A Question for My Readers: Lenz’s Law, Magnetism, and Copper

August 29th, 2014 by Roy W. Spencer, Ph. D.

This has nothing to do with climate…that I know of.

I have a question for any readers who know about how magnets and conductors work, like in an electric motor.

There are some cool videos showing how you can drop a powerful magnet through a copper tube, and it falls very slowly. Here’s my favorite, which involves a huge magnet and an even more impressive piece of copper (see especially 0:40 to 0:44):

According to Lenz’s Law, the electrical current generated in the copper by the falling magnet generates an opposing magnetic field which slows the fall of the magnet.

I’ve ordered a small version of the disk magnet, 1.5″ by 1″, which is supposed to hold 146 lbs (!). Rather than getting 2″ diameter copper tube to drop it through, I plan on wrapping copper wire (say #12 or #14) around a piece of PVC pipe.

My question is this: How can I wrap the wire to maximize the slow-fall effect? Or is a solid copper tube going to provide the maximum effect? Does the amount of copper (thickness of the tube wall, number of copper windings) have an impact?

I suppose I can answer this with some experimentation, but I’d like to know a little more about the variables (which I assume are all transferable from electric motor design) ahead of time.


115 Responses to “A Question for My Readers: Lenz’s Law, Magnetism, and Copper”

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  1. ossqss says:

    That is cool stuff. Thanks. I don’t know the answers to the questions, but you are only a couple steps away from making a Faraday flashlight 😉

    • yeah I bought one of those a few years ago…hardly generates any light.

      • JohnKl says:

        Agreed! My wife and I have an LED one as well. They supposedly can help you in an emergency, where batteries may be lacking. Realistically does it seem like a good idea in an emergency where one’s time is likely limited to consume precious seconds shaking a flashlight?! Imo, better to have an LED flashlight (lower energy consumption) with a good set of batteries.

        Have a great day!

        • Damian Crudele says:

          My wife has a flashlight that has a crank on it, and it takes very little cranking to produce a very nice light source for a good period of time. She bought this cheap, likely at a dollar store or WallMart or somewhere like that.

          BTW, I have some #9 copper wire that I retrieved from a section of high voltage cable, it seems to me, if you want to wrap some copper wire, the larger the wire, the easier the job. Anything smaller than #12 wire is going to be an endless task of wrapping. #9 is too stiff to work with, but #12 should work fine.

          Then, just wrap your pvc nice neat wraps, line next to line, until you reach the end of the pipe and then go up a level and wrap over the wrap you just laid down…. make a coil. That way you can make it nice and thick, and you should be able to run the end of the copper wire into a LED light and make it light up when you drop your magnet.

  2. lemiere jacques says:

    wonderful questionn ,questions about magnetism and magnets must have been the trigger for many people to study physics.

  3. Thanks, Dr. Spencer. The demonstration was very good.
    I think the copper pipe and the magnet will get warmer to account for the “gravity suspension” energy surplus.
    And, to substitute the solid pipe with wire, the coil winding has to be short-circuited. If not, a voltage difference will appear between the ends (this is a generator, like an electric motor can be).

  4. Jim Curtis says:

    Your direction of wrap will generate a strictly axial magnetic field inside the tube which is good (assuming that you short the two ends together). It would be better if you could short the ends of each turn end-to-end locally since it would minimize resistance and maximize current (put the windings in parallel instead of in series).

    • Jim, isn’t the extrapolation of your suggestion just a solid copper tube then? Or not? A bunch of copper rings vs. a solid copper tube…which is best?

      • Jim Curtis says:

        A solid tube would be best (one high conductivity turn).

        • Jim Curtis says:

          … because its the highest conductivity you can get per pound of copper.

          • Jim Curtis says:

            And the ring would allow you to get rid of the tube PVC pipe so you wouldn’t have to make the ring with a larger diameter than absolutely necessary (a shorter turn has less resistance).

          • Jim Curtis says:

            Plus there’s the “closeness” factor (between the fields of the copper current and the magnet — harder to quantize but even more important). Magnetic fields close on themselves and so are not even visible at a sufficient distance. Witness a motor or your power line transformer intense magnetic fields in the core but almost none externally (actually not a good example because of the presence of a lot of iron).

          • Jim Curtis says:

            Plus a solid tube would give the best conductivity in all directions which might help keep the magnet from lazing over and taking the orientation that has the least interaction with the copper (and falling the fastest). I dont think a lot of thickness is important since the current will tend to concentrate on the inside of the tube (least resistance around the turn). You do need some thickness for good conductivity, but I have no idea where to draw the line.

          • Jim Curtis says:

            Maybe you could embed it in a paper sled that holds it in the orientation it most dislikes when it is allowed the freely tumble through.

          • Jim Curtis says:

            Magnetic attraction (north to south poles) is stable. Magnetic repulsion (north to north poles) is not. Sounds like youre doing repulsion. Should be some interesting experiments there.

          • Jim Curtis says:

            I have some small x 1/8 inch disk neodymium (powerful) magnets with an axial field (a lot of disk refrigerator magnets have transversal fields, along a diameter line). So I bought some 5/8 (9/16 ID, a little sloppy) copper tubing yesterday to play with. Its impressive how stable the orientation (flat) is as it falls, although it tends to spiral down always touching the pipe. Takes about 6 seconds to fall 33 inches.

          • Jim Curtis says:

            Also, it seems to prefer to spiral in clockwise (anti-cyclone) direction viewed from above. Does that mean its a climate denier, or just that some of the materials came from the SH?

          • Jim Curtis says:

            So fascinated by the orientation thing that I had to try others. So I hot glued a toothpick tail to the magnet to force the issue (or just to add ballast for apples-to-apples comparison). The results for a 33 fall (plus some human reaction time) are:
            0 deg (the preferred flat orientation): 7.8 sec,
            45 deg (half way): 6.8 sec,
            90 deg (edge first): 2.8 sec,
            So, the orientation with the slowest fall (maximum interaction with the copper?) is the preferred one totally counter-intuitive to me. There must be a principle there somewhere.
            P.S. How do those little atoms dynamically solve all those field equations (and communicate the solution to each other) so they know how to behave.

