Demagnetizer

Specification

Technical parameters

Overview

A demagnetizer drives a coil with alternating current to produce a strong alternating magnetic field. The magnetised part is placed in that field and slowly withdrawn, so the field it experiences falls to zero while alternating — which walks the magnetic domains back to a random, neutral state.

Magnetism is one of the most common causes of a sudden large rate error. A magnetised hairspring's coils attract each other and effectively shorten the active length, so the balance oscillates faster and the watch gains — often by minutes per day. Everything else about the movement can be perfect and the watch will still be unusable.

The procedure is easy to get wrong in one specific way: withdrawing too quickly. If the part leaves the field before the field has decayed, you leave residual magnetism behind. Withdraw slowly, over several seconds, and keep the part moving away in a straight line.

What matters about it

• Fixes the most common cause of a sudden large rate gain
• Works on a complete movement or a single part
• Withdrawal speed determines whether it works
• Result should be verified, not assumed
• Coils heat — duty cycle is limited

Practical notes

• Withdraw slowly over several seconds, in a straight line.
• Verify with a compass or magnetometer afterwards.
• Treat the whole movement rather than guessing which part is magnetised.
• Check for a magnetic source in the environment — a speaker, a clasp, a phone case — or the problem will return.

Compatible with