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Newbie question - Magnetic fields vs Rolex

  1. Apr 17, 2009 #1
    Hi forum, I know it's an odd topic header, but please bear with me, especially as I'm a newbie here and useless at Physics.

    I felt this would be the best place to look for advice and understanding on magnetic fields.

    I ask as I am a wrist watch fan...and since the release of a Rolex claiming to be resistant to magnetic fields of 1,000 gauss there has been much debate over magnetic fields and their impact on watches amongst fellow watch enthusiasts, which I would like help clarifying.

    I have two questions:

    1) Various watch manufacturers use a variety of measurements for the magnetic fields that their watches can resist, can these (listed below) be directly compared, and if so what are the conversions?


    For example, some claim to work in fields of 4,800A/m which sounds impressive but my initial conversion in to Gauss attempts suggest it is not!?

    2) Is there a simple experiment I could use to measure the magnetic field of household objects such as laptops, televisions, stereos etcetera? This would allow watch fans to understand what impact these may have on their timepieces, and perhaps explain why some watches lose significant time after being left near such household goods.

    Thanks for taking the time to read this and any help you can offer.
  2. jcsd
  3. Apr 17, 2009 #2
    The magnetic field B (Tesla) is related to H (amp-turns per meter) by the relation

    B = u0H where u0= 4 pi x 10-7 Henrys per meter
    and B(Gauss) = 1 x 104 times B(Tesla)

    so B(Gauss) = 4 pi x 10-3 x 4800 AT/m = 60 Gauss

    I once lost a good watch because I stuck my hand in a big magnet, and the minute hand was pulled off. Typical MRI machines are about 15,000 Gauss, so my recommendation is take your Rolex near magnetic fields.
  4. Apr 20, 2009 #3
    Thanks for that Bob, as I mentioned I'm no physics expert and certainly won't be going anywhere near a MRI!!!!!!!! But how about household objctes such as laptops or big speakers?

    Is there a easy/simple way to measure magnetic fields without needing specialist equipment?
  5. Apr 25, 2009 #4

    Gib Z

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    There are some experiments one could do, but none of them would be accurate or reliable enough for a decent quantitative measure of their actual magnetic fields, for fields as weak as normal household objects at least. Generally even those objects that have magnets in them, like speakers, have the magnetic field pretty much completely contained inside the case. The Magnetic Field's strength decreases as distance from the magnet increases. If I am 1m away and the strength is X, 2m away it'll be X/2.

    I've got a pretty decent magnet that, when I put on top of an old watch, moves the hands, so its strong enough for this test. I put it next to all my speakers, big and small, and there is pretty much no interaction, definitely not enough to actually effect a watch. In fact, the only real way I can see this happening realistically is you keeping your earphones near your timepiece for extended periods of time, which could happen if your hand is in your pocket. Over a period of months, it may cause a noticeable change.
  6. Apr 25, 2009 #5

    Gib Z

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    Ahh sorry, thats X/4, not X/2. Fields radiated in 3 dimensions all follow the inverse square law.
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