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I Resistance from fingertip to Earth wire

  1. Dec 21, 2016 #1
    i connected one probe of digital mutimeter to the earth terminal of domestic ac supply and held the other probe. it showed resistance of 1.8Mohm .
    but when i touched that end (one i'd held) to the floor. it displays 1( beyond 200Mohm ). why so ? shouldnt resistance be lesser?doesnt any current flow if the live wire (directly , no human between) touches the floor?
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    Last edited: Dec 21, 2016
  2. jcsd
  3. Dec 21, 2016 #2
    Using only your reported measurements, and without seeing exactly what you were doing, I would have to guess that your body had a path to ground other than the floor. In fact, the floor would seem to be an insulator - offering no current path at all.
    1.8Mohm is a common low-voltage resistance across the body. It indicates moderate pressure on the volt-meter probe and very little perspiration. So I would guess that you were also leaning on some other object - such as a metal appliance or a floor board radiator.

    There is another possibility. Depending on exactly how your multi-meter works, you may have had some capacitive coupling to the AC power. If the meter acted as a diode, it could have caused a slight voltage drop on the probe that registered as a current.
     
  4. Dec 21, 2016 #3

    phinds

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    Possibly has to do with the facts that (1) the floor contact is a dry contact with VERY little surface area actually in contact (2) your skin is a wet contact and there is a lot of surface area making contact and (3) somehow the fact that your feet are a very large area make for a relatively conductive contact.

    Basically, I'm saying that it's all because the probe touching the floor actually barely touches anything and is a very high resistance contact. Try the same experiment by putting the probe into a small area of water on the floor and let us know what happens.
     
  5. Dec 21, 2016 #4
    but no i wasnt , you could try it yourself
     
  6. Dec 21, 2016 #5
    Just as you responded, I edited my response to add this:

    There is another possibility. Depending on exactly how your multi-meter works, you may have had some capacitive coupling to the AC power. If the meter acted as a diode, it could have caused a slight voltage drop on the probe that registered as a current.
     
  7. Dec 21, 2016 #6
    it worked! mutimeter shows 2.3 Mohm in water
    also tried with a steel plate between floor and probe, shows roughly 12 Mohm
     
    Last edited: Dec 21, 2016
  8. Dec 21, 2016 #7

    phinds

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    Oh good. I always love it when I'm right (because I so seldom am :smile:)

    And do you understand why that is?
     
  9. Dec 21, 2016 #8
    must have to do with contact surface area.
    liquids provide larger contact surface area. occupy all unevenness
    same does with sweat on human skin
    a flat steel plate doesnt provide that large a surface area of contact
     
  10. Dec 21, 2016 #9

    phinds

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    Exactly. This is a valuable lesson about electronics.
    Yes, but you'll notice that it does a lot better than the probe alone. Exactly the same principle.
     
  11. Dec 21, 2016 #10
    why does the contact surface area matter?
    what is the lumped circuit abstraction for probe- skin- floor system vs the probe- floor system (direct contact)
     
    Last edited: Dec 21, 2016
  12. Dec 21, 2016 #11

    phinds

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    Why do you think it matters? Obviously you now know that it DOES matter, so what is your explanation?
     
  13. Dec 21, 2016 #12
    i dont have one
     
  14. Dec 21, 2016 #13
    I think it matters because it makes the resistance of the contact smaller. In the lumped circuit abstraction you have to put a resistance in between the probe and the floor (or in between the feet and the floor) that represents the resistance of the contact. The larger the contact area the smaller this resistance is.
     
  15. Dec 21, 2016 #14

    phinds

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    Well, think about this. You have a long rod of, say, medium resistance carbon and it is 1 cm diameter. You measure the resistance. Now you take another rod of the same material and the same length but a 2 cm diameter. Will the resistance be the same? Why or why not?
     
  16. Dec 21, 2016 #15
    not same
    cause more charge flows in same time through cross-section normal to field vector
     
  17. Dec 21, 2016 #16

    phinds

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    Right. And more surface area is more cross section.
     
  18. Dec 27, 2016 #17
    can you explain exactly how capacitive coupling occurs , in this case?
    where do the coupled electric field lines start and end?
     
  19. Dec 28, 2016 #18
    to summarize all possibilities -
    1.
    2
    3
    4.
    One of the problems of measuring high resistance with a DVM on circuits that can themselves generate small bio-voltages (body for example) is that readings can be fairly meaningless
    5.
    body acts as an antenna picking up all sorts of signals, from mains to local radio to motors and many more things. This injects voltages into a high impedances setup like this
    6.
    also the actual potential of the earth terminal and the point of earth that is measured to may not be equal, IOW a very small current could be present.
     
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