Looks can be deceiving. With its almost perfectly symmetrical layout and architectural construction, it would be easy to dismiss the Arnold & Son Constant Force Tourbillon as anything more than just another handsome tourbillon from the revived brand. But in fact, the Constant Force Tourbillon is arguably Arnold & Son’s most accomplished timepiece to date and definitely one of the more interesting tourbillons I’ve seen of late. Here’s why.


To better appreciate what the Constant Force Tourbillon brings to the table in terms of horological merit, one must begin with the problem it attempts to solve, and that’s constant force or more precisely, achieving isochronism.

One of the biggest hurdles in mechanical watch movements is energy transmission and the effect A constant force mechanism is meant to alleviate such variances by transmitting energy from mainspring barrel to the escapement in quantifiable and exact “doses” rather than a continuous stream that starts out strong then slowly lessens as the mainspring unwinds and the torque drops.

When I come across the term “constant force”, the first image that comes to mind is that of a miniature bicycle chain in a watch movement, the so-called “chain and fusée” mechanism.  While visually quite impressive given its sheer scale, the fusée and chain setup isn’t actually all that efficient as a constant force mechanism. Besides the fact that just the movement of the chain causes a lot of friction and energy loss, it cannot be adjusted or regulated. You’re stuck with what you get.

And that’s probably why this Arnold & Son’s piece is somewhat overlooked: it doesn’t have an immediately identifiable constant force mechanism, and definitely no chain to be seen anywhere in the movement. That is, until you take a closer look at the small seconds open sub-dial… I’ll get back to that.


As Arnold & Son’s head of movement development Sebastien Chaulmontet explained to me, a constant force mechanism is not unlike a water filter. If you place the filter close to the source, by the time the water reaches your glass it would have lost some of its purity along the way. By filtering the water just before its final destination however, you’re guaranteed a cleaner, more thoroughly filtered drink. In the same respect, having a constant force mechanism integrated into a movement’s regulating organ makes a lot more sense than anywhere else.


That’s why Arnold & Son took a two-prong approach in trying to achieve constant force and isochronism in this piece, and it all starts with the barrels and mainsprings. Instead of having two barrels to simply double the power reserve, the second barrel feeds energy solely to the first barrel as its mainspring unwinds, ensuring that the energy output to the gear train is more, you guessed it- constant. Such a feature can be found in other Arnold & Son pieces, such as the Time Pyramid. As such, only one of the barrels is actually directly connected to the gear train.

The Constant Force Tourbillon has a power reserve of 90 hours when fully wound, though it’s theoretical power reserve last for well over 100 hours. The system simply stops when the power reserve and torque drop to a certain level, again providing a more constant energy flow throughout the duration of the power reserve.


Now this is where it gets really interesting, in a multiple patent sort of way. But first, here’s a video demonstrating the Constant Force Tourbillon in action:

The second and truly outstanding constant force mechanism lies in the small seconds sub-dial jus opposite the tourbillon. Instead of energy flowing directly from the mainsprings and gear train into the escapement, it is first regulated by a unique kind of remontoir. This patented mechanism features a spring that winds then releases fixed bursts of energy into the tourbillon every second. The anchor-shaped device makes a full rotation every minute in one second increments, driving the small dead-beat seconds hand as it goes along.

Coming back to the barrels and theoretical power reserve I mentioned, the remontoir stops “charging” altogether once the power reserve drops below a certain level. This ensures that the power output from the constant force mechanism to the tourbillon is always the same, at the cost of a few hours of power reserve.

The real advantage with this setup is that unlike a chain and fusée where the chain cannot be regulated, the spring can be adjusted and fine-tuned so that the tourbillon escapement is fed the same precise amount of energy it requires for optimum isochronism throughout the duration of the mainsprings’ wind-down.

Another noteworthy details include the actual shape of the tourbillon’s balance wheel, which isn’t exactly round but more of a battle-axe shape with 4 GyroMax-style adjustable poising weights and gold masses on each end, which I imagine offers better inertia and aerodynamics than a standard round balance wheel.

There’s a certain paradox between the smooth rotation of the tourbillon escapement and the jumping action of the constant force mechanism and dead-beat seconds that adds to the overall visual appeal of the piece that doesn’t lack for animation in the least.


To me at least, the Arnold & Son Constant Force Tourbillon represents the culmination of the ambitious brand’s technical prowess while solidifying their design identity with easily recognisable symmetrical movements. Not only is the constant force mechanism well thought out from the winding all the way to the escapement, the design and overall execution is. It’s easy to go overboard when you’re fitting in that much horological goodness into one watch, yet Arnold & Son have succeeded in keeping it a very balanced and, in spite of all the visible bridges and components and the lack of a dial, a clean and legible piece.

The Constant Force Tourbillon is limited to 28 pieces in red gold and is priced at $197’500 USD.


More information on www.arnoldandson.com