Demagnetizing a mechanical watch

A mechanical watch that has been near a magnet can stay magnetized after the magnet is gone, and it usually shows it by running fast, often by many seconds or minutes a day. Demagnetizing puts it right. The watch is passed through an alternating magnetic field that is then withdrawn gradually, which leaves its steel parts without the magnetism they had picked up. It takes minutes, the case stays closed, and the watch usually returns to its former rate. If it does not, magnetism was not the whole story, and the movement needs a closer look.

What magnetism does to a movement

The parts that keep time are small, and several are steel. The most sensitive is the hairspring, the fine coil that makes the balance swing back and forth at a steady rate. Once it is magnetized, its coils attract one another and can cling together, which shortens the part of the spring that works and makes the balance swing faster. The result is usually a large gain, sudden and often erratic, though a loss is possible too. Magnetized steel in the escapement adds its own irregularity. Seiko’s guidance on magnetism describes the same thing: a magnetized hairspring makes the balance unstable, the watch gains or loses, and it stays magnetized after being moved away, until it is demagnetized.

Everyday magnets are enough. Seiko names phones and their speakers, the speakers of televisions, computers and tablets, handbag clasps, magnetic tablet covers, hair dryers and electric razors. A watch left on or beside one, overnight on a speaker or in a bag against a magnetic clasp, is the usual story.

How to tell whether a watch is magnetized

Two simple checks point to it. The first is the rate. A watch that has kept good time and suddenly gains heavily, with nothing else changed, is a likely case, and a week of measurement, as why a mechanical watch runs fast describes, gives a figure worth having. The second is a compass. Hold an ordinary magnetic compass still on a table and bring the watch slowly past it, around the case and close to the crown. Ordinary steel can nudge the needle slightly; a needle that swings sharply, or turns to follow the watch, means something in it is magnetized. A phone’s compass app reacts to the same thing, but less dependably.

A watchmaker can confirm it with a magnetism detector and on a timegrapher, which shows the rate and how far the balance swings, its amplitude, in several positions.

How demagnetizing works

A demagnetizer is a coil that produces an alternating field, reversing its direction many times a second. In it, the watch’s steel parts are magnetized one way and then the other, over and over. What matters is how the field ends. If it weakens gradually, because the watch is moved slowly away or the device fades it out, each reversal is a little weaker than the last, and the parts are left with no magnetism of their own. If the field stops abruptly while the watch is still in it, the watch can be left magnetized, sometimes more than before. That is why the technique matters more than the device.

Devices differ. Some are pads that the watch rests on while a button is held, and is then drawn slowly away from; some are tunnels the watch passes through; some fade the field out by themselves. Each comes with its own instructions, and they are not interchangeable.

Trying it at home

Inexpensive demagnetizers are sold for home use, and on a mechanical watch that is plainly magnetized, used as its instructions say, one can work. A few precautions help.

• Confirm magnetism first, with the rate and the compass. Magnetism mostly makes a watch gain; a watch that loses time, stops, or runs erratically whatever its position is more likely to need a service.
• Keep the demagnetizer away from bank cards, other watches and phones while it is on, and do not use one near anyone with a pacemaker or another implanted device.
• Check again afterwards with the compass, and measure the rate for a day or two.
• Leave a quartz watch, a watch of real value, and any watch that does not respond to a watchmaker.

If the gain remains after a careful attempt or two, stop there. The cause may be a hairspring disturbed by a knock, oil on the hairspring or a regulation that has drifted, and a demagnetizer reaches none of them.

Quartz watches are different

An analog quartz watch drives its hands with a tiny step motor that works by magnetism, so a strong field can upset it: while it is close to the magnet it may stop, lose or gain. Unlike a mechanical watch, it usually recovers once it is moved away, and only the time needs resetting. Seiko advises having one demagnetized only after an extremely strong field. A digital quartz watch has no motor, and magnetism generally does not affect it.

What a watchmaker does

A workshop checks first, so that demagnetizing is a conclusion rather than a guess. The watch is tested for magnetism, demagnetized on a bench unit and tested again; then it goes on the timegrapher, in several positions, to confirm that its rate and amplitude have come back. If they have not, it needs a diagnosis rather than another pass, and any further work is a separate decision. Demagnetization is part of the bench’s movement work at Viattori: the assessment and the estimate are free, nothing is done before you approve it, and small repairs are usually ready in one to two business days.

Keeping it from happening again

Distance does most of the work. Seiko suggests keeping a watch more than five centimetres from magnetic products, and not leaving it beside one for long: off speakers and laptops, away from magnetic phone accessories and tablet covers, and out of bag pockets with magnetic clasps. Some watches resist magnetism far better than others, through hairsprings of non-magnetic materials or a soft-iron inner case, and ISO 764 sets the test a watch must pass to be called antimagnetic. That is a level of resistance, not immunity, so the same habits apply.

Questions

How do I know if my watch is magnetized?

A sudden, large gain in a watch that used to keep good time is the usual sign. To check, bring the watch slowly past a magnetic compass held still: if the needle swings sharply or follows the watch, part of it is magnetized. A watchmaker can confirm it on a timegrapher.

Can demagnetizing damage a watch?

Not when it is done properly. The watch passes through an alternating field that fades gradually, which leaves its steel parts unmagnetized without touching the movement, and the case stays closed. Done carelessly, it can leave the watch still magnetized, so the check afterwards matters.

Can a quartz watch be magnetized?

A strong field can stop an analog quartz watch, or make it lose or gain, while it is near the magnet, because its step motor works by magnetism. It usually recovers once moved away, and only the time needs resetting. Seiko advises demagnetizing one only after an extremely strong field.

Will demagnetizing fix a watch that runs slow?

Usually not. Magnetism mostly makes a watch gain, though it can cause a loss. A watch that loses time, stops or runs erratically in every position is more likely to need a service, and demagnetizing it only rules magnetism out.