🚀 go-pugleaf

jeroen@dxcern.cern.ch (Jeroen Belleman) writes:

>So? Can anyone suggest a way to measure the potential of the sphere?

In the Nuclear Physics lab where I used to work we had a Van De Graff
which could go up to 7 Mev.  There were two methods of measuring
the terminal voltage.  One was with the analysing magnet.  ie. There
was a magnet that bent the beam at right angles and by measuring the
magnetic field (we could do that very accurately using NMR) we knew 
what the terminal voltage was.  In fact there was a slit the beam had to
go through and by comparing how much of the beam hit one side compared
to the other generated a control signal which controlled a corona
discharge on the inside of the tank containing the terminal.  Thus
the analysing magnet actually controlled the terminal voltage.

Off hand I would say this is not too practical for you unless you want
to generate a beam.  The second method involved what we called a
"generating voltmeter".  This was plate mounted inside the tank 
facing the terminal.  There was a grounded chopper plate rotating
in front of it so an AC signal was generated on the plate the amplitude
of which is proportional to the terminal voltage.  Calibrating this
voltameter was easy (for us) - we calibrated it from the analysing
magnet.

I would suggest that you could still use the same setup.  For
calibration you could run your VDG at fairly low voltage which is
directly measureable (say like about 30 KV) and note how much
AC signal one gets at the generating voltmeter  - it won't be much 
but it should be enough to get a reasonable calibration.

The column was built in layers of conducting and insulating plates.
There was a high value high voltage resistor between each layer the
purpose of which was to keep the field gradient more or less uniform.
(We were accelerating particles)  The current down this resistor chain
would be proportional to the terminal voltage.  Although we never used
it for that you could.

Lets assume that you are getting about 500 Kv and that you could draw
1 UA without loading it too much.  If I've done my calculations right
a string of 50 10000 Mohm resistors would be just about right.  Just 
remember when dealing with resistors of this high value a finger print
on the resistor will change its value.  You also have to be careful
what you are using to mount them on.

You could also use such a resistor chain to help calibrate a generating
voltmeter at lower voltages.

	Jim