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Re: Van de Graaf Generator

On Tue, 10 Oct 2000, Michael Edmiston wrote:

I believe the reason humidity affects van de Graaff generators is not
because of air discharge,

One common fallacy is that the conductivity of humid air is immensely
greater than the conductivity of dry air. However, "conductive" means
"contains mobile charges."

Air becomes conductive when it contains ions. It becomes conductive when
it contains charged dust or charged aerosols. But why would the presence
of single H2O molecules affect conductivity any more than, say, the
presence of helium, CO2, or even O2? The origin of the misconception is
clear: humidity causes surfaces to become damp and conductive, and while
this is associated with humid conditions, it's not the air itself that
supports the leakage currents. Unfortunately the misconception about
"conductive wet air" has crept into textbooks, and their voice of
authority is hard to combat.


but because of surface discharge along the
insulating column.

I note that under humid conditions, rubbing rabbit fur upon rubber
surfaces generates no detectable e-field at all. Even those tape-peeling
demonstrations stop working when humidity is high enough. Therefore I
speculate that when humidity is high, the contact-electrification
"electrophorus" effect at the VDG rollers is shorted out. If so, then
there is no corona on the charging combs, no charge on the belt, and no
leakage down the vertical column.

Those "Winsco" tabletop VDGs only work in humid conditions if their felt
rollers are baked for a long time by hot air from a hair dryer. Once the
rollers are dry, the high voltage appears. It has been my impression that
slight dampness on the vertical column does create leakage which lowers
the output voltage, while slight dampness on the roller surfaces makes the
high voltage vanish utterly.

When trying to persuade a VDG to operate, I concentrate the hot air stream
on the non-metal roller while the machine is running. The inner belt
surface and the roller surface become dry, and sparks can be drawn from
the upper comb assembly. Then I dry the column as much as possible.


Separate topic: I suspended a metal pan by nylon fishing line and charged
it to about 100V with respect to ground. This was outdoors on a humid
weekend. I tried to detect the falling voltage caused by humidity surface
leagage, but found nothing. The voltage on the pan was rock steady. The
next evening I measured the voltage, and it had only decreased by a few
percent! It seems that three 30cm lengths of nylon single-strand fiber
have quite a small surface area, and quite a high resistance even during
very humid conditions. I guess the sphere of an 'ideal' VDG would be
suspended by fishline strands from above.



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