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Re: [TCML] Spark dynamics on Jacobs Ladder

Gary, I long ago decided that it was electrostatic attraction and repulsion through performing the same experiments you just did!

I also noticed that the first movement accompanied the spark switching from top to bottom. I doubt it has anything at all to do with the AC frequency.

On Jan 18, 2008, at 8:21 PM, bartb wrote:

Hi Gary,

Good test. I've always seen the oscillation on JL's simply due to a single end connection. I personally have no clue why this occurs (funny, we've all built JL's and don't have a definitive answer on this simple thing).

From memory (two Halloweens ago), it seems repulsion of the wires was the driving force and when the wires came close together it appeared due to the mechanical "let go" of the repulsion. It's only an old memory now, but I do remember thinking about this very issue. As the arc travels up the ladder, the motion is somewhat captured but not completely (it appeared to be repulsion with mechanical energy wanting it to go back in). When the arc is quenched at the top, the mechanical energy swung the wires inward where the arc started again. The wire swing did get stronger as the heat enabled the arc to roll up faster along the length. I think heat is directly involved in this mechanical action by it's travel time from start to quench, but there is a limit to how high the temp of the wires can get and this is what "appeared" to stabilized the amount of swing.

Just my regurgitation of that particular Halloween when I had the JL in full swing. I was controlling the coil running behind the ladder which just made for a cool effect of the whole night.

Take care,

Lau, Gary wrote:
Hi Bert,

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:

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?

Gary Lau

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