      • Mel Amato says:


        Basically, you will be making a generator. Wrap as many turns of insulated copper wire as practical around the tube. A voltage will be induced and will appear at the ends of the coil leads when you drop the magnet through the tube. That induced voltage will be proportional to the product of the number of turns and the speed of the falling magnet. Strip the insulation off the 2 ends of the wire and wrap the ends together (soldering them would help) to form a short circuit. The induced voltage will cause a current to flow in the coil which will set up a magnetic field opposing the motion of the falling magnet. The force will be proportional to the product of the number of turns and the current. Using a larger diameter copper wire will result is a lower electrical resistance in the coil circuit and, hence, in a large current for a given induced voltage. This should result in a larger opposing force and a more slowly falling magnet. In summary, more turns of increasing diameter insulated copper wire will result in a slower falling magnet. Hope this helps. HAVE FUN!!

    • Jim Curtis says:

      Maybe you could do your wrap and then solder some short wires parallel to the axis to the each layer and you build up layers to in effect short each turn to itself.

  5. lemiere jacques; Yes, they were. See:
    The Great Magnet, the Earth, W. Gilbert (Commemorating the 400th anniversary of “De Magnete” by William Gilbert of Colchester. In 1600, four hundred years ago, William Gilbert, later physician to Queen Elizabeth I of England, published his great study of magnetism, “De Magnete” – “On the Magnet”. It gave the first rational explanation to the mysterious ability of the compass needle to point north-south: the Earth itself was magnetic. “De Magnete” opened the era of modern physics and astronomy and started a century marked by the great achievements of Galileo, Kepler, Newton and others. David P. Stern).
    See http://istp.gsfc.nasa.gov/earthmag/demagint.htm

  6. PA says:

    Well, if you use a very thin-walled plastic tube almost the size of the magnet to minimize the air gap, in theory it should work.

    I would wrap full layers top to bottom/bottom to top until you have at least 6 or 8 layers of radio wire.

    Leave the starting end exposed and when the final wrap comes back up to the start – tape it off and short the ends together.

    This is a long solenoid.

    If you applied DC power to the two ends (after the magnet dropped to the bottom) it should shoot the magnet out of the tube if you get the polarity right.

  7. Bryan says:

    Better to wind insulated copper wire .
    The more turns the better.


  8. Johan says:

    Not exactly my expertise, but found this paper that gives time of flight in terms of magnetic permeability, conductivity, length and radius of the pipe and of the mass and magnetic dipole moment of the magnet.
    The authors ignore thickness of the pipe and radius of the magnet, for reasons explained in the text.

    Have fun!


  9. Fulco says:


    This reminds me of the good old days at the Pieter Zeemanlab and the spectacular demonstrations electromagnetism of prof Gersdorf.
    See also:


    This one is more fun, we did the same experiment in the Pieter Zeeman lab in Amsterdam with a large aluminium ring and a large capasitor.
    It went through te ceiling at 10 meters.

    By the way you need a massive thick cilinder, winding a copper wire will probably not do.

  10. KevinK says:

    Dr. Spencer, a solid conductor will always work the best (ie a solid one piece tube of copper).

    Given that may be cost prohibitive for your budget the next best thing would be a solid thin wall copper tube wrapped with sheets of copper (kind of like putting a roll of paper towels back together).

    Ideally you would want to solder all the surfaces together.

    Wrapping individual wires is the next poorest choice, they will want to be uninsulated and ideally soldered together over the entire surfaces.

    The wrapping should go around the circumference (like a roll of paper towels).

    A greater wall thickness will increase (to a point) the repulsive force.

    Bare un-insulated copper in a coil will oxidize over time and make your coil less effective.

    And a coil of round wire will have “fill factor” concerns (ie round wires will have small air gaps in between). For high performance electromagnets square wire (yes it is made in square cross sections) is used to eliminate the air.

    A reference for small quantities of copper: http://www.onlinemetals.com

    Cheers, Kevin

    PS: a solid piece of copper that size prices out at about $750 depending on the alloy selected, but it will make one heck of a door stop if you get bored with the magnet.

    • CC Rick says:

      A normal piece of 3/4″ copper water pipe works fine with the small button super-magnets available. I found some on eBay for a few $ and the pipe is a few dollars max at a local hardware store. I think the thinner wall sells locally as schedule 40 – the thicker as schedule 80 but I may have that wrong. Works like a charm. Use it in my physics classes every term.

  11. Svend Ferdinandsen says:

    It is the same effect that brakes an old fashion DC motor with fixed magnetizing.
    If you spin it up and short circuit the terminations it could eventually break apart.
    I have an old washing machine DC motor, and you can hardly turn it when it is short circuited. It is nearly an ideal motor/generator, so it starts running at 1.5V up to 400V.

  12. Sparks says:

    Roy, use copper sheeting or Copper Flashing, it will cost about the same as a roll of 2.5mm wire and less work is involved, use 22 Mil or above and simply wrap it around a template to get the desired size and you’re all set to experiment.
    The good thing about the copper sheeting is that you can add or remove layers. Keep in mind, the thinner the sheeting is the faster you will have to spin the magnet, a good alternative to copper sheeting is aluminium sheeting, I’m sure the same effect is produced with it.

  13. KevinK says:

    Also, it will work with Aluminum, just less effectively.

    And any heating in the Copper would yield less energy than that required to lift the magnet above the tube in the first place. Since the magnet would hit the table with less energy (compared to dropping it without the copper tube)the total energy (collision energy + tube heating) is the same as just the collision energy without the tube.

  14. Werner Brozek says:

    With questions of this type, it is best to think of extreme cases and then go from there to figure out what is best.
    So first of all, what would happen if we only had a single turn of copper? Obviously not much, so the more copper, the better.
    What happens if the coils are 10 metres in diameter? Again, not much, so the closer the coil is to the magnet, the better.
    What happens if cardboard is used instead of cooper? Nothing. So the better the conductivity, the better. So use silver instead of copper as soon as your Exxon check arrives.
    What happens if the magnet falls very slowly? Not much. So make it fall as fast as possible before hitting the coil to maximize the effect of Lenz’s Law.
    What happens if the magnet is very weak? Very little, so make the magnet as strong as possible.

