Aluminum and magnets

[tedmich]

neato!

Magnet versus aluminium experiment › Tricks (ABC Science)

[Jim Darr]

Classic "Eddy Current" demonstration. Always very cool to watch these videos. Check the web for more.

[Joe Gwinn]

neato!

Magnet versus aluminum experiment › Tricks (ABC Science)

It would be fun to try this with the foil turns insulated from one another by a strip of saran (food cling) wrap. The effect will vanish, for lack of a shorted turn. Wonder if they will figure it out.

Alternately, use a piece of solid-metal aluminum tubing from the hardware store.

[David Schwab]

Also works with copper.

[Jim Darr]

Also works with copper.

Yeah, all the previous demonstrations I've seen were with copper tubing. This is the first I've seen with aluminum foil. Joe raises a good question in insulating the aluminum wraps and seeing what happens. Thoughts anyone?

[Mike Sulzer]

Yeah, all the previous demonstrations I've seen were with copper tubing. This is the first I've seen with aluminum foil. Joe raises a good question in insulating the aluminum wraps and seeing what happens. Thoughts anyone?

As he said, it will not slow down. There is no low resistance path for the current to flow around. That web page says that the longer the tube, the slower it falls. Is this really true no matter how long you make the tube?

[Joe Gwinn]

That web page says that the longer the tube, the slower it falls. Is this really true no matter how long you make the tube?

With solid-metal tubes, the length of the tube does not matter, so I'm guessing that the length is needed to get adequate connection between turns of foil.

I suppose how tightly the inner turns are wound also matters.

[g1]

That web page says that the longer the tube, the slower it falls. Is this really true no matter how long you make the tube?

The length of the roll (unrolled), not the length of the tube. It's worded a bit awkwardly. So a 150 metre roll will give a slower result than the 50 metre roll.

[Jim Darr]

The length of the roll (unrolled), not the length of the tube...So a 150 metre roll will give a slower result than the 50 metre roll.

That makes sense. Will have to measure the "drop rate" on a new roll of foil and then again as the roll gets used.

[David King]

So does the fact that it's a roll of foil or a sold tube of the same thickness and diameter make the slightest difference?
If we sliced down the full length along the side of the tube or roll of foil we've effectively cut the circuit, stopped current flow and the magnet would then drop significantly slower?
I have my doubts about that because if i just slide a neo magnet down a steeply sloped aluminum plate, it will slide down very slowly.

I like Joe's idea to unroll the Alu foil and lay a layer of saranwrap over it and roll it back up making a coil. Would that behave any differently if we shorted the start and finish together?

[Mike Sulzer]

So does the fact that it's a roll of foil or a sold tube of the same thickness and diameter make the slightest difference?
If we sliced down the full length along the side of the tube or roll of foil we've effectively cut the circuit, stopped current flow and the magnet would then drop significantly slower?

The roll is almost like the solid tube because the layers are in conductive contact. The magnet will drop faster if you cut down the current; it is the current that makes a magnetic field that "pushes" on the magnetic file of the magnet just as two magnets with the correct orientation push each other away.

[Mike Sulzer]

The length of the roll (unrolled), not the length of the tube. It's worded a bit awkwardly. So a 150 metre roll will give a slower result than the 50 metre roll.

Well, they already have used the term "thicker" for that. Very poorer wording.

[Mike Sulzer]

With solid-metal tubes, the length of the tube does not matter, so I'm guessing that the length is needed to get adequate connection between turns of foil.

Yes, that was my point. Although perhaps the length (of the tube not the unrolled length) does matter somewhat for very short tubes because the spatial derivative of the magnetic field of the falling magnet does not result in as large a time derivative of the current in the aluminum for a very short tube. That was the only way I could justify the statement I objected to, and I admit it is a bit of a stretch!

[J M Fahey]

I'm painfully aware of that phenomenon and it bites me every day.

I make and repair speakers and most common by far aftermarket voice coil type is the one wound on an aluminum former .

Strong and cheap and stands maximum power (that's why it's so popular) BUT it's also the one that "self brakes" most.

One of these days I'll film and post here a video, showing how much.

Funny thing is that it does have a slit which cuts the "big" shorted turn (the tube is made out of a single turn of rectangular aluminum, wound over a former, and ends do not meet leaving a 1 mm gap) but whay many don't realize that you still have smaller shorter circular paths, imagine twirls, all over the place.

The experiment poster claims "it works on rare earth magnets, not on ferrite ones" .... well, it does, just it's not that evident, just because he's using raw magnets in open air, but in a speaker system which tightly focuses and concentrates magnetic field in the VC gap, it's very apparent.

When I put a speaker frame face up on the bench and drop an aluminum former VC into the gap, it takes 3 or 4 seconds to drop 1 inch but if the speaker is still unmagnetized (I magnetize them at the shop, usually after reconing, to minimize problems) the VC drops like a stone ... literally.

Also paper, kapton, fiberglass or nomex former VC drop like a stone BUT if I short the VC wires it drops slower.
Not the full 3 or 4 seconds of the aluminum one, but , say, 1 second, which is noticeably slower than "stone speeds".

And why do I say "it bites me?"
Because aluminum former VC , once quite popular, Eminence for one used them a lot in the late 70's , are terrible for guitar.

That strong self damping kills big way the resonant frequency peak which in tube amps gives rise to thump, and also brakesn them at high frequency which makes them dull.
Worst of both worlds.

That's why paper / kapton / nomex are favorites.

Some big PA woofers use them, because "dull" does not scare them anyway, and low frequency extra damping is figured into the TS parameters and automatically taken care of in box calculations.

Today's most heat resistant VC, used only in woofers and subwoofers, is wound on matte black anodized aluminum.

[Mike Sulzer]

That is an interesting way to measure the former effective resistance: compare the speed with which it and the shorted voice coil with an insulating former drop. And I like the reminder of the difference between a tube output stage (especially with no feedback used) and a solid state: the high impedance looking back into a tetrode or pentode allows the voice coil the freedom to move while the low impedance of the solid state "controls" it, that is, unless a circuit with intentional high impedance is used.