Re: van de graaff tube (fwd)

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---------- Forwarded message ----------
Date: Tue, 30 Nov 2004 08:16:00 -0800
From: Ed Phillips <evp@xxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: van de graaff tube (fwd)

"Assuming you are spraying the lower pulley at 5-7 kV DC, the 2.5 inch
wide
belt will deliver charge to the upper terminal at an approximate rate of
12-15 uA DC.  If you are using a self-excited design, the max charge
rate
will be approx 3-4 uA DC."

	I realize you're "in the business" and should know better than I, but
the current you quote for a self-excited machine sounds low to me.  I'll
have to look up my old notes but I have a self-excited machine using a
belt just under 2" diameter which gave something like 15 microamps. 
Far, far from an optimum design.  The column is grey PVC, 4" OD and cost
a small fortune when I built the thing back around 1963 or a year or so
later.  The pulleys are about 7/8" OD and turned at a pretty high speed
through a leather belt/pulley system running off a 3450 RPM motor. 
Original upper terminal was a "more or less" 15" sphere made from
aluminum mixing bowls, later replaced with an oblate I got from Frank
Lee.  I asked him about his experience with separately-excited machines
and he told me the current wasn't necessarily greater than self-excited
but that things were much more controllable.

	I have a smaller machine I bought from Science First which uses a belt
about 1-1/4" wide which puts out about 6 microamps.

	Antonio has published some notes on VDG output current limits but I
haven't bothered to compare notes with my old measurements.

Ed