The hairspring is the spring that returns the balance. Its stiffness sets the period, so anything that changes its effective stiffness changes the rate — including magnetism, which changes nothing you can see.
Hairspring: stiffness, magnetism and rate
issue ISSUE 01checked 2026-09-16responsible Escapement Papers (a named bench, not a person)
tape card: rate faults traced to the spring
| Symptom | Likely cause | Check |
|---|---|---|
| Runs fast, then settles | Magnetised, or a coil touching | Compass test, then demagnetise |
| Position-dependent rate | Coil not flat, or stud rotated | Look at the spring from the side |
| Slow and low amplitude | Weak or tired spring | Amplitude across the wind |
| Erratic, then normal | Coil rubbing on a bridge | Clearance under the balance |
Stiffness is the only thing the spring controls
The hairspring is a flat spiral that supplies a restoring force proportional to how far the balance has turned. Its stiffness, with the balance's inertia, sets the period. Every legitimate rate adjustment either changes the effective length of the spring or changes the inertia of the balance. Nothing else in the watch is a rate adjustment.
| Fault | Mechanism | Typical rate effect |
|---|---|---|
| Magnetised | Coils attract each other, shortening the effective spring | Runs fast, sometimes very fast |
| Coil touching itself | Effective length shortens | Fast and unstable |
| Stud rotated | Spring is not centred, breathing is uneven | Position-dependent error |
| Fatigue or corrosion | Stiffness falls | Slow, with falling amplitude |
Why magnetism is the fault that fools people
A magnetised hairspring looks normal, moves normally, and produces a rate error that comes and goes. The mechanism is simple: adjacent coils attract, the spring behaves as if it were shorter, and a shorter spring is stiffer, so the watch runs fast. Demagnetising restores it in seconds, which is why the bench's first question about a sudden rate change is whether the watch has been near a magnetic clasp.
A rate that is wrong by tens of seconds per day and improves when the watch is placed on a demagnetiser is a magnetised spring. A rate that is wrong by tens of seconds and does not improve is not.
Boundary: the hairspring is not the mainspring
The mainspring stores energy in the barrel and drives the train. The hairspring is part of the oscillator and drives nothing. The two are both springs, both flat spirals in some movements, and completely different parts with different failure modes.
The misreading worth naming: blaming a low amplitude on the hairspring. Amplitude is set by the energy the escapement delivers and the friction in the train. A tired hairspring changes the rate; it does not, by itself, starve the balance of energy.
The bench's contestable claim
The bench holds that every watch with a sudden rate change should be demagnetised before it is regulated, because regulating a magnetised watch moves the regulator to compensate for a fault that will disappear. The claim is contestable and it is testable in about a minute.
References
- Ref 1Hairspring, Horopedia – Watchmaking Encyclopediasupports the description of the hairspring and its function
- Ref 2Hairspring, Fondation de la Haute Horlogerie encyclopediasupports the rate behaviour attributed to the hairspring
- Ref 3Correction of the daily rate, Horopediasupports the rate-correction procedure that a spring fault invalidates
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Set this page down Set this page down when you have ruled magnetism in or out. It is the cheapest test on the list.