Resistance from fingertip to Earth wire

[hackhard]

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 ? shouldn't resistance be lesser?doesnt any current flow if the live wire (directly , no human between) touches the floor?

 

[.Scott]

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.

 

[phinds]

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.

 

[hackhard]

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.

but no i wasnt , you could try it yourself

 

[.Scott]

but no i wasnt , you could try it yourself

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.

 

[hackhard]

Try the same experiment by putting the probe into a small area of water on the floor and let us know what happens.

it worked! mutimeter shows 2.3 Mohm in water
also tried with a steel plate between floor and probe, shows roughly 12 Mohm

 

[phinds]

it worked! mutimeter shows 2.3 Mohm in water

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

also tried with a steel plate between floor and probe, shows roughly 12 Mohm

And do you understand why that is?

 

[hackhard]

And do you understand why that is?

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 doesn't provide that large a surface area of contact

 

[phinds]

must have to do with contact surface area.
liquids provide larger contact surface area. occupy all unevenness
same does with sweat on human skin

Exactly. This is a valuable lesson about electronics.

a flat steel plate doesn't provide that large a surface area of contact

Yes, but you'll notice that it does a lot better than the probe alone. Exactly the same principle.

 

[hackhard]

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)

 

[phinds]

why does the contact surface area matter?

Why do you think it matters? Obviously you now know that it DOES matter, so what is your explanation?

 

[hackhard]

Why do you think it matters? Obviously you now know that it DOES matter, so what is your explanation?

i don't have one

 

[Delta2]

why does the contact surface area matter?
what is the lumped circuit abstraction for probe- skin- floor system vs the probe- floor system

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.

 

[phinds]

i don't have one

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?

 

[hackhard]

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?

not same
cause more charge flows in same time through cross-section normal to field vector

 

[phinds]

not same
cause more charge flows in same time through cross-section normal to field vector

Right. And more surface area is more cross section.

 

[hackhard]

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.

can you explain exactly how capacitive coupling occurs , in this case?
where do the coupled electric field lines start and end?

 

[hackhard]

to summarize all possibilities -
1.

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.

2

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.

3

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 doesn't provide that large a surface area of contact

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.