From: king@reasoning.com (Dick King)
Subject: Re: resisting electromagnetic radiation
Organization: Reasoning Systems, Inc., Palo Alto, CA
Distribution: sci.med, sci.med.physics, sci.engr.biomed, misc.health.alternative
Date: Mon, 11 Jan 1993 19:14:00 GMT

In article <1993Jan8.112501.6871@doug.cae.wisc.edu> kolstad@cae.wisc.edu (Joel Kolstad) writes:
>In article <1993Jan8.082352.12105@mr.med.ge.com> hinz@picard.med.ge.com (David Hinz (hinz@picard.med.ge.com)) writes:
>>Denise M. Cuevas (denise@isl.stanford.edu) wrote:
>>: 
>>: Has any research been done on blocking electromagnetic radiation? I
>>: assume that planting trees between my house and the power lines
>>: would be ineffective. How about buttressing the walls of my house with
>>: some sort of repulsing material?
>>
>>A Faraday cage is used to block RF leakage, can't really speak for 60Hz 
>>though.  Basically, what you need is a conductive box, with no gaps,
>>surrounding that which you want to keep  the RF out of (or in).
>
>That's the right idea.  You want to use a big box that "looks" like a solid
>metal conductor to the incoming radiation.  There are a lot of geometrical
>dependencies that come in here, but in general, if the spacing between
>holes in a perforated metal sheet is considerably less than the wavelength
>of the radiation, the perforated sheet will "look" almost like a solid
>metal sheet.
>
>This is how those grills on microwaves work: The little holes in the grill
>are spaced closer together than the wavelength of the microwaves, so the
>microwaves stay inside and don't fry you.  On the other hand, light has a
>much much higher frequency than microwaves, so the wavelength is much much
>shorter, so the grill doesn't look like a conductor at all, and light
>passes through the grill very much unaffected, so that you can watch your
>turkey roast.
>
>Unfortunately (from your perspective), the wavelength of 60Hz is absolutely
>huge, and you're going to need a solid metal box to keep it out.
>
>						---Joel Kolstad


That's not the reason why a Faraday Cage can't keep out 60HZ.

There are two problems:

  1: The lower the frequency of the radiation, the more conductive the Faraday
     Cage material must be.  At 60 Hz., only a superconductor would do.
     Loosely put, the rate at which a magnetic field penetrates a conductor is
     proportional to the material's resistivity, and a magentic field can
     penetrate even silver in milliseconds.

     How does a conductor resist magnetic penetration?  A current that opposes
     the magnetic field is induced in the conductor.  Why does it penetrate?
     Because shortly after the field is turned on [but fails to penetrate
     because of the induced current] the induced current dies out because of
     the material's resistance [unless the material is a superconductor in
     which case the current does not die out].

     If you made a Faraday Cage out of superconductor it would work just fine.

  2: There is fringing near the holes*.  You could make a Farady Cage with huge
     holes, miles in diameter, but you would have to stay about a diameter away
     from each hole.  So if you had a 100 mile cube Faraday cage with 10 mile
     diameter holes you would have an EMF-free volume 80 miles cubed.

"Mu Metal" might work.  It resists magnetic fields by other means than allowing
an opposing current to be induced, so it needn't be superconducting.


-dk


* There is a flavor of scanning light microscope that can see features
  substnatially smaller than the wavelength of light.  It works by feeding the
  light through a tiny hole.  No light passes through, except when there is
  material on the other side of the hole that interacts with the fringing field
  in some manner.

  This was described in a maddeningly brief column in Scientific American [NOT
  an article], and filtered through my aging memory ;-( .