Re: VanderGraaff safety (fwd)

[Bob Sciamanda <trebor@VELOCITY.NET>]

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