🚀 go-pugleaf

In article <1992Apr10.085039.10688@dxcern.cern.ch> jeroen@dxcern.cern.ch (Jeroen Belleman) writes:
>Can anyone suggest a method to measure the output voltage? I do not
>think suitable probes exist.

There isn't really an "output voltage."  The potential on
the sphere is a function of the charge it holds and the
capacitance between it and the grounds around the room.

>So far, I get fat sparks from a polished 4cm diameter metal pipe over
>a little more than 10cm. The more diffuse sparks from the pipe's end
>easily exceed 50cm. You can make you own voltage estimates from that.

Drat.  I forgot the breakdown voltages for air.  The
potential at the time of the spark is of course just the
distance (cm) times the breakdown voltage (KV/cm).

As you can see, by varying the capacitance (a smooth surface
nearby; a sharp surface at a greater distance) your generator
can operate at different potentials.

If you integrate the charging current to get the charge you
can determine the capacitance.

You should put the entire apparatus in a large faraday cage
(perhaps you can wrap the room in foil :-)) and measure the
total displacement current to get the net charge--this also
gives you a roughly calculable basal capacitance.

				--Blair
				  "Watch where you point that thing..."

In article <10390@inews.intel.com> bhoughto@hopi.intel.com (Blair P. Houghton) writes:
>In article <1992Apr10.085039.10688@dxcern.cern.ch> jeroen@dxcern.cern.ch (Jeroen Belleman) writes:
>>Can anyone suggest a method to measure the output voltage? I do not
>>think suitable probes exist.
>
>There isn't really an "output voltage."  The potential on
>the sphere is a function of the charge it holds and the
>capacitance between it and the grounds around the room.
>

So? Can anyone suggest a way to measure the potential of the sphere?
The capacitance of the sphere is hardly affected by any grounds around
the room. The nearest ground is about 70cm away, normally.

>>So far, I get fat sparks from a polished 4cm diameter metal pipe over
>>a little more than 10cm. The more diffuse sparks from the pipe's end
>>easily exceed 50cm. You can make you own voltage estimates from that.
>
>Drat.  I forgot the breakdown voltages for air.  The
>potential at the time of the spark is of course just the
>distance (cm) times the breakdown voltage (KV/cm).
>

No sir, it's electric field strength that gives the breakdown voltage
between two electrodes, not simply their potential difference and
distance. For a given distance, sharp electrodes will cause
breakdown for much lower potential differences than blunt ones.
Besides, the capacitance change caused by a small nearby electrode
is negligible. ('Small', that is to say, compared to the size of
the high voltage electrode.)

>As you can see, by varying the capacitance (a smooth surface
>nearby; a sharp surface at a greater distance) your generator
>can operate at different potentials.
>
>If you integrate the charging current to get the charge you
>can determine the capacitance.
>

The charging current is constant and independent of the terminal
voltage. (Excuse me, potential.) It's breakdown of the air
insulation that limits the potential. The relatively simple
geometry of the setup enables me to calculate the capacitance.
It's about 18pF. If I integrate the 9uA charging current over
the time it takes to get the first discharge (about 1 second),
that gives me a peak potential of 500kV. I don't really believe
that. That's why I'm looking for a different way to measure it.

Jeroen Belleman (Getting a sore throat from breathing too much ozone)
jeroen@dxcern.cern.ch
jeroen@cernvax.cern.ch

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

In article <jim.703295980@st-michael> jim@cs.UAlberta.CA (Jim Easton) writes:

   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.


To lightning research types this instrument is known as the field
mill. It is a cleaver way of integrating a chopper circuit into the sensor.
It is also used in industry to measure man-made electric field
potentials -- they are used on military helicopters with a high votage
transformer to get the potential of the helicopter equal to the
ground to null-out static buildup. I have seen military surplus units.


Regards,
Albert Boulanger
aboulanger@bbn.com