Most changes in a watch’s rate are small and gradual. Magnetism is neither. A watch that was running at a few seconds a day starts gaining minutes, sometimes a great deal more, and it happens between one day and the next.

It is also among the most ordinary things that can happen to a watch, and among the quickest to put right.

Why it makes a watch gain

The part that matters is the hairspring, a fine coiled spring that governs how fast the balance wheel swings back and forth. Its length sets the pace. A longer active spring swings slower, a shorter one swings faster, and the whole business of timekeeping rests on that length staying put.

When the spring becomes magnetised, its coils attract one another. That pull restricts how freely the spring can breathe in and out, and under a strong enough field adjacent coils cling outright. Either way the spring behaves as though it were shorter than it is, the balance swings faster, and the watch runs fast in proportion.

That is why the symptom is so distinctive. Not a drift of a few seconds but a gain of minutes, arriving suddenly.

What magnetises a watch

Almost nothing dramatic. The fields involved are small and completely commonplace.

  • Laptop and tablet cases with magnetic closures, and the magnets inside the machines
  • Speakers, including small portable ones and the ones in a phone
  • Magnetic phone mounts and charging pads
  • Cabinet and handbag catches
  • Fridge doors, tool holders, and anything else that closes with a satisfying click

Modern watches vary a great deal in how well they resist this, and some are built with materials that shrug it off almost entirely. Older watches, and plenty of current ones, are not.

You do not usually get to identify the culprit, and it does not much matter which it was.

The complication worth knowing

A sudden large gain has a second common cause, and from outside the case it looks identical.

A hard knock can leave a coil of that same hairspring caught on the regulating pins, or touching a neighbouring coil. The effect matches magnetism exactly, since the active length is shortened, the balance swings faster, and the watch gains hard. Same symptom, entirely different cause, entirely different remedy.

This is why guessing is the actual hazard here. Somebody who assumes magnetism, buys a demagnetiser and runs the watch through it will have achieved nothing if a coil is physically caught, and may have made things harder to diagnose.

What it looks like in the numbers

If you keep a record of your watch’s rate, magnetism is unmistakable, which is one of the quieter arguments for keeping one.

An ordinary series drifts gently. The readings sit in a cluster, moving a little with the seasons and with how the watch is worn. Magnetism does not do that. One reading sits with the others and the next is somewhere else entirely, ten or twenty times further from zero than anything before it.

The size and the suddenness together are the signature. A watch that has crept from +4 to +9 over a year is telling a different story from one that went from +4 to +240 over a weekend.

That distinction is only available if you have the earlier readings, and it is much more useful to a watchmaker than a description of how the watch has been behaving.

What to do

Take the watch to somebody who works on watches, and take the record with you if you have one.

Say when the change happened, how large it is, and whether the watch was dropped or knocked around the same time. That last detail is the one people forget to mention and the one that most helps separate the two causes.

Demagnetising, when that turns out to be the answer, is quick and cheap. It is one of the happier trips to a watchmaker, and the watch usually goes back to its previous rate as though nothing had happened.

What it does not tell you

A rate can show you that something changed suddenly and by how much. It cannot tell you whether a field or a knock was responsible, and neither can this page. That is a question answered with the case open.

Why your watch runs fast or slow covers the ordinary, gradual causes, which account for most of what a rate does most of the time.