Multilayer magnetic shielding

Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:

All 'turns' which pass through the toroid center ARE turns. ANY 'turn' which remains outside the core center is NOT a turn, and does not couple. It is for eddy current abatement. Toroids as a rule do not emit greatly as compared to conventional transformers with laminated e-cores for example.

Mu metal shielding does not shield against magnetic fields... NOTHING can. It merely redirects it (diverts) thereby attenuating it both externally introduced fields from 'getting in' and those generated by the 'shielded' device from getting out.

So multiple layers of mu metal shielding WILL 'shield' (read divert) better. But actual shielding from slow moving magnetic fields does not happen. It merely 'attenuates' by diverting.

Phil Hobbs wrote in news: snipped-for-privacy@electrooptical.net:

Two masks are better too.

Superconductors certainly do shield against magnetic fields, and words like 'divert' and 'attenuate' mean an imposed B field is diminished in a region of interest. Shield is a good word for this effect.

Phil was specifically talking about progressive and non-progressive winding s. Any "progressive" winding on a toroid is - incidentally - a single turn in the plane of the toroid (which usually doesn't matter, but does create a stray field). You can largely cancel that stray field by adding an equal a nd opposite field with a single turn of wire around the toroid.

A non-progressive winding will do the job rather more perfectly. I first ca me across this point in

It came from the British National Physical Laboratory and one of the author s - Bryan Kibble - is the Kibble in the Kibble mass balance. It didn't matt er to me at the time but later I came across an ingenious scheme for measur ing the conductivity of a fluid by dunking two - stacked non-progressively wound toroids in the fluid and measuring the current induced in the second toroid when you drive the first, because only the current induced in the fl uid circulates through both toroids and can excite the second.

True, But we all know that.

Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:

I never said nor inferred that it was not known by some of the folks here or anywhere else.

That's true of DC but not AC. AC causes eddy currents, which dissipate the field energy.

Cheers

Phil Hobbs

Magnetic shielding is 'different', in that you can't impede the flux, you can only steer it. I had a case where little ferrite RF transformers were saturated by the fringing field of large 1.2T magnets in a particle accelerator. I sandwiched the transformers between two soft iron discs, aligned parallel to the field, so that most of the flux ran through the discs, and very little was left between them. It was very important to align the plane of the discs with the field. However, the shielded volume could be left open at the top and bottom without loss of effectiveness.

Jeroen Belleman

Again, this is true only at DC, because there are no magnetic monopoles. Eddy currents absorb AC magnetic fields. (That's how transformers work, after all.)

I had a case where little ferrite RF transformers

Cheers

Phil Hobbs

You know that, and we know that, but it doesn't stop some from looking.

Here is a lecture in The Royal Institution, where Michael Faraday once taught. It is long and boring, but at one point they claim to have trapped some in a crystal that causes the pitch of a detector to change when brought near. Very unconvincing. There are probably other mechanisms that could do the same thing:

The Physics of Magnetic Monopoles - with Felix Flicker

Magnetic fields have been observed in intergalactic space (by Zeeman splitting of dipole-forbidden spectral lines). That places a very stringent upper limit on the abundance of magnetic monopoles. I haven't looked into it in 30 years or so, but iirc the limit was so low that the idea of somebody finding one in his lab is _a priori_ exceedingly unlikely.

Cheers

Phil Hobbs

I don't see how a monopole could exist. If you had a monopole, a magnetic field would emanate from it. But where would it terminate? If it wrapped around and entered at the other end, it would no longer be a monopole. So what happens to the magnetic field?

There are all sorts of electric monopoles, such as protons and electrons. Maxwell's equations _in vacuo_ are entirely symmetric between E and B, so in a universe with magnetica monopoles, there would be no long-range static B fields, just as there are no long-range static E fields in ours.

Cheers

Phil Hobbs

The magnetic field is an abstraction, based originally on alignment of iron whiskers (or compass needles). Gravitational field lines do not terminate, every mass is a kind of gravitational monopole. There aren't any observed magnetic monopoles (but Blas Cabrera once got a suggestive signal...), yet it would be easy to add them to the Maxwell equations.

I don't understand your definition of static E fields. They are used in cathode ray tubes (CRT), Van de Graaff generators, and are created naturally by walking across a rug. As far as I understand, the field strength amplitude varies with the inverse of the distance, so they as long-range as any other, such as light. Electrons and protons are point sources subject to the Coulomb force, which is why fusion is so rare in the sun.

As far as symmetric Maxwell's equations, just because they say something is possible doesn't mean it actually happens. Witness the failure of zero or negative logarithms, or the rarity of magnetic monopoles. There has to be some other explanation for the rarity.

But if a magnetic monopole could exist, what happens to the magnetic field?

And if there can be a North magnetic monopole, there would have to be South magnetic monopoles. What happens if these are brought together? Would two equal monopoles repel each other, and two unlike monopoles attract each other? If so, there would have to be a field emanating from them. If so, what happens to the field when they are isolated?

None of these issues are addressed in any discussion of magnetic monopoles that I am aware of.

Magnetic fields are a bit more than an abstraction. They are used in motors, transformers, rail guns, fridge magnets, compass needles, and a host of other applications.

I finally found the answer. Quote:

"Gauss's law for magnetism states that no magnetic monopoles exists and that the total flux through a closed surface must be zero."

"Gauss' Law for Magnetism"

The net magnetic flux out of any closed surface is zero. This amounts to a statement about the sources of magnetic field. For a magnetic dipole, any closed surface the magnetic flux directed inward toward the south pole will equal the flux outward from the north pole. The net flux will always be zero for dipole sources. If there were a magnetic monopole source, this would give a non-zero area integral. The divergence of a vector field is proportional to the point source density, so the form of Gauss' law for magnetic fields is then a statement that there are no magnetic monopoles.

Not so; we use magnetic forces, not fields. The field is a mathematical abstraction from the forces, just as the vector potential is an abstraction from the fields.

Abstractions, like other theories, don't spin motor shafts. They do allow us to extrapolate; for instance a positive monopole will attract a negative monopole, because stored field energy decreases when they move closer together. Decrease in field energy, and energy conservation, tells us that this motion causes work.

We know the energy storage argument because of the abstraction, the field, even though we haven't ever seen any monopoles.

Magnetic monopoles cannot exist. See

Gauss' Law for Magnetism

The net magnetic flux out of any closed surface is zero. This amounts to a statement about the sources of magnetic field. For a magnetic dipole, any closed surface the magnetic flux directed inward toward the south pole will equal the flux outward from the north pole. The net flux will always be zero for dipole sources. If there were a magnetic monopole source, this would give a non-zero area integral. The divergence of a vector field is proportional to the point source density, so the form of Gauss' law for magnetic fields is then a statement that there are no magnetic monopoles.

Gauss's Law for Magnetic Fields

Magnetic Monopoles & Gauss' Law for Magnetism

Oh, when I said 'we haven't ever seen', that includes Gauss. Gauss was a mathematician, not an observer. He was working with an absence of evidence, but not from evidence of absence.