[GreenKeys] UPPER and LOWER case

[drlegendre .]

Hey Jeff,

I'm well aware of mu-metal and the concept of using high-permeability
materials for 'shielding' magnetic fields.

But again, the term "shield" isn't exactly accurate - in the sense that it
gives the impression that the field is simply blocked, or deflected - and
that's not the case.

This might seem like a pedantic point (I might even agree..) but physicists
aren't typically comfortable with the usage. The 'shield' constrains the
field within the shield material itself - and in fact interacts very
strongly with the field. If the source of the field is a permanent magnet,
the use of mumetal shielding actually increases the B of the magnet.

It's a funny sort of shield that actually increases the flux of that which
it's designed to 'block'. So yes you can 'shield' a field, and in loose
vernacular, the term makes sense - but it doesn't accurately describe
what's really taking place, and it's led a lot of folks down the rabbit
hole trying to "gate" magnetic fields.

On Sat, Sep 5, 2015 at 1:36 AM, Jeffrey D Angus <jdangus at att.net> wrote:

> On 9/5/2015 1:19 AM, drlegendre . wrote:
>
>> WTH is "tempest" magnetic shielding? Being that you can't actually
>> /shield/ magnetic fields, the concept is even more dubious.
>>
> Tempest was the anal-retentive paranoid design engineering about keeping
> others from "listening in" by listening to the magnetic or electronic
> pulses
> from machinery. When you're dealing with the spook agencies, you have a
> lot of money to play with, and the paranoia to justify it with.
>
> And yes, actually you can shield magnetic fields using mu-metal.
> <https://en.wikipedia.org/wiki/Mu-metal>
> Most oscilloscopes have mu-metal shielding around the CRT to prevent
> stray magnetic fields (like the power transformer) from distorting the
> trace.
> And commercially...
> <
> http://www.magnetic-shield.com/custom-mumetal-zero-gauss-chambers.html?gclid=CjwKEAjwgqWvBRC2kajZjqeOn0oSJADqBi-ldi-xsnLy-MwT7fjrTXMQeb06dl6zXgGmQPpyJIq8LBoCcG_w_wcB
> >
>
> --
> Jeff-1.0
> wa6fwi
> http://www.foxsmercantile.com
>
>
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