Steel components in a watch can become magnetized when exposed to a magnetic field, and the most likely source is an owner who unknowingly gets near a permanent magnet or high power electrical cable during normal day to day life.
Magnetism can also be introduced during your service to the watch through tools and ultrasonic cleaners.
Magnetism of steel components, mainly the balance cock or steel hairspring will affect the rate of a watch. In extreme cases the coils of the hairspring will stick together as the balance vibrates.
When this happens, the watches rate will Gain considerably, possibly as much as an hour a day due to the effective shortening of the hairspring.
In 20th century and later watches, the balance and spring are generally made from non-metallic alloys. In some recent watches the balance and balance spring are made of silicon. For the most part, these would not be affected by magnetism.
There is no consensus on a reliable method to check for magnetism and is highly likely to introduce magnetism by the very test itself or using an ineffective demagnetizer when no magnetism was present. A small compass can be placed on the watch and may twitch, but this cannot be considered a reliable test.
If you suspect the balance spring is magnetized, start by thoroughly cleaning the spring to eliminate the possibility of any sticking caused by residual oil on the hairspring. Once that has been done, gently push the coils together to see if there is any tendency for them to stick together.
As a General Rule
- All mechanical watches with steel balances should be demagnetized with a quality demagnetizer following the manufacturer’s instructions after service.
- Watches containing electrical components should never be placed in a demagnetizer. If magnetism is suspected the watch should be disassembled so that all the electronic components can be removed.