Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electric Discharge Question

  1. Dec 6, 2006 #1
    I have just now seen this long exposure photo on http://en.wikipedia.org/wiki/High_voltage" [Broken]from an electric discharge. Is it just me or does it appear that each discharge is taking an almost exact path down the length of the arc.

    I need to know if this is just a coincidence, and if not, what is causing this.

    Attached Files:

    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Dec 6, 2006 #2
    I'm not sure exactly what you mean by "an almost exact path down the length of the arc"?

    What you're seeing is the electrons passing through a gas, exciting atomic transitions and causing the atoms to radiate. If there's a smoothly varying potential inside the chamber, then they of course would follow similar trajectories.
  4. Dec 6, 2006 #3

    Meir Achuz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The first arc ionizes molecules in its path. Subsequent arcs would tend to follow this ionized path, giving similar shapes. A slight movement of the air would explain why the shapes move.
    Last edited by a moderator: May 2, 2017
  5. Dec 6, 2006 #4


    User Avatar
    Science Advisor

    It looks to me like there must eb a pocket of air that didn't ionize, and the arc went around it. I would be interested to know in which direction (left-to-right or right-to-left) one must view these exposures in order to be seeing the arc's progress in chronological order. If right-to-left, then it looks as if the arc straightend out over time.
  6. Dec 6, 2006 #5
    Does someone have a tesla coil and a camera (with long exposure) to try this? It would be cool to see the pictures with and with out a fan on.
  7. Dec 6, 2006 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    My former professor of electromagnetism had a funny and deadly device: two separating pieces of rigid metal wire going up in a kind of large V (height: about 30 cm, upper separation, about 15 or 20 cm). With a kind of tesla coil directly plugged on the mains (:bugeye:) the thing started sparking where the wires were close (down part of the V) and the arc created as such, heated the air, which moved the ionised path upward in the V until it made an almost half circle connected to the upper extremities of the V before breaking up, at which point a second arc started off down under at the V again...
    One better didn't touch the metal wires ! :surprised
  8. Dec 6, 2006 #7
    You know you're in a community of physicsts when someone seriously asks if someone else has a tesla coil just lying around to play with.
  9. Dec 6, 2006 #8


    User Avatar
    Science Advisor
    Gold Member

    It's called a Jacob's Ladder, and you can make one with a neon sign transformer. Once the arc is started across the small gap at the bottom, it will travel upwards (due to heated air) and expand. It was a favorite prop in hollywood in movies like Frankenstein with boris karloff.
  10. Dec 9, 2006 #9
    Not with a neon sign transformer I think.
    It has too small current output to maintain stable arc travelling upwards.
    Potential high voltage transformers are used for creating Jacob's Ladder effects.
  11. Dec 9, 2006 #10
    Matter of fact you don't have to touch them to be electrocuted.
    Arcs can jump across sufficiently small gap between a body and a hot wire if the voltage is high enough.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook