The Permeability of Aluminum

[Pano]

So is Aluminum (AKA Aluminium) anti-magnetic or something? Does it have a Permeability of less then 1? It's not supposed to.

But here is an odd thing. After chatting with BudP about transformers and inductors, I was doing some quick experiments on the effect of a metal core on a coil. Took a small 0.2mH air core inductor and placed various metal things in the gap. Was able to raise the inductance to over 0.4mH with large mass of steel, like a socket from a ratchet.

Copper, brass, tin, wood, plastic, flesh and blood had no effect. But aluminum? It caused the measured inductance to drop. The more massive the aluminum piece, the bigger the drop. Why? Is this just an artifact of my inductance meter, or is the aluminum actually causing a drop in inductance?

[PMA]

Eddy currents? What was the setup exactly?

[jcx]

solid copper likely will show even more effect

high electrical conductivity metal's conduction electrons get shoved arround by time varying magnetic field - and their resulting motion (Eddy currents) causes them to generate their own magnetic field that opposes the external one

another veiwpoint is that a closely fitting conductive ring or tube forms a shorted turn of a transformer with the coil - the open and shorted turn inductance measurements may be used to calculate the mutual inductance/coupling constant

[Pano]

I can see the shorted coil thing. all the aluminum I have is tubes of some sort. Nothing solid. That may be it.
So the steel tube should also be a shorted turn, but that effect is overwhelmed by the mass of the steel.

Set up is simple. Just a little air core coil like this:


Metal inserted in the center hole.

[Andrew Eckhardt]

It's certainly strange that the copper and brass showed a different effect. Ideally for shorting a field you'd like solid. Depending on how you're measuring you could be getting some resonance that is affecting it. Looking at the current or voltage waveforms might give a clue. Incidentally, for very low inductance coil tuning often threaded aluminum tuning slugs are used. It's light and cheap and of course has much better stability and response than any permeable material, so they make the coil a little bigger and then tune down.

[jneutron]

Originally Posted by panomaniac

So is Aluminum (AKA Aluminium) anti-magnetic or something? Does it have a Permeability of less then 1? It's not supposed to.

But here is an odd thing. After chatting with BudP about transformers and inductors, I was doing some quick experiments on the effect of a metal core on a coil. Took a small 0.2mH air core inductor and placed various metal things in the gap. Was able to raise the inductance to over 0.4mH with large mass of steel, like a socket from a ratchet.

Copper, brass, tin, wood, plastic, flesh and blood had no effect. But aluminum? It caused the measured inductance to drop. The more massive the aluminum piece, the bigger the drop. Why? Is this just an artifact of my inductance meter, or is the aluminum actually causing a drop in inductance?

The measured inductance is indeed being lowered by the aluminum, it is not an artifact of the meter.

Originally Posted by panomaniac

I can see the shorted coil thing. all the aluminum I have is tubes of some sort. Nothing solid. That may be it.
So the steel tube should also be a shorted turn, but that effect is overwhelmed by the mass of the steel.

Set up is simple. Just a little air core coil like this:


Metal inserted in the center hole.

The mass of the steel is not the player here.

The conductivity of the material will dominate the effect. The principal involved is called Lenz's law. It is the nature of the conductive objects to try to exclude the time varying fields from the metal. (loosely stated of course, don't get me started with equations....

Here's a link:
HyperPhysics
Look under E/M, then Faraday's law...lenz is the second group.

For your specific results, you have to consider the geometry of the objects you are placing within the field. The biggest effect will occur when the field lines penetrate the metal perpendicular to the best conductivity. How well the geometry of the metal can setup it's internal currents is the dominant factor.

This happens all the time for me, the inductance vs frequency for coils I work with and test will be altered by the surrounding metal objects..typically the inductance will fall off with frequency as a result, and the magnetic field of the coil will really start to phase lag. Also in general, the meter will pick up a higher series resistance if you use it's Ls/Rs function. The meters are not able to distinguish the difference between the coil electrical resistance and the dissipation that results from the external metal taking the field.

But, that's just my guess...y'all's gonna need to speak to a real magnetic field guru if y'all needs ta know the truth..me, I'm just a country doctor...Jim..

Cheers, John

[Pano]

Great stuff John, thanks!

Magnetic circuits are fun. I'll continue to play with this, if only for my own amusement.

[Don Hills]

Just a small data point... I started my electronics career in the tube era. The RF and IF tuned circuits in radio & TV receivers usually used threaded ferrite cores ("slugs") that could be screwed in and out of the coil formers to alter the tuning. These had a bad habit of seizing, making them liable to breakage if you tried to force them. So rather than try to alter a slug's setting to see if it was correctly tuned, we used a tuning wand - a plastic rod with a small ferrite slug on one end and a small brass slug on the other. Inserting the ferrite slug would lower the resonant frequency, and inserting the brass end would raise it. If both actions caused the performance to get worse, the coil was already correctly tuned and the coil's slug could be left alone.

[Elvee]

Aluminium is in fact a paramagnetic material, which means it is very slightly magnetic:
Paramagnetism - Wikipedia, the free encyclopedia
Its relative permeability is 1.000022, and in theory, it should increase the inductance of a coil.
But in its massive form, the shorted-turn effect resulting from its high conductivity is overwhelmingly dominant, making the paramagnetic effect unnoticable.
Aluminium - Wikipedia, the free encyclopedia
The effect might be detected with a very finely powdered material, at very low frequencies to minimize the effect of eddy currents.

[Pano]

Don: What a clever troubleshooting idea. So simple, so cool.
Elvee. Yeah, after looking up the permeability of Alu, I figured it was so close to air that it shouldn't make a difference - but it did. So it's the shorted turn, not the permeability that counts.

Maybe I need to make an AF oscillator with the coil. Then I could have fun listening to the charges different metals and shapes make.