PSN-L Email List Message

Subject: Re: Another mu-metal/permalloy question
From: ChrisAtUpw@.......
Date: Tue, 13 Jun 2000 14:24:47 EDT

Dear Meredith,

>  For a shielded enclosure that contains a strong neodymium
>  magnet (mass), would that internal magnetic field, enhance
>  the overall shields ability to help attenuate the EXTERNAL
>  magnetic fields influence?  To me, it would seem to help
>  attenuate the external fields it would somewhat
>  create a polarised shield according to the orientation of the
>  internal magnet.

    Suppose you have a hollow sphere of Permalloy etc. with a magnetic field 
detector inside and then apply an external magnetic field. At low external 
fields, almost all the field passes through the Permalloy and the internal 
sensor sees practically nothing. As you increase the external field, the 
Permalloy gets towards magnetic saturation and some reduced field will 
penetrate the sphere. At high external fields, the Permalloy will be 
completely saturated and there will be a high field inside the sphere.

    If you had two concentric spheres, you would be able to apply a higher 
external magnetic field before it started to penetrate the inner sphere.  

    For your apparatus, I think you need an outer shield of Permalloy etc. to 
reduce the effect of the low external fields and an internal shield to screen 
the Permalloy from the core magnets. This shield might be silicon iron or you 
might get away with mild steel. You might need two layers.

    Hope this qualitative description helps.

    Chris Chapman

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Larry Cochrane <cochrane@..............>