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RE: [TCML] Spark dynamics on Jacobs Ladder
I would have bet money that you were right about the EM Lorentz force force being the answer, but a couple of quick experiments I just did make me think otherwise.
I took a length of thin #30 wire to connect the free tops of the JL electrodes, so that they can still move unencumbered, just shorted. I pulsed the AC power to the 15/30 NST to match the mechanical period of the electrodes, but I couldn't get any kind of sympathetic motion. This makes me think that EM is not the force.
Then I removed the #30 wire, and moved the electrodes just slightly further apart so that no spark occurred between them. Now by timing the AC pulses to the NST, I WAS able to get the electrodes to oscillate, although for some reason, my timing mostly seemed to be OUT of phase with what would increase the amplitude. Electrostatic attraction is definitely at work.
Looking more closely at the motion in the operating JL, the motion seemed to get its kick principally as the arc broke at the top, although I still saw oscillation if I positioned the electrodes such that the arc stayed at the top - not sure why that might be.
Regards, Gary Lau
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Bert Hickman
> Sent: Friday, January 18, 2008 1:20 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Spark dynamics on Jacobs Ladder
> Hi Matt,
> Possibly, except that Peter is correct - the EM force on the electrodes
> is actually repulsive and not attractive since the current is flowing in
> opposite directions. The magnitude of the repulsion force scales with
> the square of the current and is inversely proportional to the wire
> BTW, for a graphic example of what these forces (at tens - hundreds of
> kA) can do to substation bus bars and standoff insulators during
> accidental short circuits, see:
> It would be interesting to compare the magnitude of the magnetic versus
> electrostatic forces for a typical 15/30 NST to see if they are of the
> same magnitude... I'll look into this a bit more.
> Mddeming@xxxxxxx wrote:
> > Hi Gary,
> > A simple explanation, understandable by elementary school kids, is this: A
> > wire carrying a current has an EM field around it. The two wires carrying
> > currents in opposite directions will thus attract each other. When the spark
> > first forms at the bottom, the two electrodes form effectively very short wires.
> > As the spark rises, their lengths becomes effectively longer and the
> > attraction stronger. The maximum attraction occurs when the spark is at the top.
> > When the spark breaks at this point, the restorative force of the spring tension
> > of the electrodes pulls them apart again and they try to oscillate at their
> > natural frequency. If you could get the voltage, current, spark travel time,
> > and distance just right, you would see significant resonant gain in the
> > movement (The pushing-a-swing analogy). The college-level explanation is here:
> > <http://en.wikipedia.org/wiki/Lorentz_force>.
> > Hope this helps,
> > Matt D.
> > In a message dated 1/18/08 11:06:32 A.M. Eastern Standard Time,
> > Gary.Lau@xxxxxx writes:
> > I hope this isn't viewed as too off-topic - I'll argue that the same physics
> > apply to TC sparks ;-)
> > I was giving a demonstration of various HV toys to a 4th grade class
> > yesterday. Among the devices was a Jacobs ladder, powered by a 15/30 NST.
> The two
> > 1/8" x 3 ft steel electrodes appeared to have been excited into a mechanical
> > oscillation, bouncing towards and away from each other, at very roughly ~
> > 1Hz. One of the students asked my why they were moving, and I had to admit
> > I didn't know the source of the force that was moving them.
> > The period of the oscillation was much faster than the arc travel time up
> > the electrodes. It's clear that the period was that of the free-standing rods,
> > and that the exciting force between them varies as a function of their
> > separation, but I don't see the source of the attraction or repulsion between
> > them. Any theories?
> > Thanks,
> > Gary Lau
> > MA, USA
> > **************Start the year off right. Easy ways to stay in shape.
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