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 <drkir...@kirkbymicrowave.co.uk> > wrote: > > On 6 March 2018 at 09:40, Dr. David Kirkby <drkir...@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 > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