    Werner Brozek (retired physics teacher)

    • alphagruis says:

      Excellent “reasoning”.

      Similarly, what happens to life and mankind if earth mean surface temperature dropped to the extreme T = 0 Kelvin limit? Not much, everything would die. So make the temperature as high as possible.

      That’s what finally led me to accept to get a check from Exxon too and promote fossil fuel burning.

  15. KevinK says:

    Oh, and there is a practical use for this effect, research “ferro fluidic damper”. These use a magnet in a magnetic fluid “ferro fluidic” floating inside a can. The can is attached to a rotating shaft (stepper motors usually). When the shaft turns (jerks) with high speed impulses the fluid is attracted to the magnet and the viscosity between the fluid and the can wall adds “drag” (that’s what it’s called when you don’t want it)or “damping” (what it’s called when you do want it) to the rotating shaft. When the motion is steady the magnet just floats inside the can with no large effect on the motion of the shaft.

    Cheers, Kevin.

  16. lemiere jacques says:

    well if you use a supra conductive stuff…

  17. TedM says:

    Pretty much agree with most of the comments here. I’m not sure that paralleling individual turns of copper wire would be better than a short circuited coil though. While it’s true that there would be a lower resistance the induced EMF would be lower so the current wouldn’t necessarily be higher. The strength of the magnetic field produced will be directly related to the number of turns in series. So I think that the gains would be offset by almost equal losses. Having said this, even though the theory is right, I would probably have to experiment to convince myself for sure, but I tend to think that the solid copper tube will give the best result. (Extremely low resistance.)

    Isn’t it nice to have ideas, theories or hypothesis that are actually testable in the home workshop.

    • TedM says:

      Above I said “The strength of the magnetic field produced will be directly related to the number of turns in series.”

      This is assuming the same current!!

  18. Thanks for all of the comments. After also looking at the suggested paper describing dimensional analysis of Lenz’s Law, I think I’ll start with a solid copper tube. I thought copper wire would be interesting because you can put a little space between the coils so you can see the magnet dropping from the side…except the lack of copper will cause the magnet to fall too fast. Maybe coiled 1/4 inch copper tubing instead, with wire strips soldered up the sides.

    • Phil R says:

      Just skimming late on a Friday and not an expert in electromagnetism, but i’ve done this before for the kids. Use copper pipe that you can get from the local home center for a few bucks. Saves time and money.

      I didn’t do this on purpose,but from a project a worked on several years ago I ended up with two sizes of magnets, so I bought two different sizes of pipe. I noticed two things. one, the smaller magnet,although the effect was evident, fell through the pipe faster than the larger magnet (probably makes sense). two,minimize the annular space. the closer you can get the I.D. of the pipe to match the diameter of the magnet, the better the effect will be.

  19. Don Sailing says:

    It just seems that using copper wire, shorted at the ends, will force the induced current to travel the entire length of the wire. The current should encounter more resistance and more loss than with a solid tube.
    While it should still work, the result should be less breaking of the magnate.

  20. AnonyMoose says:

    I think you inferred this… there’s a 3-D induced current and resulting magnetic field involved in this situation. You can’t replace a solid surface with a coil (and the right-hand rule).


    Notice the illustration of the surface effect on a sheet, and further down is an illustration of what is induced inside a thick sheet. Note that by intentionally using a thin sheet you could force certain behavior (I don’t know which is best).

    The result is that if you move a magnet at 30 MPH over a sheet of aluminum, the induced currents will repel the magnet. If you move the magnet over coils, you’ll get a different effect.

    However, I don’t know the exact differences in the forces. If your goal is to slow down the falling magnet, I don’t know which will work better. Induced current in a coil will build up a magnetic field along the entire length of the coil, in addition to local forces on the magnet.

    I’m sure that the orientation of the magnet also matters. Does a long bar magnet dropped lengthwise have a greater effect than a short one, or a horizontally multipolar magnet?

  21. steve fitzpatrick says:

    Have you seen how a permanent magnet simply floats above a bowl shaped superconductor? It is the copper/magnet effect in the video taken to its logical conclusion.

  22. CS says:

    For what it’s worth, I once talked to a science toy store owner that made and sold a lighter version of these. He said figuring out the difference between what made for an “ok” one and a good one initially took him a lot of trial and error. I don’t know if he had any technical guidance–I think for his purposes it may have been all trial and error and perhaps with off the shelf items (for resale)—but FYI anyway.

  23. Milton Hathaway says:

    My intuition leads me to believe that it probably requires a few thousand ampere-turns to slow the falling magnet significantly. Since the conducting ring is a shorted single-turn, this would mean that the current flowing is a few thousand amps. If you were to measure the voltage induced across a cut in a thin section of the conducting ring when the permanent magnet is moving slowly through the ring, I suspect you’d measure a fraction of a volt.

    Applying ohms law, R=V/I, this means that the conducting material needs to have a very low resistance, down in the micro-ohms.

    I suspect where all this leads is that it’s hard to build a demonstration that shows a dramatic speed reduction while retaining good visibility. I agree with your assessment of the ‘FAT copper pipe’ video – that’s the best demonstration of the effect I have seen. (The only thing that detracts from it is the apparent need to put a significant spin on the magnet to control it’s orientation in space, which makes it less of a pure demonstration, since another physics phenomenon is then thrown into the mix.)

    I have noticed that the effect can be observed by sliding a cylindrical magnet (magnetized axially) down an inclined plane of half-inch aluminum plate stock. But as you incline the plate more toward the vertical (to illustrate that the effect isn’t caused by friction), the magnet jumps away from the plate.

    Maybe the effect would still be impressive with a heavy aluminum U-channel with a Plexiglas front?

  24. Mac says:

    The current induced is around the ring, perpendicular to the direction of fall. Wire will have much more resistance than fat pipe. Aluminum would likely work and would be much cheaper/easier to find in a fat tube compared to copper. It would be cheap enough that several sizes might be compared. Have fun.

