dkl65:
Just so you know, we are having a conversation about the Earth and electricity ...
And Uranus I believe. 
Uhh... where did everyone go? Now we're not talking about anything! 
This convo isn't over yet
What does "convo" mean? I searched it on Google, and found only "convoy".
Maybe they should have a catagory for that.
Google gave up on "catagory"
What does "convo" mean? I searched it on Google, and found only "convoy".
Someone used it as "conversation". I have changed that word in my post.
P.S. I searched it on Google and I found "short for 'conversation'".
Someone may have said it, I only skimmed thru but if the arduino is connected to the earth ground then the absorbed emf from your body (originating from the mains) probably likes to go thru ur arduino to the earth ground
when on a battery the arduino ground is no better than you so the current you pass on to the arduino has nowhere to go, both the arduino + and - are fluctuating with that emf, which the arduino doesn't see as much
Sorry winner but that is simply not true.
It seems to make sense to me, but then again I never really went to school for this stuff lol
Maybe it may be like this: the human body has some capacitance like 100pF. The human body can absorb and release 50/60 Hz EMF from the mains. Mains is wired to the Earth. When the Arduino is connected to the Earth, they have a common reference point, and the Arduino is very sensitive to the mains EMFs. But what I don't get is that, how can I be Earthed enough if I am not standing on the Earth? The path can't be through an insulator.... I also don't get why a near insulator can be used as a common reference point: Car battery, Transformer, Inductance Charger, Mains Power, etc. - #9 by system - General Discussion - Arduino Forum, etc.
Well in rf doesn't the circuit consist on the actual air waves and the earth?
also I think since capacitors work by a gap(dielectric watever), you act like a capacitor to the ground, ur body which is slightly more conductive, air gap(shoes,floor watever) then the conductive ground
its actually amazing how conductive the actual ground is, if you test continuity from say the ground in an outlet to the actual dirt(stick the probe in the dirt yes) you actually get a short
Form an other thread by the same poster:-
I still don't get why if you stand on the Earth and touch to high voltage wire, you get hurt.
Simply you don't unless you provide a path to ground. Have you ever seen birds perched on a high voltage wire? They are not harmed because there is no path to ground.
If you are insulated from ground you can happily touch a high voltage wire without feeling it. It is while you are touching the high voltage wire that if you touch a ground as well you get a severe shock. Most of the electric shocks I had as a teenager came about from touching a ground, in this respect grounds are as dangerous as high voltages. Touch the two and it is painful.
The path can't be through an insulator
As has been pointed out there is no such thing as an insulator. Electrical resistance covers the second widest range of values in the whole of Physics (for extra credit what measured parameter covers the widest range).
It only takes a few mA to start to feel an electric shock, and a high voltage can push this through quite high resistances.
the Arduino is very sensitive to the mains EMFs
No it is not, no more or less than most electronic circuitry.
if you test continuity from say the ground in an outlet to the actual dirt(stick the probe in the dirt yes) you actually get a short
No you don't there are several ohms between a good earth and an electrical earth. When I was a radio ham I had a good earth consisting of a biscuit tin (U.S. read cookey tin), filled with coke (the coal derivative not the drink) and soil. Buried and watered it produced a good ground. I think there was about 8 ohms between that and the third pin on the mains socket.
Tru its not perfect, but still quite amazing, I didn't believe the guy at work when he said it works, then in a back garage on the opposite side of the house where the service is bonded to the ground at the water main and ground rod he got 2ohms with his fluke meter
but that reason of not having a return path that's why the battery would have less apparent noise I think, the induced voltage from your body is also being induced into the board and battery so no current flows
if you put an oscilloscope on a battery terminal and don't connect the ground clip you can see the absorbed emf the same as if you put your finger on it, obviously not as much as 150lbs of water but similiar effect
the induced voltage from your body is also being induced into the board and battery so no current flows
What you are describing is a differential input effect. Or rather a common mode input.
Electromagnetic pickup does not work like this because the pickup is not balanced.
Then, who's explaination answers this problem: why does Arduino picks up more EMFs when powered by USB? I know it is due to grounding, but was my earlier speculation wrong?
why does Arduino picks up more EMFs when powered by USB
I think your question is flawed, it does not pick up more. You are playing with floating inputs, which by definition involves undefined states.
That floating input behave differently under different conditions is hardly surprising. You can't generalist from your simple test that are not done under controlled conditions, in a non screened environment.
How can my input be floating if I have it tied to GND with 3.6M ohms? That is a weak connection, so I can meausure the presence of an EMF. It works pretty well with a battery! Did you see my earlier link: http://arduino.cc/forum/index.php/topic,102717.0.html ? I have debugging prints in the link. It isn't floating. Arduino is more sensitive with USB power! Shouldn't it be something to do with Earthing?
How can my input be floating if I have it tied to GND with 3.6M ohms?
That is floating.
But not to an extent that it hardly ever reads 0.
But not to an extent that it hardly ever reads 0.
That means it is floating.
You were mislead. Okay, I accept the fact that my pin is floating a bit. That is required to measure EMF. When battery powered, it reads 0 until I put it near an appliance. When I touch it, it still reads 0. USB power: when I touch it, it reads >300. When I don't, it is 0.
Evidence of mislead:
I used a (almost) double negative when you quoted me ("not" + "hardly ever"). You only took note of "hardly ever". That means that it reads 0 all the time until I put it near my iPod charger. That is intended. 
I am just confused about how Earthing (USB) can affect the readings, as you saw previously.