Re: Measuring sub picoamp DC by electrical or chemical means

On Mon, 15 Oct 2007 10:48:29 -0700, Paul <energymover@xxxxxxxxx>
wrote:

I'm trying to think of methods that may detect ~ 1/2 picoamp DC
without applying any appreciable *bias* current.

I have a passive analog amp meter that with the aid of a microscope
can accurately detect down to 100 pA. Close, but no cigar.

One possible method is to place a thin wire on a float (perhaps
Styrofoam). The Styrofoam would float on liquid. One end of the wire
would dip in a liquid bath that will conduct an appreciable amount of
electricity. The other end of the wire would dip into another liquid
bath consisting of the same type of liquid. Both liquid baths would be
the input. The total resistance could be as high as a few megaohms.
Over the wire on Styrofoam is a magnet. Current flowing through the
wire will produce a rotational force on the wire, thereby causing the
floating wire to slowly rotate.

Perhaps another method is to direct the 1/2 picoamp DC current through
some type of material that may cause a slow chemical change. Would
flowing the current through some type of photographic film work?

Any ideas is greatly appreciated.

Regards,
Paul Lowrance

National Semi has some premium parts with low fA bias currents
and very low offsets (<<100 uV). You might want to buy a dozen (or
more) and select the best of the lot. You could use one as a low gain
I>E followed by an autozero part for gain. An offset adj would be
required most likely.

Thermoelectric currents would be a problem as well, as would
stray capacitance. Another problem is that many parts are available
only in SMD.
.

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