From: SFPhysics@aol.com
Date: Wed Feb 19 2003 - 09:01:23 PST
From: SFPhysics@aol.com Message-ID: <4b.2b013675.2b851263@aol.com> Date: Wed, 19 Feb 2003 12:01:23 EST Subject: Re: Van de Graaffs and Wimshurts...
Greetings Eric:
As always Ron Wong's diagnosis is first rate. To his fine treatment I would
just like to add the following.
The first rule of working on any static electricity generator is not to touch
the parts with the bare hands. The oils and salts from your hands act to
discharge the generator while it is trying to work. Belts for Van de Graaffs
should be changed while you are wearing gloves, either latex lab gloves or
cotton film handling gloves. This also goes for the rollers and much of the
demonstration apparatus.
If the motor is running just warm that is nothing to worry about. The "rule
of thumb" is that "if you cannot hold your thumb on the motor for 15 seconds,
it's too hot." Motors for the smaller Van de Graaffs are shaded pole motors
which operate by having a shorted turn near each pole and they do run hot.
Larger machines have different types of AC induction motors that should run
cool. You don't say which yours has but I am assuming the smaller units
commonly sold by science supply houses. As Ron points out the larger motors
will run hot when overcoming excessive mechanical resistance.
Cleanliness is required for static production. When was the last time the
collection sphere was cleaned of all the human fingerprints with alcohol?
The vertical belt tube should be cleaned on the outside with alcohol,
denatured ethyl is best, and never with drug store isopropyl because they add
mineral oils to it which again will detract from machine operation. Have the
rollers been cleaned of all the old rubber belt debris? When applying oil to
bearings, rollers, and shafts, has the oil been just lightly applied so it
does not travel to the static generating surfaces? While these machines are
simplicity itself, care for them involves some procedures requiring good
maintenance techniques.
To answer Marc's question of availability for generators. The smallest basic
Van de Graaff sells for under $150 from the full price science supply houses.
I bought mine directly from the maker for $90 on sale several years ago.
The Wimshurst machines are not cheap. Occasionally one goes cheaply on eBay
but you will have to service it and can expect to get one that is not running
well. The supply house machines that are brand new seem to have a price
around $400 at the lowest. A quick look at eBay today reveals two machines,
one currently at $158.00 from Welch and the second from Edmunds complete with
some demonstration materials for $36.00 (but it is a small machine). I have
bought a lot of my equipment from eBay but always expect to service it before
it becomes fully operational. Science teachers have neat toys but they don't
always take care to their toys!
Lastly Eric, where are you? If you are in the Marin-Sonoma area I can come
and look at your machine.
Al Sefl
Trying to stay retired...
But is the only guy on the cruise ship going to Alaska who asks for a sample
of glacial ice...
And the only guy at the Trinity Site who brings his own scintillation
counter...
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