Constant Force Tourbillon

Constant Force Tourbillon

A trifold approach to chronometry
Its historical roots, aesthetic construction and true-beat seconds ascribe the Arnold & Son Constant Force Tourbillon to the tradition of ultra-accurate marine chronometers. Its practicality – in terms of both readability and accuracy – has been clearly proven.

When John Arnold developed marine chronometers that would mark the history of navigation in the 1780s, he had to create high-precision systems that had both a long lifespan and an appropriate display scale to ensure the utility of the chronometry they provided. He therefore decided on a constant force mechanism paired with true-beat seconds. 

Constant force involves inserting an energy-storage sub-system into the gear train just before the escapement. Supplied at regular intervals, this secondary reservoir produces extremely regular torque as it is released in consistent, small doses. It therefore differs from the barrels’ energy flow, which is delivered over a long period of time and naturally fluctuates. The balance spring thus receives an invariable source of energy, hence the name “Constant Force”.

The true-beat seconds is a consequence of this mechanism. As the constant force system is reloaded 60 times per minute, it is possible to display a second that jumps forward from marker to marker by simply attaching a hand to the winding wheel. Despite being a common feature of quartz watches, it remains a rare feat in mechanical timepieces.

On maps

This precise reading proved extremely useful for maritime positioning in the 18^th century. The difference between the local time (measured by the position of the sun) and the reference time (kept by a clock) gave the distance travelled along the east-west axis of the earth. Or in other words, a vessel's longitude. The creation of marine chronometers allowed captains to know their position and, more importantly, to know how far away they were from a deadly reef or shallow waters. 

Given the scale of transoceanic navigation, every second counted, as each represented a potential error of hundreds of miles. There was no point in a timepiece being accurate if its information display was not equally as precise. Because a true-beat seconds hand stays immobile for an entire second, its readings were all the more accurate.

In its design

230 years later, Arnold & Son continues to use this device in a model that incorporates all existing solutions for supreme chronometric results: the Constant Force Tourbillon. Its A&S5119 calibre adopts the same characteristic layout as John Arnold's marine chronometers. It testifies to the mechanism's inner arrangement through technical prowess and symmetry, which go hand-in-hand with performance and precision. 

The two series-coupled barrels are located at 11:30 and 1:30. When the force supplied by the first barrel falls below a predefined threshold, the second barrel reinforces it to smooth out the central – and longest – part of the torque. In addition, the A&S5119 calibre features a patented constant force wheel at 7:30, which is supplied with power every second. It can be seen under the true-beat seconds hand (a complication that is naturally also included) and can be identified by the anchor that acts as the winding mechanism's satellite carrier – a symbol that is precious to Arnold & Son in relation to its maritime heritage.

This constant force wheel supplies the regulating organ of the Constant Force Tourbillon, a one-minute tourbillon that beats at a frequency of 3 Hz and is located at 4:30. By rotating and adopting every position in order to establish an average, the system reduces the detrimental effects of gravity on the parts in the regulating organ. As well as lending prestige to the watch, it also constitutes a source of chronometry with high added watchmaking value.

In history

The combination of the three approaches with the barrels and the escapement in the space results in almost zero variation in amplitude over the entire power reserve of 90 hours – an extremely rare achievement. And yet, such consistency is a major determining factor in rate precision.

Measurements made using the Constant Force Tourbillon show an average daily rate variation of around 3 seconds. This is an error rate of less than 4 in 100,000. Once again, this Arnold & Son timepiece illustrates the power that the cultural and historical heritage of chronometry continues to wield.

Technical specifications

Functions

• hours, minutes, true-beat seconds
  

Movement

• Calibre: A&S5119, hand-wound mechanical
  
• Jewels: 39
  
• Diameter: 36.80 mm
  
• Thickness: 6.00 mm
  
• Power reserve: 90 hours
  
• Frequency: 3 Hz / 21,600 vph
• Decoration                          
  Rhodium-plated palladium bridges, grey NAC-treated main plate, hand-polished and -chamfered bridges, hand-polished and -chamfered circular satin-brushed wheels, gold chatons, bevelled and mirror-polished screws, hand-chamfered and mirror-polished tourbillon carriage.                                       

• Dial:
  Anthracite chapter ring, applied hour-markers

Case

• Material: 18-carat red gold (5N)
  
• Diameter: 46.00 mm
  
• Crystal: Domed sapphire with an anti-reflective coating on both sides
  
• Back: Sapphire crystal
  
• Water-resistance: 30 metres / 3 ATM
  

Strap

• Material: Hand-stitched bottier alligator leather
  
• Clasp: Pin buckle, 18-carat red gold (5N)
  

Reference                           

• 1FCAR.B01A.128R
  

Limited edition                    

• 28 pieces    

Astronomy, Chronometry and World Time

Arnold & Son's three founding principles

Throughout human history, measuring time has always referred to the stars. It was by observing certain stars and understanding their cycle that the first calendars were established with impressive accuracy. It took several millennia before this precision was enclosed in a timepiece like the ones designed by John Arnold. 

The golden age of maritime explorations and discoveries ushered this precision into a new technical ideal – determining longitude at sea. Its immediate corollary was the identification of local time, which changed constantly as the observer moved along an east-west axis. Astronomy, chronometry and what we now call world time are thus inextricably linked within one and the same question, to which John Arnold and his son devoted their lives, their art and their genius. 

