Re: VanderGraaff safety (fwd)

[brian whatcott <inet@INTELLISYS.NET>]

> 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

It is a commonplace that Europeans are sometimes electrocuted where the mains
is 230 volts, and that American houses sometimes burn down where the mains
is 117 volts.   I like this experimental setup.   One could I suppose ask
for a 1:1 isolating transformer in front of the variac (*), a series
resistor for each electrode (80kohms each?) and if one were obsessed with
safety, a 'deadman's handle' arrangement so that the unelectrified hand was
required to be touching the voltage adjustment in order to enable the
electrodes (as a measure designed to forbid hand to hand contact with live
electrodes needed for through the chest current)
Brian

(*) An easy 1:1 isolating setup is two bell transformers arranged back to back.
brian whatcott <inet@intellisys.net>
Altus OK