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Inverse Faraday Cage?

  1. Jun 15, 2009 #1
    Well we can use a Faraday Cage to keep magnetic fields from entering a certain area, but I was wondering if the same method could work for containing magnetic fields.

    I know we use metal cores to concentrate magnetic fields, but is there a way to actually trap them?
     
  2. jcsd
  3. Jun 15, 2009 #2

    mgb_phys

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    I think you mean a Faraday cage for electric fields.
    Yes you can make a shield from a low permeability metal to contain a magnetic field, the alloy used is called mu-metal. Although in practice it's trickier and not always as effective.
     
    Last edited: Jun 15, 2009
  4. Jun 15, 2009 #3
    Yes, electric fields. My bad.

    What makes this trickier then a regular faraday cage?
     
  5. Jun 15, 2009 #4

    mgb_phys

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    Strong fields will saturate it.
    You have to make the screen almost air-tight to stop any fields leaking in (as you do with a Faraday cage) but any machining, hammering, cold-working of the metal will lose it's permeability so you need very careful handling and lots of re-annealing.
     
  6. Jun 15, 2009 #5
    A Faraday cage (e.g., copper screen or sheets) is good for dc and ac electric fields, and for only ac magnetic fields (in electromagnetic radiation). Mu-metal, often in separated layers, is required for dc and low frequency ac magnetic fields. Because of its very high mu, mu-metal is saturated by low B fields (longitudinal) and H fields (transverse), so layer(s) of soft iron are needed to reduce high B fields.
     
  7. Jun 15, 2009 #6
    That does seem a bit trickier.

    I ask because I have plans for a few summer projects to keep myself amused. One if them is a stereo system, and I want to isolate as much noise from the power supply as I can.

    I figure instead of building a cage around the rest of the system to keep fields out, why not just trap the fields in the power supply?

    Another is a Power bus that has circuits controlled by Triacs, and I want to keep the fields trapped in the power cords, and the device itself so that noise doesn't enter my microphone when I record.
     
  8. Jun 16, 2009 #7
    I have found that 1/r2 is a good shielding; put the power supply in another room and run dc to the stereo. I also found that using dc on the filaments of the preamp tubes (12AX7 hi-mu triodes) helped.
     
  9. Jun 16, 2009 #8
    My amplifier design uses an IC, not tubes. Although that would be kind of cool. I like working with older technology for some reason.
     
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