Thank you very much everybody who answer to my questions.

Mr Birtelmer, I think your answer is adequate on classical physics domain.
But what about the quantum mechanical explenation of that fact?

I've seen a documentary related with QM which named "What the bleep do we
know?" There was a scene which the camare had zoomed into the electron
stage. While the boy have been bouncing the ball to the ground nothing was
touching each other. Either the ball or his hand. The electrons were pushing
eachother and the electromagnatic repulsive force were beating the force of
gravity..

On 5/22/06, Marcel Birthelmer <marcelb.lists@gmail.com> wrote:

> I'll try, but I'm very possibly wrong, please correct me.
>
> 1) When the circuit is closed, think of the two contacts approaching
> each other slowly. At some point they will be close enough so that the
> voltage different is higher than the breakdown voltage of whatever is in
> between them, like air for example. This then causes the air to ionize
> which forms a conductor channel, and that's a spark - the brightness
> depends on, among other things, how much current is drawn through this
> channel, which is why heavier loads spark more.
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