Re: VanderGraaff safety (fwd)

[Karl Trappe <trappe@PHYSICS.UTEXAS.EDU>]

Most of the resistance which has been discussed here has been "contact
resistance" which varies tremendously with skin moisture, etc.

Some years ago, we used to do the demo:  "Students as resistors in series
and parallel".  We measured the resistance of one student by getting the
voltage and current as has been described.  Then we placed several students
in series to observe the series law of resistance.  By switching *who* was
holding *who's* hand, we could make the whole series law fail utterly.
[this was a very valuable learning experience for students, as it caused
them to question what they had just learned/memorized about series
resistances]. Humor was readily had at their expense as we contemplated
whether John and Mary would get swetty palms when holding each other's
hands (and unspoken variations on this theme).

To confirm the contact resistance aspect of this demo, we added beakers of
salt water for each student to first wet their hands in, and later to use
to submerge their hands while making the series measurement.  Resistance
drops by at least an order of magnitude.

*However*, as some of you might guess, we tried to be more efficient in
using the beakers of salt water (it was a nuisance to bring out a beaker
for every pair of hands).  Without thinking, we "shared a common beaker for
left and right hands, making the conduction directly across the person(s).
...not funny!  Kind of like sticking the voltage source in the bathtub!

Bottom line:  R for a human is a series combination of body resistance plus
the two contact resistances (and other layers of skin, etc if you want to
get more specific).  Contact resistance is extremely variable.

PS  you also get the "human battery" effect which modifies the voltage you
are "applying" if you use different material clips (copper alligator on one
and zinc alligator on the other, etc), ie, you are a coper-zinc brine pile.
For VDG voltages this is not significant, of course, but for getting your
guestimate of the value of the person's resistance, it is significant.
Karl

> At 16:26 1/5/99 -0800, Bill wrote:
>
> > 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!! ...
> > William J. Beaty
>
> Using a wall wart set to 12 volts  (unregulated - so it floats to 18.5
> volts off load) and capable of 300 mA, I attempted to repeat Bill's
> striking test:
> but my DVM whose minimum reading is 100 microamps, would not register any
> value above zero. So I selected a Simpson's d'arsonval movement with fsd=50
> microamps.
> Using the modest area of the meter terminal screwthread and a battery clip
> connected to the wall-wart I read 24 microamps at an applied potential of
> 17.6 volts from hand to hand.
> This represents (I promise you I did not fudge the data) 7.3E5 ohms.
>
> I can easily acknowledge that the conductance is (unsurprisingly) a
> function of skin area and pressure. And my electrodes were evidently much
> smaller than Bill's. Still, the difference is quite striking:
> Bill: 17 kilohms: Brian 730 kilohms.
>
> I would be most interested in any other determinations of skin resistance.
>
> Brian
> brian whatcott <inet@intellisys.net>
> Altus OK

Dr. Karl I. Trappe                              Desk Phone:  (512) 471-4152
Physics Dept, Mail Stop C-1600                  Demo Office: (512) 471-5411
The University of Texas at Austin               Home Phone:  (512) 264-1616
Austin, Texas 78712-1081