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Physics of Jacob's Ladder

  1. Feb 14, 2012 #1
    In lecture last week my professor asked us why the electric arcs formed by the Jacobs Ladder demonstration device rose up into the air. I answered that the magnetic field created by the metal rods exerted an upwards force on the moving charge in the arc. My professor told me this was getting ahead and that the cause had to do with the heating of air particles due to ionization.

    Was I correct in believing magnetic force has a significant impact on the arc? Or is it negligible? Thanks PF.
  2. jcsd
  3. Feb 14, 2012 #2


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    Afaik, the magnetic effect should cause the current loop to repel - hence, get larger diameter. (Which is what you said) The arc will have neutral charge so there should be no net Coulomb forces so I guess that the 'electrical effect' would be magnetic.
    I guess that the expansion of the volume of heated plasma could also have an effect but, as I keep saying to people, the actual Numbers Count (!!) so you'd have to do the actual sums on heating and magnetic effects and find the greater one.
    Go on then - don't hang around - just do it. Ha Ha.

    Thinking further, if the volume (when heated) has to increase and the parallel currents within the arc will trend to keep the moving charges together (pinch effect?) then it would have to get longer - moving up to the more spaced out wires. But the magnetic effect (attractive this time) would also seem necessary to account for the phenomenon.
  4. Feb 14, 2012 #3


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    “When the device is turned on, the full voltage initially appears across the bars causing an arc to form at the closest point, where the electric field is highest. The voltage then drops to a low value because the impedance of the arc is low. The arc heats the air, and the hot air begins to rise because it is less dense than the surrounding air. As the arc rises, its impedance increases, and the voltage between the bars rises. Eventually the arc gets so long that the voltage is not sufficient to sustain it, and the arc goes out. Then the voltage rises to its full open-circuit value, and the arc restrikes at the bottom. There is also a magnetic force causing the arc to rise, but the dominant effect is the heating of the air as can be demonstrated by tilting the whole device on its side.”
  5. Feb 15, 2012 #4


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    CONVECTION!!! Of course. Not having played, hands-on, with a good Jacobs Ladder, I hadn't taken on board the 'upward' motion that you get.
    What a good explanation.
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