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Author: Subject: Electromagnetic/magnetic screens materials?
kazaa81
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shocked.gif posted on 8-10-2005 at 08:06
Electromagnetic/magnetic screens materials?


Hallo to all,

I've made some small Pb ingots, 100g each, 1cm thick, to test if a magnet would have effect on a piece of iron on the other side. With surprise, I've seen which magnetic attraction passes in Pb blocks! I thinkle which it wouldn't pass because Pb is a screen even for X and gamma rays.
What material can screen magnetic radiations like the ones of magnetite?

Thanks at all for help
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12AX7
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[*] posted on 8-10-2005 at 09:08


Well duh... lead is just another non-ferromagnetic metal... electromagnetic radiation isn't particle radiation like nuclear radiation!!

If you want to "block" magnetic fields you have to shunt them away by using magnetically "conductive" (low reluctance) materials such as annealed low-carbon steel and special alloys including "mu metal".

Tim




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kazaa81
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thumbup.gif posted on 8-10-2005 at 09:17


Thanks, 12AX7!
I don't know what is "mu metal" composed of, so if you could elaborate more......
So, to block magnetism would mild iron (non tempered) work also? If you've ideas about other materials, please post them.

Thanks at all for help! ;)
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IrC
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[*] posted on 8-10-2005 at 10:06


It is quite simple. That which sticks to a magnet can be used to block a magnet. Mu metal is typically 80 percent nickel and 20 percent iron in an alloy.
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kazaa81
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thumbup.gif posted on 8-10-2005 at 11:07


You mean blocking a magnet by sticking it to something or because of radiations absorbment?
Also, for blocking a magnet (specially if it's big enough to catch your fingers) is preferable avoiding something who will stick to it because of the violent attraction who generates as magnet reach with its radiations radius the object.

Thanks at all for help!
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DrP
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[*] posted on 13-10-2005 at 08:25


Type 'Magnetic Shielding' into Google - lots of stuff about it there.


Its not radiation, it's an electromagnetic field. Therefore try a fine mesh made from the mu metal.

[Edited on 14-10-2005 by DrP]
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DrP
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[*] posted on 14-10-2005 at 02:56
Interesting?


Hi,

I don't know if this is interesting or not but this guy has been working to replicate ion wind effects for home made 'lifters' he recons he experiences some interesting magnetic shielding effects arroung his lifter. Could be bol***s but who knows.

Might be fun to build a lifter anyway - think I'll give it a go when I get time. (type 'ion lifter' into google)

http://paranormal.about.com/library/media/audio/Anomalous-EM...

[Edited on 14-10-2005 by DrP]
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[*] posted on 14-10-2005 at 04:28


IMHO there is nothing better than ferrite. Like the other materials it concentrates the magnetic field lines into itself (the ferrite) so there is less field elsewhere.
Oh and the link does indeed look like bollocks (one of the guys calls himself merlin!)

[Edited on 14-10-2005 by Quibbler]
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12AX7
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[*] posted on 14-10-2005 at 12:57


Quote:
Originally posted by Quibbler
IMHO there is nothing better than ferrite. Like the other materials it concentrates the magnetic field lines into itself (the ferrite) so there is less field elsewhere.
Oh and the link does indeed look like bollocks (one of the guys calls himself merlin!)


The degree to which magnetic field lines are concentrated in, and shunted by, a magnetic material depends on its relative permeability. Some ferrites get very high mu, but all pale in comparison to cheap annealed low-carbon steel, and a variety of more expensive, high performance ferrous/nickel alloys (e.g., "mu metal" with mu up to 200,000, IIRC).

Remember that a magnetic material has a saturation limit, above which it won't block a field. For incident shielding this isn't a problem but if you have to shield, say, a superconducting NMR magnet, you will need several layers to reduce the field. In this range, distance is better than shielding since the internal pressure on such shielding will be in the many ton range.

As for lifters, there's a lot of BS around them but they do work on physical principles: the reaction force of moving air. I don't like the websites because, besides the junk science often associated, they're all the same. Someone sees one page and regurgitates their "progress". Sure, it might be a different shape, but that isn't going to change the efficiency or anything.

Tim




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Quince
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[*] posted on 17-10-2005 at 02:23


Look here:
http://www.lessemf.com/mag-shld.html
The foil saturates way too easily, but the MagnetShield (some sort of high-nickel-proportion alloy) worked in my case to shield transformers (for the kind of shield wrapped around the edge of the core, not the leakage flux band parallel to the coils that should be copper).
The metglas has over a million permeability, which is nuts, but it's very expensive and again, saturates too easily.

For all of these, you really should make a closed surface (at least 2-D loop) for good performance. Otherwise the magnetic field lines just go around it. I didn't have any problems soldering the MagnetShield, but all of these should be re-tempered after bending to their final shape, and sharp creases avoided; use as rounded edges as you can. BTW, beware of nasty flesh cuts... If the shield saturates, try to increase the distance between it and the source, to avoid needing a second layer.

Terrible website, I couldn't get the online order to work and had to do it by email (though I recommend you don't actually send a CC# by email as it's not secure, better to call for that).

[Edited on 17-10-2005 by Quince]




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[*] posted on 17-10-2005 at 16:41


YES!! Thanks for that link Quince.

Now I can order some un-insulated mu metal wire for my grandmother to knit into an anti-mind control beanie hat for me!!

:)
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IrC
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[*] posted on 17-10-2005 at 17:02


A better shield is to use the material such as Mu metal in concentric spheres, as with fields over one Tesla mu metal (or other metals) doesn't work, and the shell method must be used. A wall with enough area to be treated as an infinite plane can be used in the same way, by having several walls with the proper spacing between them. The spacing follows a formula which would be the same for a plane as it is for spheres. As example, a solid wall of mu metal say two inches thick does not shield as well as three walls a few mm thick which are spaced apart properly. This can be used to gain even better shielding using less materials and saving lots of money.

http://www.amuneal.com/pages/magshield-formula.php

http://farside.ph.utexas.edu/teaching/jk1/lectures/node52.ht...

[Edited on 18-10-2005 by IrC]
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12AX7
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[*] posted on 18-10-2005 at 00:24


Or use several layers of silicon steel followed by one or two of mu metal. SS has a practical saturation around 1.2T (that's what most power transformers run at).

Tim




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