1. The Hairspring and Its Job
In a mechanical watch, the balance wheel swings back and forth, and the hairspring returns it to its centre position each time. The regularity of that swing sets the rate of the watch. Any change in the spring's shape or centre of gravity changes the timing, so the geometry of the spring matters as much as its material.
2. What the Breguet Overcoil Does
The Breguet overcoil is a terminal curve at the outer end of the hairspring. It raises the outer coil above the rest of the spring and bends it inward, so that the spring expands and contracts concentrically rather than off-centre. Abraham-Louis Breguet developed the form in the eighteenth century, and it remains widely used in fine watches.
Hairspring features and their roles (qualitative)
| Feature | Purpose | Effect on timing |
|---|---|---|
| Terminal curve (overcoil) | Keeps the spring concentric | Reduces error caused by uneven breathing |
| Flat, even coils | Consistent spring force | Supports isochronism across the swing |
| Precise collet attachment | Fixes the spring centrally | Helps keep the centre of gravity stable |
3. Isochronism and Why It Matters
Isochronism means that the balance takes the same time to complete a swing whatever its amplitude. A perfectly isochronous spring would keep the watch steady as the mainspring winds down and the amplitude changes. No real spring is perfect, so watchmakers combine geometry, material and adjustment to get as close as possible.
4. Checking and Adjusting
Watchmakers check the spring by observing the rate of the watch in several positions and at several amplitudes, then adjust the balance or the spring to reduce the differences. Careful regulation is slow work, and the results depend on the whole movement, not on the spring alone.
5. Reading Claims About Accuracy
Accuracy claims in watchmaking should be read with care. Rates are usually given as seconds gained or lost per day, measured under specific conditions. Ask which positions and temperatures were tested, and whether the figure was measured on the finished watch or on a component.
6. Thermal Compensation
Temperature changes affect a hairspring and balance in two ways: the spring becomes softer or stiffer, and the balance changes diameter slightly. Watchmakers counter these effects with thermal compensation, using bimetallic balance rims or alloys chosen so that the changes partly cancel. The method is a matter of balance and spring design, and the results are judged on the complete movement.