# Re: mostly off topic, but woodworking relevant

• posted on March 2, 2008, 2:13 pm

They're called eddy fields. Because energy is created, energy elsewhere has to be reduced (speed).

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• posted on March 2, 2008, 2:49 pm

Funny, my brother's name is Eddie. Marc

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• posted on March 2, 2008, 8:41 pm

I bit and tried , pretty cool

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• posted on March 3, 2008, 2:41 am

I bet if you drop him down a big copper pipe, Eddie wouldn't fall as fast as a piece of wood either.

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• posted on March 3, 2008, 11:01 am

Thanks Maxwell, You made me laugh with a mouthful of coffee this morning. That was really funny and I'm passing this on to Eddie. Marc

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• posted on March 3, 2008, 11:40 am

The concept is generally know as the eddy CURRENT. The changing magnetic field due to the motion of the magnet induces an eddy current in the pipe; the eddy current in turn induces a magnetic field the same way any current old induces a magnetic field. There is a relative minus sign so the field due to the eddy current opposes the field from the magnet -- no perptual motion machines. There is nothing special about the fields.
Energy isn't created nor lost. The gravitational potential energy of the magnets is converted into kinetic energy and (presumably) heat in the pipe. If the pipe has poorer conductivity the speed restriction should be less dramatic -- anybody got an aluminum pipe? If you repeat the exercise with a copper pipe but put a slit down one side what happens?
hex -30-

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• posted on March 6, 2008, 2:27 am

I bet :)

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• posted on March 6, 2008, 11:46 am

--
Best regards
Han

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• posted on March 6, 2008, 4:48 pm
Han wrote:

Depends. Some SS is attracted by these magnets, such as SS knife blades. I had a similar length of stainless pipe, but a little larger than the diameter of the other two. It slowed down the magnet less than the aluminum. I was using a spherical magnet from Lee Valley.
I think the braking depends on the electrical conductivity, so theoretically a silver or gold tube would really slow it down. I didn't have any lying around the shop, so couldn't test it.
--
Gerald Ross
Cochran, GA

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• posted on March 6, 2008, 5:21 pm
"Gerald Ross" wrote:

SFWIW, if it is magnetic, it isn't stainless.
300 series S/S is not magnetic.
304 is probably the most common grade.
400 series is not truly stainless since it still has some austinite, is magnetic, and is used for things like springs, knife blades, etc that need to be heat treated.
Lew

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• posted on March 6, 2008, 7:19 pm
Lew Hodgett wrote:

Not being a metallurgist, all I can say is it has "Stainless" on the blade.
--
Gerald Ross
Cochran, GA

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• posted on March 6, 2008, 8:10 pm
"Gerald Ross" wrote:

Then it must be true<G>
Lew

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• posted on March 6, 2008, 5:32 pm

Aluminium only conducts about 2/3rds as well as copper. Therefore, to match the electrical conductivity of your copper tube, the aluminium one should have a wall thickness of about 30+% more than the copper tube's. BUT, that increase in thickness, only manifests itself at a further distance from the magnet, so even if the conductivity were to be equal, the copper would still have an advantage. Especially if the copper tube was manufactured by the Monster Cable people. But then again, I could be talking shit.
r

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• posted on March 7, 2008, 4:08 am
The thickness of the metal matters. The thicker it is, the better the braking effect. If you don't have a copper tube (especially one that's close to the dimensions of your magnet), but you do have a flat aluminum plate, try inclining it steeply and sliding the magnet down. Try different thicknesses of metal if you have them and you'll see.
- Owen -
wrote:

Aluminium only conducts about 2/3rds as well as copper. Therefore, to match the electrical conductivity of your copper tube, the aluminium one should have a wall thickness of about 30+% more than the copper tube's. BUT, that increase in thickness, only manifests itself at a further distance from the magnet, so even if the conductivity were to be equal, the copper would still have an advantage. Especially if the copper tube was manufactured by the Monster Cable people. But then again, I could be talking shit.
r