Inverse Faraday Cage: Containing Magnetic Fields?

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Discussion Overview

The discussion revolves around the concept of using a Faraday cage or similar methods to contain magnetic fields, exploring the feasibility of trapping magnetic fields as opposed to merely shielding against them. Participants discuss the technical challenges and materials involved, particularly in relation to practical applications in electronics and audio systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants question whether a Faraday cage, typically used for electric fields, can be adapted to contain magnetic fields.
  • Others suggest that low permeability metals, such as mu-metal, can be used to shield against magnetic fields, though they note practical difficulties in achieving effective containment.
  • Concerns are raised about the saturation of materials like mu-metal in strong magnetic fields and the need for careful handling and re-annealing to maintain their properties.
  • One participant proposes a practical application involving a stereo system, suggesting that trapping magnetic fields in the power supply could reduce noise interference.
  • Another participant shares their experience with shielding techniques, mentioning that distance and specific configurations can help mitigate noise in audio equipment.
  • There is a mention of using older technology, such as tubes, in amplifier design, contrasting with modern integrated circuits.

Areas of Agreement / Disagreement

Participants express varying opinions on the effectiveness and practicality of using materials like mu-metal for containing magnetic fields. The discussion remains unresolved regarding the best methods for achieving effective magnetic field containment in practical applications.

Contextual Notes

Limitations include the dependence on specific material properties, the challenges of maintaining permeability through handling processes, and the unresolved nature of the proposed applications in audio systems.

Lancelot59
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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?
 
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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:
Yes, electric fields. My bad.

What makes this trickier then a regular faraday cage?
 
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.
 
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.
 
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.
 
Lancelot59 said:
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.
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.
 
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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