[volt-nuts] Bohnenberger electrometer

[Dr. David Kirkby]

Sorry this is not precision voltage measurement, but it is not unrelated.

As a radio club project, we are building a simple electroscope, with no
active components. The gold leave variety would work, but two bits of
alluminum foil do too.

My plan was to go one better, and build a Bohnenberger electrometer. Does
anyone here have experience of building a Bohnenberger electrometer, or
know much about them? They are similar to the gold-leaf electroscope, but
have one leaf sitting in an electric field. So the leaf moves one way or
the other, depending on the polarity of the charge. Or some other
electrometer, that does not use any active components - no ICs or
transistors.

I believe the original design was built with some sort of battery, but my
intention was to use a capacitor and charge it up. I'm not sure of what
sort of electric field / energy / stored charge is required though. I have
two obvious options for a voltage source based on what I can find at home.

1) 2 nF 15 kV capacitor which I can charge to about 4.2 kV easily (Q = 5.4
uC)
2) 2200 uF 400 V electrolytic capacitor which I can charge to 400 V (Q=0.8
C)

Clearly capacitor 2 stores a lot more charge, but for any given spacing of
plates, capacitor 1 creates am electric field 10 times higher than
capacitor 2.

Obviously if gold leaf or aluminum foil moves, that takes energy. Will that
come from the charge one puts on the top of the electroscope, or will some
come from the capacitor, so discharging the capacitor? If the latter,
capacitor 1 might discharge quite quickly due to its small capacitance,
whereas 2200 uF of capacitor 2 would not.

I do not wish the unit to be mains operated. I don't mind charging the
capacitor from the mains, but I want it to be standalone, independent of
any supply voltage.

Dave