common is what’s known as a reverser mechanism. This system uses two disks that sit between the rotor and mainspring. Essentially, it allows only one disk to rotate whilst stopping the other disk. When the rotor spins in the opposite direction the disk’s roles are reversed. IWC have their own patented system called a Pellaton movement which uses cam and pawl arms to translate the rotor movement, regardless of which way it spins, into the rotation of a single winding wheel.

We’ve mentioned that the automatic rotor is weighted. This weight is important as it ensures that the rotor freely spins. The total weight of the rotor affects how easily it spins and how much power it can provide to the mainspring. In a watch with a particularly heavy rotor you can often feel the rotor moving throughout the day, which if too drastic can be a bit alarming.

In more advanced calibre’s brands have adopted a micro-rotor. The micro-rotor is far smaller than the watch diameter and sits as part of the movement rather than on top of it. Because of the reduced size the micro-rotor requires additional weight to ensure it still spins with the same efficiency. Hence why many micro-rotors are made from dense materials such as gold or platinum. As a result of the rotor being embedded in the movement, watches with micro-rotor’s are usually thinner. The Parmigiani Fleurier Tonda PF highlights this nicely with the calibre being just 3mm thick. The additional benefit of the micro-rotor is that it doesn’t obscure the view through an exhibition caseback. Instead allowing you to admire more of the movement and its finishing.

The obstruction of a rotor over the movement is something brands have tried to address over the years. Rather than use a micro-rotor, brands like Audemars Piguet have tried to improve the design of the traditional rotor. To mark the 50th anniversary of the Royal Oak they released a number of models which had semi-skeletonised rotors with ‘50 years’ cut into the middle.

Elsewhere brands like Carl F. Bucherer have led the way in peripheral technology. A peripheral rotor is one that orbits the periphery of the movement and prevents any obstruction at all. Where the peripheral rotor differs from centrally mounted and micro rotors is that they are not anchored or fixed to the movement. In 2009, after years of hiatus, Carl F. Bucherer revived excitement in the peripheral rotor with calibre CFB A1000. The CFB A1000 uses a system of ball-bearing rollers for increased efficiency and to aid rotation of the rotor with as little friction as possible. Other brands have since followed suit with Vacheron Constantin creating their own peripheral rotor in the Harmony Ultra-Thin Grande Complication Chronograph. This split-second chrono movement uses a solid gold rotor that orbits the outside of the movement. This increases the diameter of the watch but not its thickness.

The evolution of the mechanical movement to automatic has had a big effect on the watch world. Bypassing the need to wind a watch on a daily basis makes mechanical watches more user friendly. If worn regularly an automatic will easily outlast a smart watch before it needs charging and will in theory outlast a quartz battery.