  25. Massimo PORZIO says:

    Hi Dr.Spencer,
    I’m not an expert in electromagnetism, but since you seems to start a new interesting experiment, instead of wind up turns of wires (which is not a convenient choice for the reasons explained by KevinK and many others above), you could try to do something a little more tricky to do, but that it could improve the braking effect.
    You should try to build up many concentric thin pipes made of a good conductor material all electrically insulated each other. This should reduce the eddy currents induced by the magnet drop, that are not concentric to the axis of falling.

    Enjoy your experiment.

    Have a nice day.


  26. After reading comments and examining the paper describing the theory, I ordered 2 copper bars to experiment with. This will allow me to vary the spacing between the magnet and the copper, and allow easier watching from the side (rather than looking down the tube). According to the theory, you want the magnet to be as close as possible to the copper for maximum effect. Might need to get a third bar in a triangular arrangement.

    I’ve also asked my brother-in-law (a physicist) for advice…I remember years ago he worked on a DoD rail gun design, which involves similar issues.

    • Massimo PORZIO says:

      Hi Dr Spencer,

      About the two bars, I believe you are right about the need of the third bar, otherwise IMHO the magnet should be thrown away from the two bars alone.
      Anyways, I believe that the pipe arrangement should performing much better than your proposed three bars arrangement, that because your arrangement can’t minimize the gap between the copper “walls” surfaces and the magnet as the pipe does.

      More, according to this explanation about eddy currents:

      The most effective magnetic field in braking the magnet fall, should be the fluxes perpendicular to the opposed surfaces, that is those radial to the falling axis in case of a round magnet. This is the reason of my former suggestion to try the concentric insulated thin copper pipes. Even if I know that it’s not an easy to set arrangement.

      Have a nice weekend.


      • Massimo PORZIO says:

        Hi Dr.Spencer,
        I thought a little more about the issue and concluded that I was plain wrong about the concentric and insulated pipes, because even if it’s true that the perpendicular fluxes are those which perform better the braking effect, any other fluxes give their contribution too, so the thick solid pipe should be the best choice.

        I also realized that I misunderstood you, and you were writing about 2 parallel bars. It should be a good choice even if the pipe should perform better because in that configuration the magnet has both the poles exposed to the conductive material.
        Since it’s the delta flux that induces the eddy currents, having both the poles exposed to the conductive material it should enhance the braking effect (the magnetic profile between the poles is smoothed, so I don’t really know how much it could be better indeed, but it should be better to some degree), while with the 2 bars you have one pole for each bar exposed only.

        Have a nice day.


    • SC-Slywolf says:

      I saw this done on some science quiz show recently (6 mo?). Teams had to quess which material would fall fastest through a metal tube with a slot for the entire length. No one quessed the magnet, and the ‘consensus’ was for ‘all the same.’

    • Jim Curtis says:

      A rail gun is a very difficult technological challenge. Your drill motor uses a lot of turns to get a high ampere-turn magnetic intensity, a lot of iron (high permeability) to translate that to a high flux density, and a planetary gear to translate that to a lot of torque. A rail gun tries to do it all in one turn with low permeability air and no mechanical advantage. It consequently has to deal with colossal (and destructive) currents especially at the switch and at the sliding contacts.

  27. Mike Flynn says:

    Dr Spencer,

    Just a minor observation, but one which may save you some money. As you wish to build a simple moving magnet generator with a shorted single turn output coil, you might find aluminum tubing cheaper than copper. The conductivity is a little less, but the mechanical properties of extruded Al tubing may well compensate.

    As you are aware, reducing the distance between the magnet and the tubing will enhance the effect you seek. There is a world of difference between hard drawn copper tubing, and the softer annealed type, if a reasonably straight and concentric bore is desired. It may be to your advantage to establish what straightness tolerance you may expect, and take that into account. You might find that something like 6061T6 treated Al would be more cost effective than hard drawn Cu.

    Depending on this and that, you may be able to observe the slowly descending magnet from the side by the simple expedient of drilling holes in the tubing. Not too many, and not too big, I suggest – at least at first!

    Live well and prosper,

    Mike Flynn.

  28. Aaron S says:

    So this is cool and im curious what would happen if gold and silver or even lead were used instead… slower fall?

  29. Gordon Robertson says:

    This phenomenon is not entirely due to Lenz’s Law, it involves the laws of induction based on the theories of Faraday and Henry. The unit of induction is measures in Henries.

    A magnet traveling down a copper tube induces an electric current in the tube which is based on Faraday and Henry. Lenz’s theory involves the currents induced, which create an electromotive force (EMF) in an inductor, the sign of which opposes the sign of the EMF that produced the initial current. In doing that, the derived magnetic fields oppose each other as well.

    Without the inductor, you’d have a short circuit, so inductance in the inductor causes a form of resistance to current flow which limits the current. That applies only to changing current, however. With a DC supply, inductance is only effective as the switch is turned on and off, or as the DC varies in some way.

    As far as Roy’s question is concerned regarding winding a copper conductor on a piece of PVC, I’m not so sure that would work as well as the copper tube. I say that because the solid copper tube allows the flow of eddy currents which in turn create a magnetic field to counter the magnetic field of the falling magnet.

    Dynamic braking on electric motors uses the eddy currents induced in an aluminum cylinder in which a coil driven by the motor turns. A motor and generator are essentially the same device but in a motor current is delivered from an external source to the motor windings and in a generator current is delivered from the generator coils to a load.

    When power is removed from a turning motor, it becomes a generator as it slows down. The current produced by the generating effect is used to induce eddy currents in a metal cylinder and the eddy currents set up a magnetic field that oppose the motion of the motor.

    Dynamic braking is very effective and seems to be a similar principle to the one used when a magnetic is dropped down a copper tube.

  30. Kasuha says:

    I believe you will have best results if you wind the wire along the length of the tube.

    It’s the same principle used in magnetic brake.


  31. Stu Miller says:


    You can get the same effect from an aluminum tube, although the rate of fall is a bit faster. The aluminum tube will be somewhat less expensive than copper.

