# Re: [TCML] Spark dynamics on Jacobs Ladder

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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
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 that
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

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