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Re: Floppies - Actual Question

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On 2025-10-07 17:16, Rich wrote:
> c186282 <c186282@nnada.net> wrote:
>> On 10/6/25 06:29, Stefan Ram wrote:
>>> Lawrence =?iso-8859-13?q?D=FFOliveiro?= <ldo@nz.invalid> wrote or quoted:
>>>> On Thu, 25 Sep 2025 13:04:59 +0200, Carlos E.R. wrote:
>>>>> At some point in the 80's, a friend at uni wanted to go home with
>>>>> his floppies kept safe (the train was electric).  We told him to
>>>>> try wrapping in aluminum kitchen foil, or perhaps in a cookie tin
>>>>> box.  I don't remember what he did exactly, but it worked.
>>>>> “Faraday cage”.
>>>
>>>     In a perfect conductor, inside the material itself (such as the
>>>     cage walls), the electric field E is zero because free charges
>>>     move to cancel any applied electric field.
>>>
>>>     Faraday's law then implies that within the conductor, the
>>>     magnetic field B cannot be changing with time.
>>>
>>>     Changing magnetic fields induce currents (eddy currents).  These
>>>     induced currents generate opposing magnetic fields that cancel
>>>     the time-varying components inside the conductor, effectively
>>>     shielding against changing magnetic fields.
>>>
>>>     But this does not imply zero magnetic field inside the
>>>     conductor's hollow interior (the cage's interior space).  Static
>>>     fields do not induce currents because there is no changing
>>>     magnetic flux within the conductor to generate an electromotive
>>>     force.  Consequently, these magnetic fields penetrate the cage
>>>     interior unaffected.
>>>
>>>     Also, real materials might not be perfect conductors.  And it
>>>     seems that, in practice, a Faraday cage can possibly protect to
>>>     some extend against fast changing magnetic fields, but less
>>>     against slowly changing magnetic fields.
>>
>>    It would take a fairly extreme static/slow-changing
>>    field to erase floppies. You can do it - I just did
>>    a couple weeks ago - using a neo magnet at point blank
>>    range - but fields from train motors, I just don't
>>    see it. High-frequency mag fields are more dangerous
>>    but even THEN would need to be pretty strong.
> 
> The drop off of magnetic field intensity by distance can be
> **approximated** as 1/(d^3) (inverse cube drop).  So yes, the field
> from the train motors would need to be extremely strong, or the OP's
> floppies would have had to be very close to the motors, for the
> electric train motors to have any impact on the floppies.

Well, some trains have motors on every carriage. Then also there was the 
thought that the overhead cable carried large currents and perhaps 
produced large fields.

It was just the fear that something might happen to the floppies, and 
what should we do to prevent hypothetical damage. We had no experience, 
and there was no internet to ask other people.


> 
>>    My guess is that we're hearing an "urban legend"
>>    here.
> 
> Yes.  There are a lot of those that go around.  And since the "foil
> wrap" did nothing, but the floppies were undamaged (and the person did
> not do an A-B test with some wrapped, some unwrapped) they delude
> themselves into belieiving the foil wrap actually did something.

I don't remember if we used just foil, or a tin box, or both. Also, I 
don't remember if there was another floppy outside. I might ask the 
other chap if you people are that interested.

-- 
Cheers, Carlos.
ES🇪🇸, EU🇪🇺;

