Mumetal Permeability: Right or Wrong?

[linux kid]

If it's a magnetic shield, then it must be permeable. Right or wrong? I would like to know if mumetal is permeable.

 

[ranger]

You are correct. Mumetal has very high magnetic permeability.

 

[linux kid]

Is the a magnet shield that is NOT a permeable?

 

[ranger]

All magnetic shielding must have the characteristic of high-permeability in order to redirect the magnet fields.

 

[marcusl]

Is the a magnet shield that is NOT a permeable?

Yes, a superconductor expels flux (perfect diamagnetism) for fields below a critical value, and can act as a non-permeable magnetic shield.

 

[linux kid]

Yes, a superconductor expels flux (perfect diamagnetism) for fields below a critical value, and can act as a non-permeable magnetic shield.

Where can I buy some, do you know?

 

[dille71]

Super conductors require very very low temperatures (close to 0 kelvin).
Not so easy to work with...

 

[linux kid]

oh, nevermind. Should have known that.

 

[linux kid]

One more question:

Just how good is mumetal at shielding? Could someone give me a simple explanation please. For example what percent of the magnetic force is reflected in a certain scenario?

 

[marcusl]

First of all mumetal doesn't reflect fields at all, it shields by guiding them through the metal. Think of when you go camping and make a little ditch around your tent to conduct water away if it rains. The ditch doesn't reflect water, it guides it along. Designing magnetic shields isn't rocket science but there isn't a simple formula either--it depends on configuration and shape of both the field and the shield, and also on the strength of the field and thickness of the mumetal. The shielding effectiveness drops dramatically if you saturate the material in a strong field, in which case multi-layer shields are used.

Here are some sites to get you started. You'll find many more with a google search.
http://www.advancemag.com/" , click on "Shield Design".
Here's a vendor that sells small quantities of mterials
http://www.lessemf.com/mag-shld.html"
and they have a design guide.

http://www.magnetic-shield.com/dynamics/works.html"

 

[linux kid]

First of all mumetal doesn't reflect fields at all, it shields by guiding them through the metal. Think of when you go camping and make a little ditch around your tent to conduct water away if it rains. The ditch doesn't reflect water, it guides it along. Designing magnetic shields isn't rocket science but there isn't a simple formula either--it depends on configuration and shape of both the field and the shield, and also on the strength of the field and thickness of the mumetal. The shielding effectiveness drops dramatically if you saturate the material in a strong field, in which case multi-layer shields are used.

Here are some sites to get you started. You'll find many more with a google search.
http://www.advancemag.com/" , click on "Shield Design".
Here's a vendor that sells small quantities of mterials
http://www.lessemf.com/mag-shld.html"
and they have a design guide.

http://www.magnetic-shield.com/dynamics/works.html"

Thanks marcus for the explanation. You bring me to more questions. In the camp scenario, the water can be redirected down a slope to a river or something. What about in the case of the fields? Where does it go, into the air? Do magnetic field build up (eg. like heat) or does it always stay at a certain level?

 

[ranger]

Thanks marcus for the explanation. You bring me to more questions. In the camp scenario, the water can be redirected down a slope to a river or something. What about in the case of the fields? Where does it go, into the air? Do magnetic field build up (eg. like heat) or does it always stay at a certain level?

The field moves through the mumetal (picture the magnetic field like field lines) because it has higher magnetic permeability than the air around it. The field lines originate from the north pole of the magnet then "flow" to the south pole, through the magnet itself then back out the north pole. Placing mumetal in the vicinity will simply create a sort of like a different path for the fields lines, but it will still flow from north to south through the mumetal. Please keep in mind that fields lines are abstract entities that are used to visualize the magnetic fields.

 

[Mk]

Where can I buy some, do you know?

You can however, get some Type IIs on eBay or any good electromagnetic effects-themed scientific toy website. You also need to rent a dewar for liquid nitrogen.

 

[linux kid]

The field moves through the mumetal (picture the magnetic field like field lines) because it has higher magnetic permeability than the air around it. The field lines originate from the north pole of the magnet then "flow" to the south pole, through the magnet itself then back out the north pole. Placing mumetal in the vicinity will simply create a sort of like a different path for the fields lines, but it will still flow from north to south through the mumetal. Please keep in mind that fields lines are abstract entities that are used to visualize the magnetic fields.

Thanks ranger. I actually knew that but I guess my mind just wasn't efficient with pulling that one out at the time.

 

[marcusl]

Thanks marcus for the explanation. You bring me to more questions. In the camp scenario, the water can be redirected down a slope to a river or something. What about in the case of the fields? Where does it go, into the air? Do magnetic field build up (eg. like heat) or does it always stay at a certain level?

1. Yes, it goes back into the air.

2. Static fields build up inside the material. Here's a site that shows the concentration of flux lines within an iron rod, exactly the scenario that Ranger mentioned above. The second diagram shows how iron or mumetal can be used to shield a device from a uniform magnetic field.
http://tpub.com/neets/book1/chapter1/1j.htm"

If the field is alternating, some can be dissipated as heat in the material. The area within the hysteresis curve gives the energy dissipation per cycle.

 

[berkeman]

i was thinking of making my own brick funace just for this, some sand, stone coal and an accordian blower lol.

Hi light_bulb,

Welcome to the PF. I'd suggest in making a post like the one I've quoted above that you consider using the QUOTE option in your post. That way, others know what specific post and topic you are responding to. For myself, I was very confused by your response, because I couldn't tell if you were responding to the superconductor discussion or the mu-metal discussion. After thinking about it, I think you were saying something about making your own smelting furnace for making some mu-metal, but I'm still not sure.

Anyway, just a suggestion to help you make yourself as clear as possible in PF posts. That's something we try to do around here as much as possible.

 

[light_bulb]

sorry about that, will respond with the proper quotes next time. btw i was talking about the mumetal as for the super condutor i have some ceramic i could lend you.