Re: VanderGraaff safety (fwd)

["L. R. Cartwright (Larry)" <physics@SCNC.CPS.K12.MI.US>]

I definitely agree that additional precautions like a 1:1 isolation
transformer and redundant resistances in each output line would be a great
idea.  "An old variac" connect directly to AC service is a catastrophe
waiting to occur, no matter how many times the device has performed safely
up to now.

The most important thing is:  ONLY use your device in a GFI protected
outlet.  Anyone whose lab doesn't have ground fault interrupt protection
needs to talk to your building administrator(s) right away.  It's code
everywhere in the US that gfi's be installed in any room with plumbing.
Gfi's aren't very expensive, and pretty effectively eliminate the
direct-path-to-ground concerns.

Best wishes,

Larry


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Larry Cartwright
Physics, Physical Science, Internet Teacher
Charlotte High School, 378 State Street, Charlotte MI 48813
<physics@scnc.cps.k12.mi.us> or <science@scnc.cps.k12.mi.us>

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On Sat, 9 Jan 1999, paul o johnson wrote:

> 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