On Thu, 11 Dec 1997, Mcorio wrote:

> If the charge is so low (i.e. only a small current available) why is it
> not discharged quickly and therefore no large gradient is maintained. I
> do not disagree with your description, but I am still not sure about the
> contradition with the mains-to-the-head analogy.

You're right.  It works like a Van DeGraf generator.  If you short it
out, you won't measure a voltage.  The voltage gradient between my head
and my toes is pretty darn small, because I'm a pretty good conductor.
However, the voltage gradient between 2 points outside separated
vertically by 6 feet is a few hundred volts.

The trick is measuring it, without shorting it out.  My cheap digital
voltmeter sees a few hundred millivolts if I lower one electrode and raise
the other, but I'm having trouble clipping the leads to the air (-:.  If I
clip each lead to a couple chunks of aluminum foil, then the measured
voltage goes up.  If I short it out, the voltage drops to zero.  When I
remove the short the voltage climbs.  The bigger the electrodes, the
faster it moves.

My meter sucks for this sort of experiment.  It's highest impedance is on
the millivolt range.  If the impedance didn't change, then it was
registering 300 millivolts on the millivolt range, it should register .3
if switched to volts.  Instead it sees .005 volts.  The voltmeter loads
the leads to heavily with only a couple of square feet of aluminum foil
for contacts.  I guess I need a better voltmeter or 20 foot long
uninsulated wires.  20 foot long wires are not practical on RC airplanes.

Some clever person figured out that ionized air is a conductor.  If you
ionize the air near the contacts with a little bit of radioactive
material, it's as if the contacts were many many times larger.

===Now I'm going Way Way Off Topic, but I can't help myself.=====

I want the thread to die, so I shouldn't mention this.  But it was too
cool not to note.  With the meter hooked up to the foil, I could detect a
running cat at 15 feet.  It is dry here this time of year, and he is a
long haired cat.  I imagine he is really charged up.  He enjoys chasing
the dot from a laser pointer.  So even though he is a cat, I could get him
to run anywhere I wanted, when I wanted, to do experiments.

If I was still, and the cat was far away, the voltmeter showed a few
millivolts.  If I had the cat run towards the negative meter lead, the
meter would register a few volts.  When he ran away, it would go negative.
At longer distances, I'd see smaller changes, but he could still change
that last digit at about 15 feet.  Since the measured voltage went up as
he approached the negative probe, and down when he went away, he's got a
significant negative charge.

I can't repeat the experiment with my other cats, they don't chase the
laser dot.

--
paulh@hamjudo.com  http://www.hamjudo.com
The April 97 WebSight magazine describes me as "(presumably) normal".