Isolation from magnetic field?

[Ulysees]

Is there any material such that if you make a hollow object out of it, the magnetic field inside is not equal to the magnetic field outside, but it is very small or zero?

 

[Dale]

Is there any material such that if you make a hollow object out of it, the magnetic field inside is not equal to the magnetic field outside, but it is very small or zero?

Iron. Depending on the amount of shielding you need you may need a lot of it.

 

[f95toli]

mu-metal is even better. It is what most magnetic shields are made from.
Also, note that DaleSpam wrote iron, NOT stainless steel. While steel is mainly made from iron (+carbon etc) there is a significant difference between the two in this case; steel is almost useless as a shield.

 

[Ulysees]

So it is the permeability μ ("mu-metal") that makes a good shield.

How thick does a mu-metal shell have to be to reduce the Earth's magnetic field by a factor of 1000?

 

[f95toli]

The shield I use are usually around 1mm and that should be enough. However, note that is usually more efficient to have two thin shield than 1 thick.

 

[TVP45]

Is there any material such that if you make a hollow object out of it, the magnetic field inside is not equal to the magnetic field outside, but it is very small or zero?

It's going to depend on the strength and frequency of the magnetic field. There is not a "one size fits all answer". If you are talking about a high frequency field, conductivity is important; if you are talking about a low frequency one, permeability is important.

In the low frequency case, which I'm guessing you meant, the higher the permeability (at that particular strength), the better. As noted, mu metal is absolutely the best, but can be difficult to fabricate. Iron and some of the iron alloys work OK but need to be much thicker. 1008 steel will work pretty well; if you need to weld it, I'd probably go to 1018 and maybe do an anneal. If you need corrosion resistance, stay away from the austenitic stainless steels; probably 430 would work OK.

If you're thinking about a hollow object (rather than, say, a box), I'd look at cast steel. It has fair permeability, can be machined, and is generally obtainable. If corrosion is a problem, plate it or use powder coat.

 

[Ulysees]

Thanks.

How do I calculate the field inside a hollow sphere of a given thickness if the only field outside is the earth's?

I think Biot-Savart's law has something to do with it, how is the average μ calculated when different materials are put in an arbitrary geometry?

 

[Ulysees]

TVP45, are you working for the military by any chance?

 

[TVP45]

TVP45, are you working for the military by any chance?

Only in the sense that a big chunk of my income tax goes to the defense budget.

As to your question about different materials with arbitrary geometries, you almost are at the point where you have to simply test them. There are FEA programs that will do this, but you may spend a lot of time (read $) and then still have to verify with tests.

You might look at the website of Magnetic Shield Corp. They make a calculator (cardboard, not online) to help with calculations.

 

[Ulysees]

This is such a good site, thank you very much. It answers everything one might need to know, on magnetic shielding, from dc to 100kHz.

http://www.magnetic-shield.com/faq/index.html

I wonder if what the site calls "A/C magnetic fields" is exactly the same thing as ELF waves. If yes, then why do they only talk about permeability and there's no mention at all of dielectric constant as in E-fields?