[volt-nuts] Bohnenberger electrometer

[ed breya]

For static bias, look up "electret" for ideas on some other possible 
options.

I would recommend against your option 2 capacitor - that's a dangerous 
amount of energy to store in something that may be fooled around with 
experimentally. Also, even though it's a lot of C, being electrolytic, 
the charge will eventually leak off anyway - probably faster than any 
charge loss from using the machine.

The option 2 (2 nF at 4.2 kV) seems more appropriate for this use, 
because of the much higher sensitivity attainable. It's charge will leak 
off too, but since it's likely a plastic or oil capacitor, the retention 
time will hopefully be OK overall.

I wouldn't want to take a jolt from either one. In the ultimate design, 
be sure to use some sort of series current limiting resistance to 
isolate the capacitor from the outside world. The R can be quite high 
(megohms, and of course suitable for the maximum voltage) since not much 
current is needed for operation, so the contact/fault hazard would be 
reduced from dangerous to a tingle. It would be good to also have a safe 
discharging method - another R - that can be switched or jammed in, to 
quickly clear the charge for safe keeping when not in use, or during 
design.

In the old days, optical methods were used for "gain," as in a mirror 
galvanometer, for instance. Putting some simple magnification and 
illumination (sun light if electricity is a no-no) in the system can 
increase the visibility of any deflection.

Lastly, regarding capacitors, a good option if available, is to use the 
nice HV oil caps that can be salvaged from older-era (before they went 
to switching supplies) microwave ovens. These are typically rated around 
1 uF, 2 kV AC. Two in series would do for up to 4-5 kV service. Since 
you don't want bleeder/balancing Rs in this application, it would be 
best to use identical caps, or slightly more complicated charging 
circuitry. They can bought new, but may be pretty spendy, depending on 
the project budget. I have dozens of them - saved from every microwave 
oven I've junked out over the years.

At 1 uF, these would have much better retention time, with hazard energy 
between the original options.

Ed