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Leyden Jar Capacitor Recharging?

  1. Feb 6, 2008 #1
    I built a leyden jar and have noticed something rather interesting:
    After discharging the jar and letting it sit overnight I can come back to it and short it out and get a tiny little spark...then I let it sit and come back later and I can get it to spark
    again. I've repeated this over and over. What causes this effect? Is it just picking up charges from the air?
  2. jcsd
  3. Feb 6, 2008 #2
    Without more information about the materials and concentrations involved I would say...No. What is likely to be happening is that over time without a connection the dielectric discharges the ions within it to the poles and when at some later time you reconnect the leads you cause the dielectric to reestablish an equilibrium. Almost as if you had a leaky capacitor in a DC circuit that emits charges initially but as the dielectric becomes charged the cap stabilizes and stops leaking for a while. Turn of the power and the cycle repeats. Best I can do without more specific info on the actual experiment.
  4. Feb 7, 2008 #3
    This is common in electrolytic capacitors, but air? How long did you leave it shorted? Do you live in a heavily polluted or a salt fog area?
  5. Feb 7, 2008 #4


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    Electrons generally move fairly slowly so it can take a while for the entire charge to redistribute.
    Also there is a quench voltage at which a spark is extinguished.
    If you are discharging the jar with a quick touch of a probe then the combination of these two items can leave enough residual charge for more sparks.
    When discharging CRTs, in TVs and Monitors, I always used a clip on ground to prevent surprises like this when working on them later.
  6. Feb 8, 2008 #5
    I'll try to give a little more detail about my Leyden Jar and the effect I've seen.

    My Leyden jar is built from a 40oz plastic peanut butter jar and uses aluminum foil for the inner and outer plates. A bolt that is connected to the inner foil protrudes from the center of the jar. Attached to the bolt is a piece of stranded wire that acts as a brush to collect charge. A piece of solid wire is connected to the outer foil and bend towards the bolt to form a spark gap. Here's a picture: http://tinyurl.com/ywhf37

    I typically charge my Leyden Jar by either rubbing a pvc pipe near the brush or by touching the brush to a large piece of aluminum foil that is placed on a TV screen.

    After discharging by touching the top wire to the bolt and holding it for a second or two I let it sit for awhile. When I come back later I can short out the two plates like before and get a tiny spark. The weird thing is that after I do this I can let it sit for another period of time without charging it again and then come back later and get it to spark again. I can keep doing this without recharging it in between. If this was from residual charge I would expect each spark to be dimmer but provided that I allow the jar to sit long enough I can usually get a spark just as bright as the one before it.

    As to TVP45's question...where I live is neither heavily polluted nor prone to salt fog
    Last edited: Feb 8, 2008
  7. Feb 9, 2008 #6
    Ah, now I got it! This is a great question. One word in your post tells the story I think.
  8. Feb 9, 2008 #7
    No word back from you. Mmmm? Has my tin foil hat blocked your transmission?

    Anyway, if I'm right (and I'm about 80% sure), the answer is on the bottom of your Leyden jar.
  9. Feb 9, 2008 #8
    I don't understand??? nothing is on the bottom just the bare plastic
  10. Feb 11, 2008 #9
    Nothing at all? No symbols? No numbers?
  11. Feb 11, 2008 #10
    Well yes there are symbols inscribed on the bottom.

    one of them looks like this: http://www.ci.monterey-park.ca.us/kids/_gfx/pete.gif
    indicating that the jar is made from Polyethylene terephthalate I believe.

    I assume this is the one you are interested in?
    Is there some property of Polyethylene terephthalate that is causing the observed effect?
  12. Feb 11, 2008 #11
    This is speculative, but...

    New, or pure PET has pretty good dielectric absorption properties, but the recycling effort results in some reuse with attendant problems, the result sometimes being jars with defects and impurities which greatly increase the dielectric absorption. As I said, pure speculation on my part, but I'd bet even money in a flash.
  13. Feb 11, 2008 #12
    Well, thank God you didn't bet me. I just read Bob Pease's (from Nat'l Semiconductor) column on soakage and it suggests even pure PET (as well as glass) should do that to some extent. MMM? I think I was too smart for my own good?

    So, short it out for a couple of minutes.
  14. Feb 12, 2008 #13
    I left it shorted for more than 13 hours. Then I left it open for about 7+ hours and then
    shorted it. I didn't get a visible spark...In the past I've let the jar sit open for much longer (12 to 24 or more hours) so I'll see what happens when I short it tomorrow...but you are
    probably right about it being dielectric absorption.
  15. Feb 12, 2008 #14
  16. Feb 12, 2008 #15
    Thank you. I appreciate your help!
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