Re: VanderGraaff safety (fwd)

[William Beaty <billb@ESKIMO.COM>]

On Tue, 5 Jan 1999, brian whatcott wrote:

> Sam asked three questions.
>
> Low Resistance:
> I measured my sinistral index to my dextral index and read about 780 kilohms.

If your capacitance to ground is about 200pF, then the decay constant for
a discharge would be RC = 160uS.  If sparks normally go "snap", we'd
expect the time length to be brief.

> I measured my sinistral index to my wife's dextral index while holding her
> other hand. This read about 1.7 megohms.

These values might depend on the ohmmeter, since unpredictable half-cell
voltages (where metal touches electrolyte) will probably contribute a
falsely high reading.  The DC creates a rechargable cell, an opposing EMF
appears, and it makes your fingers taste like corroded metal.  A better
technique might be to apply a potential thats >> than any half-cell
voltage.  Connect a DC supply to a pair of metal objects, grasp the
objects, then measure series current.  Can do!

With a regulated DC supply and a DVM in series, all clip-leaded to two
large jeweler's screwdrivers, the resistance between my one hand and the
other is:  19.9v/1.16mA ~ 800 ohms!!  If I grabbed 120VAC, I'd feel 150mA!
My palms are not very hard or dry, so this is probably a lower resistance
than for an average adult.  The palms of the students might have about the
same order of resistance.


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