Re: Van de Graaff

[Roger Key <rogerk@maxwell.phys.csufresno.edu>]

> I am about to do the Van de Graaff generator demo.  I have tried it before
> with some smaller devices and I did not have much success.  Now I have a
> generator with a rather large sphere. A student of mine told me that it
> works better when one has hair jelly on his/her hair.  Is it true?  How
> about the length of time a person needs to wait in order to get discharged.
>  (!!)  Would shoes with rubber soles provide enough insulation? 
> ..........Well, if I don't see anything signed by me anymore, you'll know
> that I didn't do it right. ;-) 
> Cheers! 
> Violeta 

Most of the advice I've seen so far has been pretty good, but I can't 
resist putting in a couple of pointers.

Success with the 'hair raising' depends on all the factors of humidity,
insulation of the student, and hair quality (dry, fine hair is best).
I usually have someone picked out of the crowd before the presentation
begins...


I should add here, that when the volunteer comes up front to do the demo,
have the VDG globe discharged and the VDG turned off.  Have the subject place
a hand on the globe _before_ turning it on.  This reduces the pain
to the volunteer subject.  After the hair raising demo, turn the VDG
off, and then have the subject release the globe first, then step down
after releasing the globe.  Again, this reduces the pain to the subject.

If you _do_ want a little unpredictability in the presentation, have the
volunteer make a fist and hold it towards you.  the vol is insulated, you
are not.... then, bring your own fist close to that of the volunteer,
and a spark will jump from your knuckle to hers ... (or maybe it's the 
other way) --- the reason for using knuckles instead of fingertips is
that you have far fewer nerve endings to feel the pain in your knuckles.
Another variation is to hold hands with several people in the audience
before making the spark jump, then observe the 'flexing' of the vol's
hair during and after the spark .... lots of interesting physics to
explore with this device.    

I should also add that I always ask if the person volunteering wears
a pacemaker or any other medical electronics.  If they do, then I don't
use them as a volunteer.  With larger machines, I've seen the demonstrator
ask anyone in the room with a pacemaker to either move all the way to the
back or to leave ... it's safer that way...

You might also try taping a beaker on top of the VDG globe and putting
styro packing peanuts in it before turning on the VDG ...

have fun . . . .

- Roger Key
 
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Roger Key                      Teaching Laboratories and Demonstrations
Cal. State University, Fresno  Department of Physics
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email: rogerk@csufresno.edu    http://maxwell.phys.csufresno.edu:8001
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