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Re: VanderGraaff safety (fwd)

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

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.

It might interest folks to know that Underwriter Labs considers the "shock
hazard" threshold to be 40Vdc (if I recall correctly, or maybe it was
45Vdc). If it's above 40V, a whole different set of product testing is
required.

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?

Maybe you'll want to add a 1:1 transformer to provide isolation from
ground. Depending on the internal circuit of the variac (and possible
miswiring of the electrical outlet,) there might still be 120VAC on your
electrodes relative to ground. If the variac's "common" is not referenced
to the neutral line of the AC outlet, and a student leans against a
grounded metal lab bench, ZAP! A 120V:120V isolation transformer would
eliminate this problem.

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William J. Beaty SCIENCE HOBBYIST website
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