This is how these three dimensions – astronomy, chronometry and world time – have come to be embodied in the House's contemporary timepieces.  Echoes of John Arnold's inventions and preoccupations, these pillars represent the foundations on which the Arnold & Son collections are based.

Chronometry: Be accurate

Rate accuracy, which is known as chronometry, is the key requirement of Arnold & Son's contemporary watchmaking. It is the standard of excellence for its collections, the first condition to be met and constantly checked, whether it is at the forefront or in the background of a watch designed by Arnold & Son. It is the most discreet of a movement's characteristics. 

When building an Arnold & Son collection, all the thinking is focused on this chronometry. The manufacture calibres are based on advanced technical fundamentals that are not necessarily the best known. One of them is the choice of small, lightweight balances capable of rapidly returning to their isochronous rate after the latter has been disturbed by inevitable everyday shocks. Another is the routine use of large barrels or even two series-coupled barrels to store the energy required for the movement to function.  They consequently provide Arnold & Son's manual winding calibres with above-average power reserves of 90 hours and more. A third is the particular attention paid to manufacturing the gear trains, roller-burnishing the pinions and polishing the gear teeth, as well as the precision of the machining and therefore the relative positions of the moving parts, a key concept in rate accuracy.

The tourbillon (or the fact of putting the regulating organ in rotation on its axis to best adjust the effects of gravity on the balance and its hairspring) was patented after John Arnold's death, but it was undoubtedly at the heart of his chronometric research and discussions with his friend Abraham-Louis Breguet, who incidentally assembled his first tourbillon on a John Arnold pocket watch in homage to this great watchmaker. Nowadays, the tourbillon has become a must in Arnold & Son collections.

While the tourbillon was not a complication in John Arnold's time, constant force underpinned the design of his marine chronometers. The regularity of the rate of the sprung balance relies on the consistency of the energy that it receives. However, this naturally fluctuates due to the circular and spiral nature of the mainspring contained in the barrel. To achieve a perfectly smooth torque, in other words a constant force, Arnold & Son uses a one-second constant-force mechanism. Housed just before the escapement, it stores up a small but always equal amount of energy in a secondary spring, the remontoire. Thus, every second, the sprung balance receives very precisely the same force to power its oscillations. These become more even, thereby creating the conditions for a high-precision rate.

Arnold & Son also works with another of John Arnold's historical chronometry indicators: deadbeat seconds, a mechanism that was indispensable to navigators at the time for calculating longitude. This mechanism advances one step each second, rather than six or eight smaller jumps in sync with the frequency of the balance. Instead of the term deadbeat seconds, Arnold & Son prefers a name whose very sound means accuracy: “true beat second”. Its jump is a signature of John Arnold's marine chronometers and a complication that is still alive and well in the House's collections.

Astronomy: Under the sky

The Pole Star, Southern Cross, astrolabes and sextant: measuring time has relied on the recurrence of astronomical phenomena in order to find long, reliable points of reference there that can be used under any conditions. The fruit of the human ingenuity, patience and dedication of countless observers and astronomers from every culture, these markers of cyclical time are foundational for Arnold & Son watchmaking.

Astronomical complications are a signature of Arnold & Son collections, with the moon as the heavenly body of choice, main subject and major inspiration. The distinctive feature of the Brand's moon phase timepieces is that they feature “astronomical” display precision as a matter of course. This term corresponds to an accumulated one-day deviation in the moon phase display every 122 years. Since setting this complication requires great finesse, the Arnold & Son moons generally have a double display with a secondary indicator on the case back next to the movement. This extremely rare display bears witness to Arnold & Son's involvement in all types of development and reflects its favoured themes, which are as much chronometric as astronomical.

Over and above their precision, Arnold & Son's moon phases attract all the light, either with a large moon opening up over half the dial, or with a 12-mm three-dimensional rotating moon, making it the largest of all moons. Whether in two or three dimensions, the moon is always treated as a small work of artistic craftsmanship, composed of materials that are rare in watchmaking such as marble or Paraíba tourmaline, or delicate such as mother-of-pearl, meteorite or aventurine glass. The Arnold & Son moon also shines at night, often with a subtle addition of luminescent material. 

World Time: Here, elsewhere, everywhere

With ocean navigation, humankind divided up the world and invented longitudes, which were calculated by comparing the local time, observed using the sun, and the time at a starting point, kept by an extremely reliable timepiece. John Arnold was one of the leading suppliers of chronometers to the British Navy. He was the one who successfully improved the reliability and simplified the production of these indispensable marine chronometers, so much so that he became a benchmark among great explorers such as James Cook and later Dr David Livingstone. The indication of several time zones is therefore integral to Arnold & Son's watchmaking identity.

As this navigation was itself inseparable from cartography, for this world time complication Arnold & Son has chosen to depict a three-dimensional terrestrial hemisphere, making it possible to tell what time it is at any point.

In parallel to this graphic vision, Arnold & Son has developed a second approach to the time elsewhere in the world: with a double tourbillon featuring two distinct rotating regulating organs, making it possible to follow time zones offset by 15, 30 or 45 minutes compared to a full hour – a freedom in terms of setting that remains extremely rare. 

Once again, and because this double display is based on a profoundly chronometric complication, Arnold & Son’s fundamental principles are interwoven. One never advances alone; there are always two – if not three – together. This is how a pillar's strength is measured: it relies on the next one, creating the conditions for a solidity that stands the test of time.