True output voltage on a Van De Graaff generator?

[Albertgauss]

Greetings,

How do you know what the true output voltage on a Van De Graaff generator is? Can it be measured? If a manufacturer advertises it as having an output voltage of 100,000 Volts, is this an idealistic, classroom quote or the true quote they have actually measured? If the output voltage advertised by the manufacturer is not the true, measurable voltage, what causes the discrepancy?

 

[gleem]

You measure it. If I remember correctly the voltage that can be maintained depends on several things like the curvature of the HV terminal, the charge rate, the leakage current, the surrounding air humidity. One way to measure it is with an AC electrostatic voltmeter basically a rotating metallic vane that is partially shielded by a grounded plate. Then the vane is exposed to the electric field an induced charge in produced in the vane proportional to the terminals potential. This produces an AC signal that can be measure with conventional electronics. However it needs to be calibrated. In a research Van de Graaff a particle beam is produced whose energy is measure by a magnetic spectrometer type of system deflecting the beam by a certain amount that can be calculated based on the field strength and the geometry of the system.

 

[anorlunda]

Try a search for "high voltage non contact voltage measurement" It returns lots of hits.

For example this one which claims to do both voltage and charge.
http://www.trekinc.com/pdf/3002-field-voltmeter.pdf

 

[Albertgauss]

@anorlunda, ok, looks good. Just did a basic web search but it is going to take me a few days to go through all that.

Does anyone have any experience with one meter or the other for measuring 100,000 volts with no contact?

I will also need a day to read the paper you suggested. Let me spend some time on that so I can give it a thorough review.

@gleem I am familiar with what you mentioned in this post, but your response is too vague. Do you have any clear pictures or diagrams that are more specific? That would help a lot.

 

[gleem]

@gleem I am familiar with what you mentioned in this post, but your response is too vague. Do you have any clear pictures or diagrams that are more specific? That would help a lot.

I'm sorry I do not have any more information. I think it is the AC version of the type of electrostatic voltmeter that @anorlunda mentioned.

 

[SF cookie]

First: Van De Graaff generators are DC, not AC.

At school, we were shown the method based on two slivers of gold foil that were connected at the top and with no voltage applied they would hang down (parallel, like). If a voltage was applied at their junction, the 'like charges repel' effect would come into play. The higher the voltage - the greater the repulsion, increasing the angle between the gold foil slivers.

They weren't calibrated but were effective at showing the Van De Graaff generator was working - without draining any significant charge.

 

[gleem]

First: Van De Graaff generators are DC, not AC.

At school, we were shown the method based on two slivers of gold foil that were connected at the top and with no voltage applied they would hang down (parallel, like). If a voltage was applied at their junction, the 'like charges repel' effect would come into play. The higher the voltage - the greater the repulsion, increasing the angle between the gold foil slivers.

They weren't calibrated but were effective at showing the Van De Graaff generator was working - without draining any significant charge.

You missed the point. The electrostatic voltmeter generates an AC signal see post #2.