  32. Winston says:

    Why go so large with the magnet? I use a 24″ long repair section of 3/4″ (~21mm ID) copper pipe you can buy at a home improvement store. Through it I drop a stack of two N40 19mm OD, 9mm ID, 10mm thick ring magnets, magnetized through their thickness. It takes about 8 seconds to drop through the length of the pipe. Very impressive effect.

    I fine-sanded the outside of the pipe to remove oxidation (make shiny) and applied a coat of floor wax to keep it that way.

  33. Bill Clayton says:

    Dr. Spencer:

    I’d suggest using a PVC pipe sheathed with copper sheeting having a soldered seam. I haven’t thought the physics through, but off the top of my head I think it would be good for the induced currents to be free to flow in whichever direction they prefer to flow, with the resistance being completely independent of direction.

  34. David Springer says:


    Geek city.

    My kind of place!

  35. bob says:

    Your best bet is to use bare solid copper ground wire. Sand off the coating and wrap it so it is shorted at all times. Tight and touching the next winding. Maybe soldering it would help too. A copper tube is going to work best though. You could wrap a thin copper pipe with the bare copper wire to reduce the resistance.

  36. Gordon Robertson says:

    If you want to see the magnet fall, how about using a donut-shaped magnet with the copper pipe inside it? In a situation like that a copper rod should work as well as a tube, maybe even better.

    The advantage of the magnet inside the tube is that the magnetic field is contained within the tube, hence more concentrated. If a donut-shaped magnet is powerful enough it should work on a copper/aluminum rod inside the magnet.

  37. Gunga Din says:

    Here’s a practical application of a magnet inside a tube.

  38. Jim Bowen says:

    As the magnet drifts down through the tube, it’s potential energy is being converted to electrical energy. If there were no electrical resistance, no energy would be lost, and the magnet would remain suspended. (If your budget is unlimited, use a superconducting pipe to demonstrate this!) The greater the resistance, the faster the magnet will fall, due to the higher rate of loss of energy. For the same geometry, a lower-resistivity material will cause a lower rate of fall. (Budgetary constraints aside, a thick silver pipe would be your best choice of non-superconducting materials. Silver has a little lower resistivity than copper. Gold and aluminum would be poorer than copper. Lead or stainless steel would be poorer yet.) The circumferential component of the eddy current is the part that produces a magnetic force along the axis of the tube, so you want the resistance around the circumference to be as low as possible ( to demonstrate slow fall). Thus, in general, a pipe will work better than wire of the same thickness. If you use wire, it will work better if arranged as a series of rings (each ring soldered or welded at the joint to form a continuous circle for current to circulate). If you use a coil and only solder the two ends together, the magnet will fall more rapidly due to the higher total end-to-end resistance in the long wire length.

    Most of the comments have been pretty-much on target, but I have added these to clarify a few points — this from a retired electrical engineer who remains fascinated by the “magic” of electromagnetism!

  39. Jim Bowen says:

    A clarification of my last posting: When I said the magnet’s potential energy is being converted…, I was referring to the magnet’s potential energy by virtue of the magnet’s mass and it’s elevated position in the earth’s gravitational field.

  40. Aaron S says:

    Jim Bowen thanks. So when they float a frog, I assume that occurs bc the water has electrolites in solution and iron in blood which add some conduction in the water (most of the frog by mass). So what happens if you dropped the magnet into water and mixed different conducting solutes into the solution? Seems like fun experiments to me at least… may e even freeze them into place. I need to order one of these magnets but i hear you can take a finger off w a big enough one if u get between it and something like iron.

  41. Bart says:

    Make sure you spin the magnet when you drop it – you’ll notice him doing that in the video. It isn’t necessary for the effect, except insofar as upright stability is necessary. But, the dynamics are unstable without it, so for all practical purposes, it is necessary. The angular momentum of the spin stabilizes the magnet so that the force lines consistently superimpose in constructive manner. Same principle behind this, which is a neat toy I highly recommend. Without the spin stabilizing it, it will just tumble through, and the force will not consistently oppose the fall.

    For your experiment, what is happening can be easily understood if you use cylindrical coordinates, with orthogonal directions dr, dtheta, and dz. The magnet produces a standard diplole pattern of flux lines which vary in z and r, producing current in your selenoid windings in the dtheta direction, which then induces a reactive magnetic field in the dz and dr directions. The induced field in dr is the important one, because your permanent magnet has internal currents which are cumulatively in the dtheta direction. This internal current in dtheta crossed with the magnetic field in dr produces your reactive force in dz.

    So, to make this work, you need the current to flow as freely as possible in your coil in the dtheta direction. You need to short the ends of the coil together to allow a circuit. Grounding them would help prevent other fields from interfering. But, do not short the individual windings – I believe that would significantly reduce efficiency, as the magnetic field induced by currents in dz is not consistently in the dr direction.

  42. Jerry L Krause says:

    Dear Roy Spencer, Ph.D. and others:

    This is an experiment. The question is: How long can you ignore what R. C. Sutcliffe and Richard Feynman wrote? Both of these scientists were recognized as eminent scholars in their field at the time they wrote. Yet, it seems that many have failed to consider what they wrote and was published in 1963 (Feynman) and 1966 (Sucliffe).

    The following are excerpts from pages 32-8,9 of The Feynman Lectures On Physics (Addison-Wesley, 1963). One interesting question is, why do we ever see clouds? Where do clouds come from? Everybody knows it is the condensation of water vapor. But, of course, the water vapor is already in the atmosphere before it condenses, so why dont we see it then? After it condenses it is perfectly obvious. It wasnt there, now it is there. So the mystery of where clouds come from is not really such a childish mystery as Where does the water come from, Daddy?, but has to be explained. We have just explained that every atom scatters light, and of course the vapor will scatter light, too. The mystery is why, when the water is condensed into clouds, does it scatter such a tremendously greater amount of light.? That is to say, the scattering of water in lumps of N molecules each is N times more intense than the scattering of the single atoms. So as the water agglomerates the scattering increases. Does it increase as infinitum? No! When does this analysis begin to fail? How many atoms can we put together before we cannot drive this argument further? Answer: If the water drop gets so big that from one end to the other is a wavelength or so, then the atoms are no longer all in phase because they are too far apart. So as we keep increasing the size of the droplets we get more and more scattering, until such a time that a drop gets about the size of a wavelength, and then the scattering does not increase anywhere nearly as rapidly as the drop gets bigger. Furthermore, the blue disappears, because for long wavelengths the drops can be bigger, before the limit is reached, than they can be for short wavelengths. Although the short waves scatter more per atom than the long waves, there is a bigger enhancement for the red end of the spectrum than for the blue end when all the drops are bigger than the wavelength, so the color is shifted from the blue toward the red.

