Date: Wed, 26 Oct 2011 16:01:11

Author: --- "Rueckner, Wolfgang"

Subject: Re: Image Current Demo


Very good ... sounds like a convincing demo to me -- Wolfgang

On Oct 26, 2011, at 3:53 PM, Adam Beehler wrote:

> Oh boy, what a day! I finally had time to mess with this "demo." This
> is what I have learned.
> Wolfgang, the connecting red wire is definitely influencing the swinging
> rod, despite it being "away." I confirmed this by moving it closer and
> farther, as well as switching sides. I can now make the swinging rod
> swing either direction or not at all depending on wire orientation.
> Cliff, the iron rod I was using is now very, very, very weakly
> magnetized. I am not sure if it was when I started. I started testing
> all kinds of iron and steel objects around my area. I walked around
> with a tiny little compass needle and watched to see its deflection
> right up against the objects. I was surprised to learn that most of the
> iron/steel objects I have just lying around were ever so slightly
> magnetized. Anyway, I finally found some that were not and used them in
> the "demo." They worked as well, i.e., the swinging rod was attracted
> to them.
> I even set up the swinging rod so that it would repel one way from the
> side wire, while at the same time placing an unmagnetized iron rod so it
> would swing the opposite way. In other words, the repulsion and
> attraction forces should be competing. It still worked - the swinging
> rod was attracted to the iron rod.
> Adam Beehler
> On 10/26/2011 6:14 AM, Cliff Bettis wrote:
>> Adam,
>> A piece of ferromagnetic material placed in a magnetic field will have an
>> induced magnetic dipole moment. It doesn't matter if the source of the field
>> is another magnet or a current carrying wire. Whether or not the
>> ferromagnetic material experiences a force or not depends on geometry (e.g.
>> there is no net force if the applied field is uniform, only a torque (again
>> depending on geometry)).
>> I am skeptical about this demonstration because the theoretical derivation
>> requires an infinite wire. In the real case, the iron rod is finite and
>> there is a current loop in at least one side of the magnetic circuit. Should
>> the iron rod be made a closed circuit so current can flow? If so aren't you
>> talking about two current loops? This reminds me of the problems that arise
>> when you try to deal with the question of what frame of reference a current
>> carrying wire is neutral in and explaining the Biot-Savart law
>> relativistically (from the electrostatic point of view). There, too,
>> calculations involve an infinite wire and it turns out that while the field
>> quantities are easily transformed from one frame of reference to another,
>> the source terms are not in realistic cases.
>> Cliff
>> -----Original Message-----
>> From: [] On
>> Behalf Of
>> Sent: Tuesday, October 25, 2011 6:44 PM
>> To:
>> Cc:
>> Subject: Re: [tap-l] Image Current Demo
>> When I first held copper up to the swinging rod, I was all excited to see a
>> slight repulsion. But no luck, instead I saw a slight attraction. I was
>> not sure what to think. I just simply tried other materials and shapes and
>> thicknesses. Same thing. Soft ferromagnetic materials attracted the
>> swinging rod conspicuously, and everything else either caused no movement or
>> initial attraction. Even when I had nothing next to the swinging rod, it
>> still seemed to move inward. Part of it is due to me physically shaking the
>> unit when I depress the "on" button, but I am not convinced that is the
>> whole story. I tried to be pretty careful at times.
>> I thought another reason could be the mini electrical arcs that can happen
>> in the small contact points mounting the swinging rod. I do not always see
>> them but I can quite often hear them. They could be jiggling the swinging
>> rod and allowing it to adjust due to gravity. (The swinging rod unit has
>> leveling feet for adjusting the unit so that the swinging rods tend to swing
>> back after attracting or repelling. I tried to adjust those feet so that
>> the one swinging rod would swing back, and away, from the fixed rod after
>> any interaction. Sometimes the swinging rod would stick or come to rest in
>> slightly different positions though.)
>> I then wondered what you mentioned. Could it be repelled from the other
>> wires or posts involved with the set-up? Short answer - I'm not sure but
>> I'm leaning towards probably not. The swinging rod should be "attracted"
>> to the post, so that isn't it. The current in the same-side, red wire would
>> be going the opposite direction and thus provide a repulsive force, but
>> since it is relatively far away from the swinging rod, I am not yet
>> convinced it is the culprit. I am at home right now and will test this
>> theory tomorrow by moving that wire even farther away. I can also change
>> sides with the swinging rod and test that. This theory would sure explain
>> it, though.
>> As for the iron rod having already been magnetized, I cannot confirm nor
>> deny at this point. In other words, I did not check. I will tomorrow. I
>> did however try several different iron objects (not all rods), and they all
>> showed attraction. True, they might all have been pre-magnetized, but that
>> would be unlikely.
>> I am also curious about the possible connection between this demo and a
>> solenoid pulling in an iron core. I always considered the iron core as
>> becoming magnetized and then attracting to the coil's magnetic field.
>> Maybe one can think of it as the iron core's magnetic field exerting a
>> Lorentz force on the moving charges in the coil. I don't know. I guess if
>> one can calculate the same resultant forces using an image current, then
>> maybe so.
>> Adam Beehler
>>> Well that certainly works and makes the point! Is the slight movement
>>> (with the aluminum and rubber rods) due to repulsion from the other
>>> wire off to the right? -- Wolfgang
>>> On Oct 25, 2011, at 3:23 PM, Adam Beehler wrote:
>>>> I was able to do the demonstration myself. Hooray! But I did it
>>>> differently. I have posted a little video I took of it here:
>>>> I do not yet have the supplies to do it Wolfgang style, so I tried it
>>>> with my "Scientific Supplies, Inc." "Force Between Two Parallel
>>>> Currents Demonstrator."
>>>> I removed one of the swinging/pivoting rods and mounted an iron rod
>>>> next to the remaining swinging/pivoting rod. It worked. The
>>>> swinging rod was attracted to the iron rod. I then replaced the iron
>>>> rod with aluminum, copper, and rubber. I was not able to see any
>>>> repulsion, but I did not see attraction either.
>>>> Adam Beehler