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Re: Fw: my $1.00 quasi-electrometer and more "electrostatics" experiments

Tesla List wrote:
> 
> Subscriber: harris-at-parkave-dot-net Sat Feb  1 22:13:37 1997
> Date: Sat, 1 Mar 1997 22:31:51 -0500
> From: Ed Harris <harris-at-parkave-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Fw: my $1.00 quasi-electrometer and more "electrostatics" experiments
> 
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> 
> Hi all,
> 
>         Due to Richard Hull's initial experimets on the production of
> excess mobile charge (in his case positive) during spark gap coil firing,
> I thought I'd try some experiments of my own. It's a very facinating
> subject, and I'm not sure we've really pinned the thing down completely.
> 
>         First problem: how to detect the charges. Since I don't have a
> Keithly 600 series electrometer like Richard, I thought I'd build my own
> "electrometer". Though it doesn't have the absolute sensitivity of the
> Keithly, I think it beats out the Keithey on the basis of a
> cost/performance
> basis ;-). It consists of a $0.30 neon bulb from Radio Shack, a couple of
> pieces of aluminum foil and a 6"x6" polyethylene "baggie". The two pieces
> of
> fiol and the baggie are used to make a low leakage 400pF capacitor to
> store the accumulated charge. The value of 400pF is in no way critical.
> The circuit is like this:
> 
> GND ---------------(neon bulb)--------------------------> detector input
>              |                               |
>              |                               |
>              ---------| |---------------------
>                        400 pF cap
> 
> Current flows into the input and charges the cap until the neon
> bulb breaks down at about 100volts (or so). The bulb flashes at
> a rate dependent on the input current. It takes Q=CVb coulumbs
> of charge to make the bulb flash where Vb is the 100Volt breakdown
> voltage. If we assume a current of I amps input then:
> 
> I=fCVb
> 
> Where f is the bulb flash rate. So for f=1flash/sec, C=400pF and Vb=100v,
> we get a current of 40 nano amps. If one reduced the cap to 40pF and
> allowed
> a flash rate of 0.1 per sec, then 400pico amps could be detected. the other
> advantage, of course, is that the this circuit is very tolerant of abuse.
> Using
> a rectified line voltage of about 150V and a 250Mohm resistor I was able to
> confirm that the flash rate corresponded to the current as predicted above.
> 
> The polarity of the charge accumulated could be determined by
> which of the bulb's electrodes light up. The negatively biased electrode
> always
> lights up for a single polarity current. If the bulb electrode which goes
> to
>  the detector INPUT glows then the input current is NEGATIVE. Otherwise
> the current is POSITIVE.
> 
<SNIP>

Ed, THANKS!! This is really a neat way to accurately measure polarity
and charging current with a robust, yet cheap(!!) tool. Looking forward
to doing some similar experiments on the disruptive coil now that I've
got the right equipment!!
 
> 
> By the way, I ordered some FET input op-amps from Burr Brown for
> $25 apiece. They have a typical  input bias cuurent of 0.040 pico-amp!
> This is getting pretty damn close to to the Keithly, eh? The number
> is OPA128LM if anyone is interested in making their own FET input
> electrometer.
> 
> Take care,
> -Ed Harris

Could you share the design you plan to use here as well?

Thanks in advance!!

-- Bert H --