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Is this a mass change or disturbed local gravity? Answered

I dropped a single 6mm dia steel ball down a 3/8 copper tube 1.29m (50.75 in) long, it took less than 1 sec to pop out the other end.
I then dropped a string of 5 X neo disc magnets, 5mm dia x 3mm thick, down the same tube and it floated down taking 7 seconds to pop out the other side.
I tried the same experiment with an aluminium tube and observed the same results.

What is causing this phenomenon?
Could it be magnetic eddy currents in non ferrous material?
Has the mass of the neo mags been changed or is it a local gravitational disturbance?



Best Answer 3 years ago

It is en electrodynamic brake using eddy currents induced into the Cu, Al, most any non-magnetic metal pipe by the motion of the magnets attempting to fall at 32'/s/s.

This induces electric (think of water eddies) currents into the copper pipe which generate their own magnetic field in opposition to the desired magnet movement. ERGO the magnet slows compared to a steel ball just pushing air down.

Works on angle Aluminum too and the thicker the tube the slower it travles.

I figured it might be eddy's, you get 1st best answer. :)

I guess its not as intriguing as I first thought, so my planned SS tube test in the morning will be mothballed.

Thanks :)

BTW the Al disk on you home watt-meter is an eddy current motor.


3 years ago

Cut a slit down the tube, and because current cannot flow, and thus no opposing magnetic field, the magnet falls normally, and you can observe a small voltage between the edges of the slit, on either side of it! A multimeter may not be sensitive or fast enough to capture it. An osciloscope may do it.

Here is an aluminum channel that has a seriously large slit and it still provides dynamic breaking because eddy currents are able to use miniscule current loops. This is how modern amusement rides are brought to a stop.

BTW, I use brass screws to hold the track in place and an iron screw to catch exit magnets.

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Interesting, I have seen the demonstration with a slit cut down the middle to stop the current, and it did work, at least with copper pipe. That aluminum has to be real thick to create that much opposing current I would think. Yeah, I have seen those "solid state" brake pads on roller coasters like some copper sheet metal strip that stick up out of the track and the magnets underneath the carts. Also have seen it the other way around. Either way I can see why they use them; they are considerably simpler, no moving parts, no actual friction to wear things away, and so probably considerably longer lasting and reliable!

I have personally observed two Al angle rods comprising an isolate interrupted square channel generating a braking eddy drag. That in my understanding is two slits for the length of the drop yet still eddy currents occur and generate opposing EMF !

I can't believe none of these answers mention Lenz's law.


Heinrich Lenz was a nineteenth century physicist. I'm not sure about Eddy Current, but his name gets mentioned a lot in the same articles.


Lenz said that "when an induced current flows, its direction is always such that it will oppose the change which produced it." in 1834

"Eddy currents (also called Foucault currents) are circular electric currents induced within conductors by a changing magnetic field in the conductor", due to Faraday's law of induction. in 1824. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field.

The term eddy current comes from François Arago (1786–1853), the 25th Prime Minister of France, who was also a mathematician, physicist who said

"eddy current comes from analogous currents seen in water when rowing using an oar causing localised areas of turbulence known as eddies
give rise to persistent vortices. Somewhat analogously, eddy currents
can take time to build up and can persist for very short times in
conductors due to their inductance."

Well, for one thing, a sphere has lower drag than a blunt body shape like that of the magnet. that *does make a difference when float down a tube, more than you might think.

to be clear, you made piston with your magnets. that has significantly more drag than a bearing. To normalize the sphere with the magnets in terms of drag, you'd have to have a very close tolerance on that sphere, almost to the point where mechancial friction could play a role. (which ,actually may also be playing apart in your magnets T.O.F as is).

What you have done is a standard school science lesson demonstration - the falling magnets generate an electric current in the pipe. That current has its own field, opposite to that of the magnets, which resists the movement of the magnet.


3 years ago

I'm waiting for the perfect day to do a few photos and then I will have an 'ible typed up on exactly what you are talking about!

Could you please remove/change this question so nobody gets the idea and beats me to it? I hope to have my 'ible up in a few days at the most, it is all written and I just need photos.

Thanks, JM1999

FEAR the Eddy ;-)



I just did a proper google search using "eddy currents in copper tube" and got 84000 results.

These are previous Instructables:





So you wont be the first and the idea has been around awhile, but you can make it good, dont use blurred pics from your cellphone. :)

Give me some time...

I am uploading the photos right now!

I will post the link when it is done :D