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Re: Floppies - Actual Question ram@zedat.fu-berlin.de (Stefan Ram) - 2025-10-06 10:29 +0000
  Re: Floppies - Actual Question Lawrence D’Oliveiro <ldo@nz.invalid> - 2025-10-06 22:08 +0000
  Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-06 23:30 -0400
    Re: Floppies - Actual Question Rich <rich@example.invalid> - 2025-10-07 15:16 +0000
      Re: Floppies - Actual Question "Carlos E.R." <robin_listas@es.invalid> - 2025-10-08 02:34 +0200
        Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-08 01:05 -0400
          Re: Floppies - Actual Question "Carlos E.R." <robin_listas@es.invalid> - 2025-10-08 10:58 +0200
            Re: Floppies - Actual Question The Natural Philosopher <tnp@invalid.invalid> - 2025-10-08 11:06 +0100
            Re: Floppies - Actual Question ram@zedat.fu-berlin.de (Stefan Ram) - 2025-10-08 12:10 +0000
              Re: Floppies - Actual Question "Carlos E.R." <robin_listas@es.invalid> - 2025-10-08 14:14 +0200
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-09 02:44 -0400
                Re: Floppies - Actual Question "Carlos E.R." <robin_listas@es.invalid> - 2025-10-09 21:10 +0200
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-09 21:36 -0400
                Re: Floppies - Actual Question The Natural Philosopher <tnp@invalid.invalid> - 2025-10-10 09:54 +0100
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-11 01:57 -0400
                Re: Floppies - Actual Question "Carlos E.R." <robin_listas@es.invalid> - 2025-10-10 13:49 +0200
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-11 02:29 -0400
                Re: Floppies - Actual Question "Carlos E.R." <robin_listas@es.invalid> - 2025-10-11 14:32 +0200
                Re: Floppies - Actual Question Joerg Walther <joerg.walther@magenta.de> - 2025-10-09 12:50 +0200
                Re: Floppies - Actual Question "Carlos E.R." <robin_listas@es.invalid> - 2025-10-09 21:11 +0200
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-09 21:37 -0400
                Re: Floppies - Actual Question Joerg Walther <joerg.walther@magenta.de> - 2025-10-10 11:19 +0200
                Phone wallet "Carlos E.R." <robin_listas@es.invalid> - 2025-10-10 13:51 +0200
                Re: Phone wallet c186282 <c186282@nnada.net> - 2025-10-11 02:30 -0400
                Re: Phone wallet "Carlos E.R." <robin_listas@es.invalid> - 2025-10-11 14:32 +0200
                Re: Phone wallet c186282 <c186282@nnada.net> - 2025-10-11 21:35 -0400
                Re: Phone wallet "Carlos E.R." <robin_listas@es.invalid> - 2025-10-12 14:48 +0200
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-09 21:17 -0400
                Re: Floppies - Actual Question Charlie Gibbs <cgibbs@kltpzyxm.invalid> - 2025-10-10 06:51 +0000
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-10 03:29 -0400
                Re: Floppies - Actual Question The Natural Philosopher <tnp@invalid.invalid> - 2025-10-10 09:55 +0100
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-11 02:10 -0400
              Re: Floppies - Actual Question The Natural Philosopher <tnp@invalid.invalid> - 2025-10-08 13:18 +0100
                Re: Floppies - Actual Question Robert Riches <spamtrap42@jacob21819.net> - 2025-10-09 03:09 +0000
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-09 02:58 -0400
                Re: Floppies - Actual Question The Natural Philosopher <tnp@invalid.invalid> - 2025-10-09 12:45 +0100
                Re: Floppies - Actual Question Robert Riches <spamtrap42@jacob21819.net> - 2025-10-10 02:53 +0000
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-09 23:44 -0400
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-09 02:45 -0400
              Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-09 02:38 -0400
              Re: Floppies - Actual Question Rich <rich@example.invalid> - 2025-10-09 15:16 +0000
                Faraday cages (was: Re: Floppies - Actual Question) Nuno Silva <nunojsilva@invalid.invalid> - 2025-10-09 17:02 +0100
                Re: Faraday cages Rich <rich@example.invalid> - 2025-10-09 18:48 +0000
                Re: Faraday cages The Natural Philosopher <tnp@invalid.invalid> - 2025-10-10 09:49 +0100
                Re: Floppies - Actual Question The Natural Philosopher <tnp@invalid.invalid> - 2025-10-10 09:47 +0100
                Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-11 01:05 -0400
        Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-08 01:52 -0400
      Re: Floppies - Actual Question c186282 <c186282@nnada.net> - 2025-10-08 01:38 -0400

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