This reminds me of a stunt this guy I met in tech school Father did. He
had built a DVM from a kit and took it home to show his folks. His father
took the dvm and showed it to his friends and plugged it into the AC at home
in the Ohms mode and it melted the probes. melt and blew the main fuse. They
guys father got away with his life. Be ware of AC Power lines!!!
Michael Johnston
----- Original Message -----
From: "Scott Dattalo" <scott@DATTALO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, February 21, 2004 9:51 AM
Subject: Re: [EE:] curious Live wire experiment


> 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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