[volt-nuts] Bohnenberger electrometer

[Dr. David Kirkby]

On 6 March 2018 at 09:40, Dr. David Kirkby <drkirkby at kirkbymicrowave.co.uk>
wrote:

> 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.
>

For what it is worth, this is my design:

http://www.kirkbymicrowave.co.uk/tmp/G8WRBs-electrometer.jpg

There's 600 V DC between two strips of PCB material. A 600 V 47 uF
capacitor was charged to 600 V. A small bit of aluminum foil, between the
plates, then moves to the left or right, depending on whether the charge is
positive or negative. The big capacitor, which is 2.2 nF 15 kV is not doing
much apart from being a structure to hold other parts. It has large lugs on
it, where multiple M6 screws can be fitted, so it is nice electrical
insulator. Its actual capacitance (2.2 nF) is insignificant when in
parallel with 47 uF.

Under sufficient applied field, and with sufficient charge, it is possible
to get the foil to oscillate from side to side like a pendulum. I believe
what happens is if a negative charge is applied to the foil, it gets
attracted to the positive plate, which causes them to touch, so the foil
receives a positive charge - the opposite of what it had before. This
causes it to move in the other direction. It is possible to get it to
oscillate back and forth. I expect, with a sufficient mass and very high
electric field, a pendulum could be made to make a clock, but with a little
bit of tin foil, the foil would clearly break quite quickly. A more
substantial structure would be required, which I suspect would need some
very high voltages.

A Google of 'electrostatic clocks' does indicate they exist, although I
have not looked into how they work. But I believe a sufficiently high
electric field could make a pendulum swing, and that of course could make a
clock.

Anyway, it was interesting playing with this.

I am wondering if there's any way to detect the polarity of a charge,
without having any power source. Clearly the gold leaf electroscope can
detect charge, but does not need a power supply. The Bohnenberger
electrometer can detect polarity too, but needs a power supply. I was
wondering if the charge could be applied to two diodes, which were each
connected to a plate. The it may be possible to charge one plate only, as
only one diode would conduct, so only one plate would be charged. The the
leaf would be repelled from whatever plate has the same charge.

Dave