In Search of the lost magnetic property.

[G1Motion]

In the 1950's while disassembling speakers from old radios I came across one pole piece that was not attracted to the magnet but concentrated magnetic flux. Recently while searching for the alloy I have been informed the memory must be false because magnetic conductivity depends on the material being magnetizable.

However I clearly remember sandwiching the pole piece between a magnet and a steel washer, turning it upside down and marveling that it fell apart as soon as the washer was removed.

If the material exists, what alloy could it be, and why do so many believe, it's not possible?

 

[IRstuff]

There are situation where magnetic fields need to be shielded from other things, and mu-metals are used for that purpose. They are attracted to magnets, but do not propagate the magnetic flux. The head actuators in modern hard drives use mu-metals to shield the rest of the drive from the powerful fields generated by those magnets.

I don't know if your recollection is accurate and whether you tested your premise rigorously.

TTFN

FAQ731-376

 

[electricpete]

was not attracted to the magnet but concentrated magnetic flux.

That sounds like a contradiction. If it "concentrates magnetic flux", I take that to mean high permeability and it would be attracted to a magnet.

I clearly remember sandwiching the pole piece between a magnet and a steel washer, turning it upside down and marveling that it fell apart as soon as the washer was removed.

Hmmm. I could do that with a piece of wood or a piece of plastic. Or a piece of paper between a refrigerator and a manget. Am I missing something?



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[electricpete]

Since you mention it was a pole piece, I can guess maybe you are talking about a high frequency application. The materials used there may appear to have permeability nearly same as air at dc / low frequency (I think), but act differently at high frequency.

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[FeX32]

I thought the permeability of most magnetic materials changed with frequency of flux application. Though some more than others.
At very high frequencies the permeability of even steel could drop near zero. Is this your thinking?


Fe

 

[electricpete]

My thinking was the reverse. DC test with simple magnet showed no attraction (other than it was clamped between magnet and steel, like any piece of wood or plastic would've been). But yet OP thought it had magnetic properties. Maybe the permeability does not get high until very high frequency. Perhaps a tuned circuit with very high permeability at resonance. But much lower at power frequency but at dc much closer to air (or at least nowhere near high enough to support the weight of the piece when held next to this magnet).

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[electricpete]

My last sentence was garbled. Correction in bold

My thinking was the reverse. DC test with simple magnet showed no attraction (other than it was clamped between magnet and steel, like any piece of wood or plastic would've been). But yet OP thought it had magnetic properties. Maybe the permeability does not get high until very high frequency. Perhaps a tuned circuit with very high permeability at resonance. But much lower permeability when exposed to dc magnet, closer to permeability of air (or at least nowhere near high enough to support the weight of the piece when held next to this magnet).

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[FeX32]

That seems feasible. Interesting thought.


Fe

 

[IRstuff]

Don't forget that it was part of a speaker; that limits the frequency to less than 40 kHz, and a resonance would not be apropos for a speaker.

TTFN

FAQ731-376

 

[MagnetDave]

To be honest, I strongly doubt the pole piece really concentrated magnetic flux, given the behavior you described.

w.r.t. the frequency dependence:

As a rule, the permeability decreases with frequency (not linearly, it's generally constant with steps down as different mechanisms drop out. It's analogous to permittivity).

This material, if it existed, would be very useful today, and I doubt we would lose track of it. My guess is that there is likely something lost in your memory, or that the properties were misrepresented to you.

 

[G1Motion]

The memory isn't false. The material exists, it was used in a speaker manufactured in the thirties or forties. ElectroVoice mentions it in an advertisement. The magnetic conductivity of the material had to be greater than one or the magnetic flux lines would have been at the wrong angle to the voice coil. One reason the property may have been lost is that popular Magnetic theory, based on World Lines doesn't explain the property, but that theory requires orders of magnitude more information than theory based on Magnetons in which magnetic conductivity and magnetization are separate properties.