    The following are excerpts from Sutcliffes book Weather and Climate (W. W. Norton, 1966).
    It would be difficult to overstress the importance of clouds as the necessary intermediary between invisible vapour and falling precipitation upon which all land-life depends, but their importance by no means ends here. Clouds which do not give rain, which never even threaten to give rain but which dissolve again into vapour before the precipitation stage is ever reached, have a profound effect on our climate. This is obvious enough if we only think of the difference between a cloudy and a sunny day in summer or between an overcast and a clear frosty night in winter. Taking an overall average, about 50 per cent of the earths surface is covered with cloud at any time whereas precipitation is falling over no more than say 3 per cent. Non-precipitating clouds are thus the common variety, rain clouds are the exception. The climatic importance of clouds lies in their effectiveness in reflecting, absorbing, transmitting, and emitting radiation, Long-wave radiation from the earth, the invisible heat rays, is by contrast totally absorbed by quite a thin layer of clouds and, by the same token, the clouds themselves emit heat continuously according to their temperatures, almost as though they were black bodies. [pp 33,34] And one cannot explain the broad features of world climate if one does not know the actual mechanisms involved. [pp 138]

    You have a choice: Do clouds reflect and absorb electromagnetic radiation (light) or do they scatter it?

    Best wishes, Jerry

    • Dude, magnets, copper wire, and copper pipe.

      I nominate Jerry L Krause’s comment as the most pointless off-topic comment of the year.

      • Jerry L Krause says:


        This blog site is advertised as being about climate and global warming. So why did you not chide Roy Spenser,Ph.D. for his blog which had nothing to do with climate and global warming.

        Your comment is no more valid than mine which is about climate and global warming.

    • JohnKl says:

      Hi Jerry L Krause,

      Since you’ve asked me to comment on cloud theories, please know Feynman’s analysis seems reasonable. Feynman claims: “One interesting question is, why do we ever see clouds? Where do clouds come from? Everybody knows it is the condensation of water vapor.” More accurately, clouds typically form from the chrystallization (ice) of water vapor if the clouds become massive & warm enough water droplets form which absorb more of the light than ice chrystals and therefore appear darker.

      Overall as I mentioned in the earlier posts, I believe that while the effects of cloud coverage have some ambiguity the overall effect is to cool the surface. Since typically the high albedo & reflectivity sends most of the sun’s visible spectrum back to space reducing overall absorption. Without clouds the surface would either reflect or absorb those wavelengths, but typically at a significantly lower albedo, thus increasing energy absorbed to heat the surface.

      Please let me know your thoughts or if you seek opinions regarding other aspects.

      Have a great day!

      • Jerry L Krause says:

        Hi John Kl,

        Thank you for moving our dialogue to a location where more eyes might see it. When I submitted my reply to Spenser’s blog which had nothing to do with climate, I was unaware that you had replied to my reply in his blog of Aug 13.

        Because of what you wrote here, I would recommend that you find a copy of Sutcliffe’s Weather&Climate. It seems that W. W. Norton has recently published a paperback of it, if you cannot find an used copy available at a lesser price. I recommend this because he has a sixteen page chapter about the microphysics of clouds. Much too much to discuss here.

        Relative to Feynman and his explanation of the scattering of light by cloud droplets and probably the ice crystals of cirrus clouds, what I understand is scattering is a phenomenon in which no energy is ever transferred to the scattering particle. Hence, I understand (conclude) cloud particles never absorb visible or the invisible longer wavelength radiation. I have tried to find another author who addresses this topic without success. Clearly I am not a theorist, but Feynman’s accomplishments clearly suggest that he is one who can be trusted.

        Now,I did encounter a problem when Sutcliffe referred to the observed fact that clouds did emit longwave radiation as if they were a near blackbody with their temperature. The solution seemed that far, far, more molecules of nitrogen and oxygen had a far greater heat capacity than the fewer cloud particles. So when a droplet emitted photons which should cause a decrease of its temperature, the many collisions of the now warmer molecules with the cooler droplet warmed the droplet without significantly lowering the temperature of the entire system being considered.

        As I write, I see another factor which needs to be considered. The environment of the cloud particles should be saturated with water vapor. So, the condensation of water vapor on a colder than average water droplet seems a better possibility than the former.

        It is things like this that a dialogue will help. I do not expect anybody to be able to jump with great experience with what I drop. Because I have yet to find anyone who has thoughtfully considered that what I write. That is why I ventured into this new activity Aug 13.

        Just some thoughts and thank you for giving me someone to write to.

        Have a good day (or evening), Jerry

        • Jerry L Krause says:

          Dear John Kl,

          Just reviewed what you wrote. It should be obvious that I do not properly proofread what I write even though I do try.

          Feynman only wrote about the scattering of visible radiation but from what he seemed (to me) clearly state, the 20 micrometer radiation should be scattered many more times intensely by a 20 micrometer diameter cloud droplet than would red photons by the same droplet. Hence, explaining what Sutcliffe wrote. That even quite thin clouds totally absorbed the longer wavelength radiation emitted from the earth’s surface. But in Feynman’s case the word absorbed should be replaced by the word scattered. No, I have no idea what a thin cloud quantitatively is.

          Again, have a good day/evening, Jerry

          • JohnKl says:

            Hi Jerry L Krause,

            You stated:

            “But in Feynmans case the word absorbed should be replaced by the word scattered.”

