Nature takes its revenge ;-)

Jim Thompson · 2015-01-05T16:59:43+00:00

Jan 05, 2015 66 Replies

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John Larkin 11 years ago

Consider the 55 gal drum. A simplified model is to take a cross-section slice in the middle, which looks like a steel ring.

Consider the magnatic conductance of that ring, along the barrel axis. The steel conducts Es better than the air inside, where Es is the net permeability of that steel at some frequency, low thousands ballpark. Compute the ratio Es*As over Eo*Aa where As is the steel area, Eo is the permeability of air, and Aa is the area of the interior of the circle. It's just like putting resistors in parallel.

As is small compared to Aa, so the steel doesn't conduct much of the field.

That's a rough approximation. Real life seems to be worse... skin depth maybe.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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George Herold 11 years ago

Yeah, I'm not sure the skin depth is at all the right number for shielding local AC fields. The derivation assumes a radiated field (far field) where the 60 Hz stuff is all near field.

The only way I've been able to block B-fields, is with a piece of mu-metal I got at a trade show.

George H.

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John Larkin 11 years ago

Concentric steel enclosures help. But mumetal can be hundreds of times better than cheap steel... if handled right.

John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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Martin Brown 11 years ago

A thin layer of a better conductor like copper or silver foil on top and in contact with the steel will help a bit or more expensive and less amenable to DIY Permalloy or mu-metal round the delicate bits.

Mu-metal is about an order of magnitude better than mild steel at shielding variable magnetic fields (but still nothing like 100%).

Regards, Martin Brown

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Bill Sloman 11 years ago

It helps if you anneal mu-metal after you shape it. Apparently it work-hardens, which does nothing good for it's magnetic properties. Enthusiastic users reputedly employed multiple shells of mu-metal for maximum screening.

Bill Sloman, Sydney

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Phil Hobbs 11 years ago

Bending a mu-metal shield, or even dropping it on the floor, will reduce its permeability by a _lot_. It also saturates really easily, as you'd expect, so in high field situations it's best to use some nice thick piece of mild steel with the mu-metal inside.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net

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