This thread would normally go into the watchmaking section, but I am thinking here more about vintage timepieces which do not run correctly and could be affected by magnetism and how to check this and what to do about it in case of need.
There are different opinions about how much a watch can be affected by magnetism, to what extend, what causes it and above all, we are told that modern materials, especially for hairspring, such as NIVAROX, would make any discussion obsolete anyway.
He, who does not believe in the bad influence of magnetism on a watch, must now be convinced: We got an opinion on the matter from one of the highest of the highest authorities, from Jaeger-LeCoultre. Read it and then follow my humble elaborate. The ones who know all this already might find at least an interest in how and what I did.
Watch magnetism | Jaeger-LeCoultre
Image 1: For testing purposes, I have taken and old watch from my late mother. Sorry Mom, it’s the cheapest I have. But if I would have screwed up, I would have given it away for a repair – promise!
Image 2: I put the watch on the micro of the timegrapher to have the present reading for comparison.
Image 3: Initial readings on the timegrapher.
Image 4: How do I know that my watch is affected my magnetism (not just recognizing that it is running fast?). Like Jaeger-LeCoultre suggested, simply take a compass and see if the needle goes astray when the watch comes near. I have somewhat refined the process by downloading a compass app on my smartphone and one which gives me also the strength (STAERKE) of the magnetic field (here in µTesla). You have to find the sensor on your smartphone by moving around a magnetic object. Calibration is not necessary here, as we do not need to have exact measurements.
Image 5: I then magnetized the watch by putting it next to a strong magnetic soure. It could be anything like a loudspeaker etc. etc. - see article from Jaeger LeCoultre. I do not recommend this thing, which does both magnetizing and de-magnetizing, for de-magnetizing watchmaker's screwdrivers. It works excellent on larger screwdrivers but does not always get rid of magnetism completely. I tried it 100 times and still could lift up a tiny screw from a watch movement.
Image 6: Back to the compass again. The needle strongly moves away and the Tesla reading has gone up (again, this does not have to be exact, it's just interesting to see how that compares to the initial reading).
Image 7: Total disaster on the timegrapher (like the watch you buy from eBay - ‘runs well, accuracy not tested’).
Image 8: De-magnetization: There are a lot of cheap devices around which might do the job or not, but if you have enough grease on the chain and allocatable to tools, don’t go for a compromise here (unless you can construct something yourself). There is just one touch of the button, no moving around of the object, no moving away at whatever speed and to whatever distance with the button still held down. It shuts off itself in fractions of a second before you have time to get your finger off the button.
Image 9: Back on the compass again. Magnetism gone.
Image 10: Readings on the timegrapher good enough for an old and serviced watch.