            Thank you for pointing this out. Personally, maybe it’s just me but while I have some understanding of absorption/emission and reflection the concept of scattering radiation makes no sense to me. To me radiation represents radiated energy which apparently travels ( supported by empirically valid experiments conducted by Young and others )in waves and not billiard ball like particles scrambling hither and thither ( for which I have yet to see any evidence ). The photon concept doesn’t resonate with me at all. Certainly my views may prove inadequate. John Von Neumann, Feynman and many others embraced quantum mechanics, but I find it less than convincing. Since I’m not a degreed physicist there remains much for me to learn and I am willing to listen to any rational explanations. Unfortunately, the argument for mass-less particles seems to fall short. If you have an explanation that clears the mist please present it. For what it’s worth, Doug’s friend Claes Johnson didn’t believe in photons either if I remember correctly although Doug apparently does.

            You also mentioned:

            “That even quite thin clouds totally absorbed the longer wavelength radiation emitted from the earths surface.”

            Certainly, but the enormously greater quantity of higher frequency/energy radiation from the sun stands to warm things up a great deal more.

            Have a great day!

  43. Bob_MI says:

    My first thought is that a magnet that can lift 146 pounds sounds like it needs a lot of respect. Like, don’t get your finger between it and a railroad track, for instance. Not a physics person, so maybe I am off base with that concern.

  44. Lewis says:

    RE: the earlier comment about rail guns. The US Navy is putting them or is going to put them on ships. 5000 + MPH at partial power.

  45. Jim Bowen says:

    In answer to Aaron S, here’s an introductory explanation of frog levitation with a magnetic field, as well as a detailed technical paper on the same subject. For those who are interested, enjoy!


  46. Curt says:

    Sorry to be late to the party here, but I think I can still add some interesting perspective, as my business is in the control of electromagnetic systems.

    If you are used to looking at these types of systems from an “engineering” viewpoint, where you are trying to do something “useful”, I think it is easy to get fooled. For example, if you wanted to use the falling magnet to create some useful energy, say to light a bulb, you would want to use turns of wire with the ends connected through the bulb.

    But if you just wrapped a wire around the cylinder many times without connecting the ends, I don’t think you would get any damping effect at all on the falling magnet, because no current would flow in the wire. (There would be a significant voltage induced across the length of the wire, but no current could flow.)

    The same is true if you wanted to use an external power source to levitate the magnet — you would want turns of wire connected to the voltage source.

    However, in this case, I think you just want the maximum possible current flow around the magnet, and for this you can’t do better than a solid cylinder of highly conductive metal. This will provide the minimum resistance, and therefore the maximum current, for the voltage induced by the falling magnet.

    Even if you think in terms of “ampere-turns”, even though there is only one “turn” in a solid cylinder, there will be so many more amperes flowing that you will maximize the ampere-turns and therefore the resulting counteracting magnetic field.

    • Bart says:

      But, if that were true, wouldn’t selenoids be made with conducting sleeves instead of windings? It’s really just the inverse problem. My thought was a sleeve might lose efficiency by stray current in the z direction. The induced current which provides the reactive field is in the azimuthal direction.

      • Curt says:


        Note my 4th paragraph: “The same is true if you wanted to use an external power source to levitate the magnet you would want turns of wire connected to the voltage source.”

        That is basically what a solenoid does, although it is fighting a spring rather than gravity.

      • Gordon Robertson says:

        @Bart…”…wouldnt selenoids be made with conducting sleeves instead of windings?”?

        Solenoids use a different approach than this experiment with a falling magnet. They actually are a true form of Lenz’s Law in the pure solenoid. Solenoid require an external current to energize the magnetic field and the inductance of the solenoid is proportional to the number of turns, the length of the turns, the permeability of any core used, etc.

        Solenoids used in relays can have a copper sleeve as well.Some relays are required (for timing purposes) to be ‘slow to open’ while others are required to be ‘slow to close’. That is done using a copper sleeve to interfere with the hysteresis effect in a coil, causing them to be slow to react.

        This experiment with the falling magnet is based on eddy currents induced in the copper by the magnet. A single tube would not supply the inductance to make an effective solenoid but it can set up a flow of eddy currents that can create their own magnetic field which opposes the magnetic field of the magnet.

        On the other hand, a solenoid with it’s turns of copper would be relatively ineffective at setting up the required eddy currents. Transformer cores are laminated for that reason, to break up eddy currents, which represent a power loss.

        • Bart says:

          Gordon – it really is just a complementary problem. For a selenoid, the emf to the coils is supplied by an external power source. In this problem, the emf is induced by the falling magnet.

  47. MJ Sheridan says:

    Dr. Spencer,

    Interesting experiment. When I saw your post, I thought that this should be easily solvable. After some work, I now think it’s solvable, but not easily, LOL. I think the key is eddy currents. In most pratical applications, these are to be minimized. Magnetic cores are laminated for this reason. In this case, The eddy currents are responsible for the observed breaking by inducing diamagnetism. The moving magnet is inducing three diminsional currents in the thick cylinder. Constructing a coil an then shorting it in various ways is simply trying to approximate a thick cylinder.

    As to material, Al should also work, but less well. The issue with AL vs. Cu is that although Al has good conductivity per unit mass, it’s not dense. Cu comes out much ahead in terms of unit volume. Since spacing is an issue, Cu is a better choice.

    I remember doing an exercise in school where we had a rectangular wire loop made up of a U shaped piece and a straight piece across the open end. The thing was inclined a 45 degrees. The material was a superconductor. As the top piece fell the the area of the rectangle was reduced. We were required to show that pendulum-like motion could result, with the top straight piece riding up and down on the U shape. I also recall that we didn’t solve the motion, except to show that it was harmonic, but not simple!

    I bring up the superconductor example because this experiment demonstrates diamagnetism. The eddy currents in the Cu cylinder create an induced diamagnetism, repelling the applied magnetic field. The term superconductor is a little unfortunate, a better term would be ideal diamagnet. At the macroscopic level, a superconductor is a material that expels all applied magnetic fields.

    Here is a static version of the effect, where a Nd magnet is floated between Bismuth plates. http://netti.nic.fi/~054028/images/LevitorMK1.0-1.mpg

    • Curt says:

      I’m afraid this has nothing to do with eddy currents. Eddy currents are generated in ferromagnetic materials (i.e. iron) by changing magnetic fields. This is just the standard generator effect from relative motion of a conductor in a magnetic field.

