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Re: VandeGraaf horror stories?



Date: Wed, 7 Jan 1998 20:56:13 -0800 (PST)
Reply-to: phys-l@mailer.uwf.edu
From: William Beaty <billb@eskimo.com>
To: tap-l <tap-l@listserv.appstate.edu>,
list physics teaching <PHYS-L@atlantis.uwf.edu>
Subject: VandeGraaf horror stories?


Does anyone here have any tales to tell about safety hazards
involving tabletop VandeGraaff generators? Parents are
leery about the 300,000 volts produced by some of these, so
I wrote up a page on VDG safety.

Leyden jars and medical implants are the biggies. Are there
other hazards I should mention? (see below)

((((((((((((((((((((( ( ( ( ( (O) ) ) ) )
))))))))))))))))))))) William J. Beaty
SCIENCE HOBBYIST website billb@eskimo.com
www.eskimo.com/~billb
EE/programmer/sci-exhibits science projects, tesla,
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VandeGraaff Machine Safety


Tabletop Vandegraaff machines are very safe. They've been
used in classrooms and Science Museums for decades with
little difficulty. I've not encountered any literature on
hazards associated with VDG machines, so I've put together a
bit here.

NOTE: I am not an expert regarding electrical safety.
This page is for your information only. For accurate
information, consult an expert in electrical shock
hazards.

VOLTAGE: electric potential or electrostatic fields by
themselves cannot harm humans. Rubbing a balloon upon dry
fur can produce potentials approaching those of a tabletop
VDG generator: 20,000 volts. The high voltage produced by a
tabletop VandeGraaff generator is about as dangerous as an
electrified balloon.

DC CURRENT: a tabletop VDG produces an electric current
below 100uA (microamperes), which is about ten times smaller
than a human is able to feel. If you lick your fingertips
and touch them to the terminals of a 9v radio battery, the
level of current in your fingers will be much higher than
that produced by a VDG machine. As far as constant currents
go, a 9v battery is more dangerous than a VDG.

SPARKS AND IMPULSE CURRENT: here is where there is possible
danger. The peak current during the spark from a VDG
machine can be as high as 10,000 amperes. However, at this
amperage the current lasts for less than a millionth of a
second. At a more moderate discharge current of 10mA, the
discharge might last up to a millisecond. This is near the
threshold of human perception of current, and indeed, if you
allow the spark to jump to a metal object held in your hand,
you sometimes can feel a small twitch in the muscles of your
arm. The pain from sparks mostly comes from the high
current density and microscopic region of burned skin where
the spark enters your skin. Small sparks might be slightly
painful, but there is far more danger from the suprise they
can cause when they happen unexpectedly.


POSSIBLE HAZARDS

* A person suprised by the small shock might stumble
and fall,
especially while standing on a rickety chair during
the usual "hair raise" demonstration.

* If you use your VDG to electrify a large Leyden Jar,
you can build
up dangerous energy levels and create an
electrocution hazard. If you touch the wrong parts of
the leyden jar, the discharge through your body can
violently tense your muscles, causing you to crash
into nearby furniture. The discharge might knock you
unconscious so you fall and hit your head. It may
even stop your heart. Don't use Leyden Jars unless
you are trained in dealing with hazardous high
voltage. Never let kids experiment with VandeGraaff
machines and Leyden jars together.

* Medical implants could be damaged. A person with a
cochlear
implant, a portable external defibrillator, or other
sensitive device must steer clear of radio
transmitters, microwave ovens, cellular phones,
electrified balloons, and VDG machines.

* The small spark from a VDG could cause a heart attack
in a person
having a serious undiagnosed heart condition.
Needless to say, this possibility gives nightmares to
public science demonstrators. It doesn't matter if
that person would have had a heart attack within a
day, if YOUR device triggered it, who will be blamed?

* If a person has flammable liquids on their skin or
clothing, a
spark from a VDG might trigger a fire. The classic
hazard is butane from a leaking cigarette lighter in
an inside pocket in a jumpsuit.

* Some older VDG machines contain a high voltage power
supply. This
supply presents an electrocution hazard if the
generator's case is opened or if the charged lower
comb is touched. Also, if the belt becomes conductive
from built up dirt, and if both combs touch the belt,
the generator's upper sphere will present an
electrocution hazard.

* And last: If you operate a VDG next to a computer,
and the
computer circuitry gets trashed, the owner of the
computer might kill you! ;)





Created and maintained by Bill Beaty. Mail me at:
billb@eskimo.com.
If you are using Lynx, type "c" to email.

It seems to me that you need to be careful if you let the
VandeGraf generator operate a discharge tube. You can
produce a lot of cold-cathode X-rays. WBN
Barlow Newbolt
Department of Physics and Engineering
Washington and Lee University
Lexington, VA 24450
Telephone and Phone Mail: 540-463-8881
Fax: 540-463-8884
e-mail: NewboltW@madison.acad.wlu.edu

"Prediction is very difficult, especially about the future."

Neils Bohr