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Re: VanderGraaff safety (fwd)



As a child I remember seeing such an apparatus at amusement parks and
carnivals - they were coin operated. It was presented in the nature of a
dare-devil contest to see who could stand the highest voltage setting
without letting go. I haven't seen them for many years and have always
assumed that these things went the way of foot X-Ray machines and were
probably outlawed, but I don't really know.

At the least I would advise some redundancy of safety precautions - at least
one resistor in each lead, so that the failure (shorting) of a single
resistor would not result in a dangerous current. One might also add
protection against voltage surges (lightning, etc?).

Bob Sciamanda
Physics, Edinboro Univ of PA (ret)
trebor@velocity.net
http://www.velocity.net/~trebor

-----Original Message-----
From: paul o johnson <pojhome@FLASH.NET>
To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
Date: Saturday, January 09, 1999 10:44 PM
Subject: Re: VanderGraaff safety (fwd)


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This van de Graff thread has been extremely interesting. It has moved me to
describe a demo I set up for my physics 2 class (2nd half of Serway).

Most inquisitive kids have briefly gotten across 120 VAC by the time
they're in
college. They know what it feels like. But most of my students have no idea
what it feels like to get an electrical shock. First we talk about it. Very
few
of us can feel anything if we put our dry fingers across a 1.5 V flashlight
battery or even a 6 V lantern battery. A few more can feel a slight tingle
by
putting our dry fingers across a 12 V car battery. But 120 V knocks us on
our
butt. Where is the threashold?

I rigged up an old variac with its output connected to two brass screws
about
two inches apart and also to an AC voltmeter. I put a 100-kohm
current-limiting
resistor in series with one lead to the screws. I ask a volunteer to place
two
fingers of the same hand on the screw heads (to avoid sending any current
through the chest) and use the other hand to slowly turn the variac up from
zero while watching the voltmeter. So the individual student is in complete
control of the voltage between the finger contacts.

With dry skin, my most sensitive students can start to feel a faint tingle
at
about 20 V while I, myself, can feel nothing at all until about 65 V. Then
we
re-try it after dipping our fingers in a beaker a pure tap water, We can
then
feel that initial tingle at perhaps 50% of the dry-finger voltage.

I've done this in about four classes, now, and my students say it is an
interesting and memorable experience. Many for the first time in their
lives
feel electric charge going through their hand. I figure the current when we
first feel the tingle is between 10 and 100 microamps.

OK, colleagues, what are your thoughts on this demo? Is it too dangerous?
Is
there a better way to let students actually feel an electric shock?

poj