On Sat, 21 Feb 2004, Omega Software wrote:

> But how can current pass from just one wire? Is it a capacitive effect?

Assuming that you weren't touching some other conducting surface, then yes
it is a capacitive effect. At Synaptics (where I work), we use ~10pF for
the human body free-space capacitive coupling. Our TouchPads measure the
capacitive coupling between a finger and the surface of the TouchPad
sensor. This is roughly 1pF (and our ASIC can resolve this down to 10's of
femtofarads!). In your case, you could approximate the LED as a wire and
your body as a two-plate capacitor; one plate tied to the LED and the
other to free space. Using that approximation, you can then estimate how
much energy your "body-capacitance" can store.

Now, as the grounded LED approaches the wire, there is another capacitive
coupling there as well. In this case, it's the capacitance between the
wire and the LED. As the LED approaches, there will be some point at which
the air-dielectric of this capacitance will break down. That distance will
depend on many factors such as the peak voltages, air humidity, and the
geometry of the conductors. A sharp point like the lead of the LED will
cause the air to break down at a larger distance than a flat conductor
(like your finger). Once the air-dielectric breaks down, the live wire
will begin to charge your body capacitance. So the arc that you saw was
most probably the current flowing from the live wire to charge your 10 pF
body. You probably immediately pulled back, at which point your body
discharged into the surroundings!

I hope you weren't at home when you did this because you're not supposed
to do these at home! :)

Scott

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