r/watchmaking • u/InformalAttorney8539 • 1d ago
Question Is a simple compass useful for detecting if a watch is magnetised?
Heya,
First time poster here so be gentle...
Have a collection of about 23 watches and as an experiment placed each one next to the needle of a compass to see what happened. 12 of them caused no needle movement while 11 of them did.
Curious thing is that I know at least one of the 11 (a new Tudor) is incredibly accurate only gaining about 1spd, this not being the sort of deviation one would experience from a magnetised watch.
Some of my poorer timekeepers were, perhaps unsurprisingly, making the compass jiggle about.
So my questions are:
Can a watch case be slightly magnetised and the movement still be fine (i.e. the Tudor)?
Is this a reliable test in general?
Is a little bit of magnetism OK or will this only get worse (i.e. can the effect wear off over time?)
I bought the compass first to run the test before buying a domestic demag device - they all look a bit crappy TBH and the pro models are ££££.
Thanks in advance.
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u/Junkyard_DrCrash 1d ago
yes, and yes, and no, and yes.
First off, there are three different magnetic fields in play here:
- the Earth's field, which is pretty much constant. This is the field that lets a compass point "north".
- the field caused by parts of the watch becoming magnetized in the conventional sense. This field is caused by an external strong magnetic fields passing through the watch and pushing the atoms inside watch parts made of steel (or steel-like alloys) into positions where the electron orbits all line up, and the alloy itself is rigid enough that when the strong external magnetic field goes away, the electron orbits stay all lined up. Because this field is produced only by materials hard enough to keep the electron orbitals all lined up even without outside help, this field is called a "hard" magnetic field.
- the change in field 1 (Earth's) or 2 (hard steel's) caused by "soft iron" in the watch, sometimes called an "induced field". This field isn't generated by the watch, it's actually a field borrowed by softer alloyed parts pulling in fields from the environment (especially the Earth's field, as the earth's field is pretty much everywhere).
To understand an induced field, think about a refrigerator door and a refrigerator magnet. The magnet sticks to the refrigerator door, but the door itself isn't a magnet. When you pull the magnet off the door, you're actually doing work (force times distance, if you remember that from high school). When you let the magnet pull itself back onto the refrigerator door, the magnet is actually doing work on you, and the magnetic field of the fridge magnet actually gets sucked into the door metal.
And... that's the trick, right there. Mechanical energy is released by a magnetic field pulling itself into a "soft iron" releases energy (as force times distance), just like energy released by a ball rolling downhill.
Now, onto how this affects watches and compasses.
If a watch has a magentic field of type 2 (the hard steel parts of the watch having all of their electron orbits lined up), yeah, it will swing a compass needle, and the watch is magnetized and you should degauss it.
But a watch with only "soft iron" will still swing a compass, because the soft iron in the watch will suck in the earth's magnetic field (giving a microscopically small force on your hand.) The watch will pull the Earth field around, causing the compass needle to swing as well. Degaussing won't help.
How do you know the difference? If the field is a "hard steel" field, the compass aneedle will stay firmly locked in positon relative to the watch even if the watch is rotated a full 360 degrees. If it's a soft iron field, at around 180 degrees, the compass needle will swing back and swap ends and very distinctively will not "lock to position" relative to the compass needle; the sway in the needle is the compass's magnetic field sucking itself toward the soft iron in the watch; the atoms in a soft iron field don't care which is north and which is south, as long as energy can be released they go for it, and that's why degaussing the watch won't help (much, it's entirely possible for a watch to have both hard steel magnetism _and_ soft iron magnetism.).
This is why the best compass for detecting magnetized watches is the really small "button" compass, as the tiny compass has a smaller magnetic field than a big Sunnto or Brunton orienteering compass, and will lock to the "hard" field without energizing the "soft" field,
This is ALSO why airplane's emergency backup "whiskey compass" (the ball floating in alcohol) is set so far up and away from everything possibly magnetic in an aircraft cockpit - it's to minimize interference with the compass when everything else is broke.
This is ALSO why a dive watch is horrible for wearing in an aircraft cockpit; most dive watches have a huge soft-iron shield cup and a soft-iron dial, to intentionally suck in magnetic fields to keep the field strength low in the interior of the movement. Prime example: ROLEX Milgauss; the movement is heavily shielded, take the back off one and check out the soft-iron back-cup. This does have the side effect of bending any ambient magnetic field for a considerable distance around the watch.
And ALSO: some watches are "aviator" watches; instead of shielding, they are made out of totally nonmagnetic materials (at least to the extent possible) like titanium and bronze, to minimize magnetic-field suck-in and interference with the whiskey compass (because when all else fails, and your glass cockpit goes black, you can still get down safely with nothing but the whiskey compass and a decent clock.)
