Van de Graaff ca.100 kv voltage

Very nice generator you have there.

could you do me a favor? Get a spray bottle of water.. and spray a stream of Mist right down between the 2 spheres while its sparking, maybe the water will explode..

probably wrecked that cd you had down there

ehehe yes probably :)

Yes, you're totally right. I'm only estimating, the electric field isn't uniform and it has lots of discharges due to corona effect. I'm estimating the voltage value only on mathematic, as you can see on my first comment.
I'm sorry but it's impossible to measure the right value because i hadn't a so large resistor...but I appreciate your comments :D
Thanks you all guys! I hope you want to continue discussing about that!!
I'm sorry for my english, but I'm an 19-years-old italian guy ;)

pretty good english I'd have to say.

your estimation was correct as far as it seems the debate has panned out...

I won't accept a measure of the electric field over specific gemoetries to be an accurate representation of the voltage. I'm sure if you had a needle on top of that van-de-graff it would just bleed corona, you might get an electric field value somewhere in the range of 'megavolts' in its local electric field.

I'm willing to send him a large resistor if only so he can measure with a volt meter and equations the accuracy of his statements.

High voltages like these are measured with calibrated gaps between spheres. If the spheres are generally larger than the gap being measured, the figure of 30 kV per centimeter is pretty close. Needles and spheres smaller than the gap will give longer gaps for the same voltage. Your 30 kV supply clearly was not sparking between large spheres.
Please Google "sphere gap tables" and you will see for yourself.

...Ok, you may be correct about that

Nevertheless, I'd STILL like to see a measurement with a resistor... but I'll accept that the 100kV estimate lends itself more towards the truth.

The problem with resistors at those voltages is just that--everything conducts!! It takes a very special resistor to have a precisely known value at 100+kV. This is why it's done with gaps--they are actually more accurate, if done right.