Just a quick post today on semi-technical watch stuff. Not really worthy of BoneDoc's 'Roff the Prof' title I'm afraid
. However, I was asked about what happens when a watch becomes magnetised, and that’s fairly straightforward. That said I am not a metallurgical engineer – so I can talk to the watch bits, and I apologise if I get the magnetism bits a little screwy!In theory any part of a watch can become magnetised, however some elements are easier to magnetise than others. As a general rule the larger and harder the metal the more difficult it is to magnetise, and the softer and finer it is the easier it is to magnetise. Of course it also needs to be a ferrous metal.
In a watch there are a number of pieces that might become magnetised in the presence of a strong magnetic field, but in most cases it makes no difference. If plates become magnetised it doesn’t affect the watch in any practical sense at all. Even if the mainspring becomes magnetised the impact is minimal – it may not be within COSC specs, but it’ll only be very slightly out.
The problem starts when the balance spring becomes magnetised. This is very easy to do because it is so fine. As I previously explained in this post - viewtopic.php?f=6&t=8009, the balance spring is a part of the escapement that controls the way that the mainspring unwinds and regulates the timekeeping of the watch.
The balance spring is a kind of torsion spring that rocks back and forth at a fairly fast rate – 4 times a second in a Breitling. The spring is a loose spiral that can be tightened / shortened or loosened / lengthened to make micro adjustments to the timekeeping of the watch using the index.
If a watch is running fast then the spring is loosened, to slow down its rate of back and forth movement and get the rate closer to 4 times a second, and if it is running slow then the spring is tightened.
When this balance spring becomes magnetized the fine metal that spirals around is attracted to itself (because it spirals the metal of the spring is close to another part of the same piece of metal. This has the effect of reducing the amplitude – i.e. the spring doesn’t move so far in each direction of it’s movement, which in turn means that it makes more back and forth movements, causing the watch to run dramatically fast. Of course the stronger the magnetism the faster it runs.
There are a number of ways that modern watch manufacturers try to prevent this from happening – shielding in some cases, or more often these days reducing or eliminating the use of ferrous metals in the balance spring. The arrival of alloys that go by amazing names like Glucydur and Nivarox helped, although these are more accurately referred to as magnetism resistant rather than anti-magnetic.
There are other potential problems caused by magnetism, but well over 90% of magnetised watches have a magnetized balance spring.
The good news is that there is absolutely nothing fundamentally wrong with a magnetised watch – as soon as the watch is degaussed then it will be as good as new. Usually degaussing is performed by placing the watch in the middle of an electro-magnetic coil through which an alternating current is passed. This effectively creates a strong magnetic field that is constantly reversing polarity which has the effect of demagnetising anything placed in the field.