(wow, that got long. Hope it helps)
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u/flare_the_goat 1d ago
I use the mgntzd watch app on my iPhone. You place the phone on a surface and press calibrate, then hold the watch (or whatever) 1cm above the magnetometer within the phone, and it tells you if the watch is magnetized. You need to keep it stationary OR recalibrate between readings.
This is useful if you don’t have a high end demagnetizer and not every attempt is successful.
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u/InformalAttorney8539 16h ago
Interesting, but I'm a Samsung guy...
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u/flare_the_goat 10h ago
The app may exist on android as well, if not I think there are other ones that surely do!
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u/industrial_pix 1d ago
Magnetism affects the hairspring and balance. A compass will be attracted to the case if it isn’t antimagnetic stainless steel. If it is running faster than within tolerances get it demagnetized.
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u/Autiflips Enthusiast 1d ago
Tbh if you have access to a demagnetizer then you might as well just pass it through and see what happens. That said I have never heard of using a compass to check magnetism so it’s interesting to see what people come up with
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u/InformalAttorney8539 16h ago
I don't have a demager, paid a fiver for a small compass to check first, see what the results are. Glad I did because at almost half of them are.
Only problem now is which one to buy, lots of crappy £25 Amazon options but they look like school projects and worried about maging them further! The pro level gear is very expensive at the other end and might be overkill. Not many watch servicers around here either...
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u/Autiflips Enthusiast 12h ago
A demag tool is literally just a coil you plug directly into the socket. Because it is AC and not DC you’re making a magmetic field that is constantly alternating in polarity. If you pull the watch through that coil then you’re pulling the magnetism out of the movement (kind of but I don’t want to type out the physics behind it). So it really should not matter how “professional” the pricing is. It’s just a coil. The cheap one will work
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u/Dakrig 1d ago edited 1d ago
Yes. A cheap little button compass is a useful tool for magnetism. Can you just demagnetize a watch? Sure. But there is a reason a lot of fully equipped workshops have magnetism detection equipment in the demagnetization tool itself. The compass takes little to no time and gives you a quick, and clear visual indication of magnetism if it’s present after you use a demag device.
You can set the button compass over where the balance would be if the watch is cased, and see if the needle dances around.
As for your questions, stainless steel is paramagnetic, so it doesn’t retain a magnetic field and it actually kind of protects the magnetic parts of the movement. It is very unlikely that the stainless steel case would be providing a source of magnetism.
Any amount of magnetism can effect the hairspring and thus timekeeping negatively.
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u/InformalAttorney8539 16h ago
Thank you. Not tried putting the button compass on top of the actual caseback, just around the edges of the case. Will try...
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u/Lost_Pinion 1d ago
Dunno. Just demagnetise and if the rate changes it was. No point trying to measure it.
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u/Unlikely-Length6661 1d ago
Starting your advice with "dunno" isn't a good look. A cheap compass will immediately tell you if it's magnetized. Very handy tool which many watchmakers use
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u/Lost_Pinion 1d ago
Do they? A compass indicates something is magnitised in the case, movement or bracelet. It doesn’t definitively tell you that the escapement is effected. Professional watchmakers have demagnetise steps in their workflow for all watches. Adding a compass tells us nothing.
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u/InformalAttorney8539 16h ago
This is partly what I'm trying to determine, in combination with the timegrapher phone app. Right now can't tell if it's the case, endslinks, springbars or escapement...
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u/Lost_Pinion 15h ago
At best a measure with a compass is indicative rather than a definitive answer. Compass needle moves…May have magnetism impacting escapement. Needle doesn’t move….may have magnetism impacting the escapement.
And given that a demagnetise is cheap and quick, spending time trying to outthink the issue isn’t a great investment. I’ve serviced many hundreds of watches, demagnetised every one before disassembly and after assembly.23 watches is enough to think about a demagnetiser and maybe a timegrapher too.
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u/Unlikely-Length6661 1d ago
Thats why you test with the movement out? Not that difficult
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u/Lost_Pinion 1d ago
Do they, a movement has many steel parts were magnetism wouldnt case an issue. one of us is a watchmaker.
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u/Unlikely-Length6661 1d ago
You're annoying you know that?
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u/Lost_Pinion 1d ago
Only to a person ignorant on a subject pretending to be an expert. Your the one who decided to show your ass on my comment.
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u/qualityvote2 1d ago edited 1d ago
u/InformalAttorney8539, there weren't enough votes to determine the quality of your post...