THE TRAINING CENTER

Behind every mechanical watch that “keeps time” lies a fundamental principle: isochronism. That is, the ability of the regulating organ (balance) to maintain a constant rate, regardless of how much energy it receives or the conditions under which it operates.
It sounds simple. It is not.

What is isochronism in practice

In ideal conditions, the balance wheel would perform oscillations with a perfectly constant period. In other words, each “tick” would take exactly the same amount of time.
In reality, however:

• The torque from the mainspring is not constant
• The amplitude constantly changes
• The position of the watch affects its behavior
• Friction changes over time and with lubrication

Isochronism, therefore, is not given — it is something that must be achieved.

The core problem: energy is not constant

The main source of deviation is the mainspring itself.
When fully wound, it delivers higher torque. As it unwinds, the torque decreases. This directly affects the amplitude of the balance wheel:

High amplitude → different behavior of the hairspring

Low amplitude → increased sensitivity to errors (the mechanism tends to “gain” time) 

A well-regulated watch must remain as unaffected as possible by these variations. This is where isochronism comes in.

Classical solutions that remain relevant

Breguet overcoil
Abraham-Louis Breguet’s intervention remains perhaps the most influential.
The overcoil allows the hairspring to “breathe” more symmetrically, reducing errors caused by changes in amplitude and position.
It is still used today — not out of tradition, but effectiveness.

Fusee and chain
A mechanical way to “flatten” the torque curve of the mainspring.
Despite its complexity and size, its principle is extremely modern:
constant force = better isochronism.

Remontoir d’égalité
Perhaps the purest expression of constant force.
An intermediate mechanism stores and releases energy at fixed intervals, isolating the escapement from variations in the mainspring.

Modern watchmaking: materials instead of corrections

Today, much of the effort has shifted from geometry to materials.
Silicon
The use of silicon has changed the landscape:

• Geometric stability of the hairspring
• No need for lubrication
• Resistance to magnetic fields and temperature changes 

Brands such as Patek Philippe, Rolex, Omega, and Ulysse Nardin have heavily invested in this direction.

Higher frequencies

Increasing frequency (e.g., 4Hz → 5Hz and above) reduces the impact of micro-disturbances.

But it is not a “free” solution:

• Higher energy consumption
• Increased wear
• Greater lubrication demands 

Constant-force escapements

Modern implementations attempt to “lock” the energy delivered to the escapement.
They are technically impressive, but remain limited to high-end constructions.

Regulation vs design

Here we see an interesting shift.
Traditionally, isochronism was a matter of regulation:

• Poising 
• Timing
• Fine adjustment of the hairspring

Today, it is increasingly a matter of design and materials.
This does not mean regulation is unimportant - but the emphasis has shifted.

A practical conclusion

A watch with good isochronism:

• Does not change drastically between full wind and half wind (low power)
• Has a more stable rate across different positions
• Is more “predictable” in its behavior

And this is something a watchmaker recognizes immediately — not necessarily from a number, but from the overall behavior.

Epilogue

Isochronism is not a feature. It is not a marketing term.
It is the result of dozens of small decisions:
design, materials, and regulation.
And perhaps that is what keeps mechanical watchmaking interesting:
that even today, there is no single solution - only better approaches.