      But while an engineered generator would have many loops of a conductor to get a higher voltage, with the ends connected through a load circuit, this just has one very big loop with the ends shorted together, so low voltage and high current.

  48. Gil Carvalho says:

    When shopping for copper tubing, you will need a thicker wall to achieve the desired magnetic effect. There are 3 standard thicknesses of copper tubing, Types K, L, and M, with Type K being the thickest. Each Type has different color printing on the outside with Type K usually being green. There might be thicker industrial options available through a specialty supply shop.

    Here’s a quick size guide with different inside diameters for each type of tubing. You could also try stacking multiple diameter tubes inside of each other to possibly amplify the magnetic effect.


  49. Mike says:

    I tried it using some 12mm dia.x 5.5mm thick neodymium magnets and some 15mm standard copper central heating pipe (for those who don’t know this is the equivalent of 1/2″ pipe before the UK went metric) ID is 13.5mm i.e. a wall thickness of 0.75mm. Works like a charm

    Because of the small annular gap can look like it’s being slowed by a ‘leaky piston’ effect but if you use a similar sized slug of something else, say steel, it drops through at a normal rate.

  50. Jim Bowen says:

    Another interesting question has to do with the length of the magnet. Assume our objective with the demonstration is to have the magnet descend as slowly as possible. I think most folks have agreed that the speed of descent will be reduced as we increase the diameter of the magnet to approach the inner diameter of the pipe. However, suppose we want some obvious spece between the magnet and the pipe so we produce a visually-impressive demonstration. Suppose we use a long hollow cylindrical pipe made of a fixed uniform conductive material, such as copper, and we use a solid cylindrical permanent magnet made of a uniform magnetic material, magnetized to the maximum strength sustainable by the material. For any fixed inner and outer diameter of the pipe, and for any fixed outer diameter of the magnet, is there an optimum length for the magnet, and, if so, what is the optimum length? To simplify the problem, let’s further assume that the magnet is spin-stabilized or otherwise stabilized as it falls by some frictionless, massless means that will keep the cylindrical axis of the magnet aligned and coincident with the axis of the cylinder.

    In the video, we observe that the longer magnet descended more slowly, but admittedly we are comparing apples and oranges since the first magnet was squarish instead of round.

    My math skills are way too rusty for me to have any hope of precisely solving this problem, but I think there is an optimum length, and I also think the optimum length for the magnet is on the same order, but not identical to, the difference between the inner diameter of the pipe and the diameter of the magnet. I think the outer diameter of the pipe will also influence the optimum result, but not nearly to the same degree as the inner diameter.

    • Jim Bowen says:

      I should have also said to assume that the magnet is magnetized in the normal direction for a bar magnet, that is, magnetized along its cylindrical axis.

  51. Jerry L Krause says:

    Dear JohnKl,

    Please check the end of Spenser’s Aug 13th blog. Jerry

  52. UPDATE: I got 3 different sizes of disk neodymium magnets (cylindrical), some copper pipe, and 2 copper plates. The greatest “slow fall” effect is for the 1.5 x 1 inch magnet falling through a copper tube which is only slightly larger than the magnet. Even though the tube wall thickness is only 1/16 inch thick, it takes about 14x as much time (about 4 seconds) to fall through a 13 inch long tube as it does for free-fall.

    I found a paper which comes up with an equation for the case of the cylindrical magnet falling through a copper tube:

    The slow fall effect is maximized when the magnet thickness is about the same as its diameter…magnets significantly taller or shorter than this will fall faster.

    One thing I found out is that these magnets really are dangerous. The 3/4 x 1/4 inch disk magnets pinched my fingers a few times and drew blood. If you allow them to attract one another unrestricted they chip apart from the force. If I had gotten a second large magnet (1.5 x 1 inch) I probably would have broken a finger or a bone in my hand by now.

    Again, they really are dangerous.

    • Jim Curtis says:

      He-he. I understand that magnets resent being made to jump through hoops by someone (e.g. a climatologist) straying far from his field. They tend to react in a manner analogous to an alarmist who is having his assumptions and misrepresentations questioned.

    • Curt says:

      I would like to echo Dr. Spencer’s warnings. These rare-earth magnets (neodymium or samarium) are easily an order of magnitude stronger than the “refrigerator magnets” most people are used to. They literally can create bone-crushing force.

      A few years ago, a colleague of mine, when fooling with these same small disk magnets, put one on each side of his nose, then found he couldn’t get them off! He almost had to go to the emergency room, but finally used a large screwdriver to pry one off, gouging his nose deeply in the process.

      A few years ago, you could buy these strong magnets as little balls, 2 or 3 mm in diameter. Kids who swallowed them got into very serious medical difficulty, as they attracted each other from adjoining paths of the intestines, crushing the intestine walls. I believe a few died. The government forced these off the market.

    • Jim Bowen says:

      Nice paper. Many thanks for pointing it out.

      Good luck with the neodymium magnets. So far, I have confined my play to the small ones to reduce the chance of injury.

      Imagine a pipe with negative resistance. When dropping a magnet into this pipe, the magnet may come shooting back up this pipe, and may proceed to a point higher than where it was released. This magnet would behave something like the bouncing balls of flubber in that old movie starring Fred McMurray.

      It takes an active (power-producing) circuit to synthesize a negative resistance. The simple tunnel diode motor (see the Amateur Scientist column in the October 1965 edition of Scientific American) was a neat example using these same electromagnetic principles. The tunnel diode served as the negative resistance synthesizing element in this example. A one-cell battery to supply the power, a couple of bias resistors, a coil of wire, and a magnet on a pivot completed the motor. No brushes were necessary. These days, tunnel diodes are scarce, but a pair of transistors and a few resistors can be configured to create the same characteristic negative resistance to operate the motor.

  53. Al in Kansas says:

    The counter force is due to eddy currents that are induced in any conductor, that is what I have always been told. This is commonly used in scales as a vibration/motion damper. Aluminum plate moving between two magnets. Look at any standard laboratory “triple beam balance”.

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