> 1-) Why do sparks exist when we make touch the (+) line to the (-) line of
> dc voltage or (phase and neutral line of AC)?

The spark is an electrical discharge through air and metal vapor that is 
started when the field between the almost touching contacts is large 
enough for the air to ionise.

> 2-) Where is the source of scent after touching the two lines to each other?

Gas (nitric and nitrous oxide which forms HNO3 with water in air) and 
several types of pulverised metal oxides and nitrides.

> 3-) (Although this one is a bit foolish question) Is it wiser to test the
> existance of the electricity on the AC line by letting touched phase and
> neutral lines?

Yes, but only once. The last time. (joking)

Peter
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