A magnetised hairspring makes the watch run fast because adjacent coils attract and the spring behaves as if it were shorter. The check takes a minute and needs a compass, not a screwdriver.
How to tell whether a hairspring is magnetised
issue ISSUE 01written 2026-08-15responsible Escapement Papers (a named bench, not a person)
tape card: magnetism against other rate faults
| Symptom | Magnetism | Other cause |
|---|---|---|
| Sudden large rate error | Likely | Regulator moved, or a knock |
| Amplitude healthy | Yes | A tired train would show low amplitude |
| Rate improves when demagnetised | Confirmed | Not magnetism |
| Position-dependent error | Possible | Poise, or a stud rotated |
The check
- Hold a compass near the watch, a few centimetres away, and watch the needle. A watch that is magnetised will pull it.
- Take a rate reading, with the lift angle set, and write it down.
- Demagnetise the watch. This takes seconds and costs nothing.
- Take the rate reading again, same position, same angle.
- If the rate has moved towards normal, the diagnosis is confirmed. If it has not, the fault is elsewhere.
Why a shorter spring is a faster watch
Adjacent coils of a hairspring normally do not touch. Magnetised, they attract, and the spring behaves as though its effective length were shorter. A shorter spring is stiffer, and a stiffer spring gives a shorter period, which is a faster rate. Nothing has changed shape, which is why the fault is invisible.
| Object that magnetises a watch | How close | Typical result |
|---|---|---|
| Magnetic clasp on a bag or strap | Touching | Rate fast by tens of seconds |
| Tablet cover magnet | Centimetres | Rate fast, may fade with distance |
| Speaker driver | Centimetres | Rate fast while nearby |
| Phone case magnet | Touching | Rate fast, sometimes intermittent |
Why demagnetising comes first
Regulating a magnetised watch moves the regulator to compensate for a fault that will disappear the next time the watch is demagnetised. The watch then runs slow. The bench's order is therefore: demagnetise, re-read, then decide. That order costs a minute and removes a whole class of error before any adjustment is made.
The bench holds that a sudden rate change in a watch with healthy amplitude should be treated as magnetism until it is ruled out. Some watchmakers will argue that the compass test is unreliable for weak fields; the bench's answer is that the demagnetise-and-re-read test is the one that decides, and the compass is only the screen.
References
- Ref 1Hairspring, Horopedia – Watchmaking Encyclopediasupports the hairspring description that explains the magnetic effect
- Ref 2Hairspring, Fondation de la Haute Horlogerie encyclopediasupports the rate behaviour attributed to a hairspring fault
- Ref 3Surface treatments, Horopedia – Watchmaking Encyclopediasupports the materials and surface context in which magnetic parts are identified
One link, one document, titled as it titles itself. No home pages, no search results.
Set this page down Set this page down when the re-read has told you whether the rate moved.