Faraday cages ( was Re: staticy plastic tubs )
ray at arachelian.com
Fri Oct 20 18:54:51 CDT 2006
aliensrcooluk at yahoo.co.uk wrote:
> Ahh, thanks. I had forgotten about Google
> (not surprising since I have been working
> 8.5 hour days for the past week and only
> getting 6 hours sleep each night).
Sounds a lot like my schedule. Are you also a sysadmin? :-)
> I don't like to rely too heavily on Wikipedia
> as (like alot of information online) you can't
> be sure what's right or wrong, unless someone
> knowledgeable in the subject can confirm it.
For things that I'm familiar with Wikipedia is almost always correct.
For things that I'm not familiar with, their external links point to
data that agrees with it.
> So if I understand correctly, the outer casing
> of a harddrive acts as a Faraday cage? My
> (4GB) Toshiba ones I use in my Amiga 600
> have circuitry on the underside, but I suspect
> that may stop it being a Faraday cage, unless
> it's stuck onto the outside of the unit.
The platters are very well protected by the metal case. There are holes
for air, (hard drives don't like working in a vacuum or low air
pressure) and of course holes for the control/data cables, so it's not a
perfect Faraday cage, but it's close enough.
Faraday cages protect the inside from static electricity (ESD) as well
as limit the amount of Radio Frequency Interference (RFI) that's able to
penetrate in or out of them.
The board isn't protected by the Faraday cage effect, but since the
cards are designed with their ground lines around the edges such that
they're less vulnerable and enough resistors to dissipate ESD energy.
Even so, ESD is dangerous to hard drives - the controllers anyway. This
is why they're shipped in nice anti-static body-bags and have the ESD
warnings on them.
This is why one bit of advice in repairing drives when they're no longer
accessible is to swap the controller card. Of course this requires an
identical drive, right down to the same firmware revision for it to work.
The controller cards are meant to be protected by the computer's case
(or external hard drive case), which is supposed to be shielded.
The magnetic media is well protected by the Faraday cage from RFI, but
even so, the platters have high coercivity than floppies, which means it
would take a very strong magnetic field to flip the bits around. ESD
doesn't do very much to the data on the platters.
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