Magnetism and Watches: Causes and Fixes

Magnetism is the most common cause of a sudden, large rate error, and it is the cheapest fault in horology to fix. A watch that was running to a few seconds a day yesterday and is gaining five minutes a day today is magnetised until proven otherwise.

What magnetism does to a hairspring

A hairspring is a coiled steel spring. Magnetise it and its coils attract each other. They clump and, in doing so, the spring effectively becomes shorter — and a shorter hairspring oscillates faster. The balance beats faster, the watch gains, and it gains by a lot: minutes per day is typical, not seconds.

The mechanism explains the two diagnostic signs. The error is a large gain, not a loss, and it is roughly consistent across positions, because magnetism does not care which way the watch is facing.

• Sudden onset — it was fine yesterday
• Always a gain, never a loss
• Large — minutes per day
• Similar in every position
• Sometimes the coils can be seen sticking together under magnification

Where it comes from

Everyday magnetic sources are stronger and closer than they were a generation ago. The usual suspects are worth knowing because the fix is useless if the source stays next to the watch.

• Laptop and tablet closures with magnets
• Phone cases and magnetic mounts
• Magnetic clasps on bags and wallets
• Speaker drivers, including small Bluetooth speakers
• Induction hobs and some kitchen appliances
• Airport and retail security equipment — less often than folklore suggests

How demagnetising works

A demagnetizer runs alternating current through a coil to produce a strong alternating magnetic field. You place the watch or part in that field and withdraw it slowly. As the distance increases, the field the part experiences falls while still alternating, which walks the magnetic domains back to a random, neutral arrangement.

The single most common mistake is withdrawing too quickly. If the part leaves the field before the field has decayed, you leave residual magnetism behind. Withdraw over several seconds, in a straight line.

Prevention and limits

Some modern movements use silicon hairsprings, which are not ferromagnetic and are therefore immune. Others use alloys with improved resistance but not immunity. Steel hairsprings are fully susceptible, and steel hand tools can carry magnetism into a movement — which is why anti-magnetic tweezers and screwdrivers exist.

A persistent case that does not respond to repeated treatment usually means a part with high retentivity, or a second magnetised component that was not included in the first pass. It occasionally means the fault was never magnetism at all — so always confirm on a timegrapher rather than assuming.