[volt-nuts] Bohnenberger electrometer

[Neville Michie]

In the lexicon of physical devices is an item called an electret.
Commercially these are used in capacitive microphones.
The common ones consist of polymer sheet that has been annealed in a voltage gradient.
An accidental example is the swarf from methyl methacrylate (Perspex, Plexiglass) which will stick
permanently to surfaces when it is turned in a lathe.
I should think that the materials like barium titanate could be used to make electrets.
A polymer film electret in a leaf electroscope would make a polarity sensitive instrument.
BTW some mineral crystals eg tourmaline are pyroelectric, when heated they become charged on
oposite faces forming an electret.

cheers, Neville Michie

> On 16 Mar 2018, at 23:52, Dr. David Kirkby <drkirkby at kirkbymicrowave.co.uk> wrote:
> 
> 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
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