# Measuring the electrical conductivity of the atmosphere

I am trying to devise a way to test the electrical conductivity/resistivity of the atmosphere through rising altitudes. The platform for the experiment is a NASA weather balloon with the allowance of a 1 kg payload. I have started by creating a sort of leaky capacitor design which involves the collection of charge on two parallel plates spaced closely together. The problem is getting a measurable reading. The output signal at this point will be on the scale of femtoamps. This requires the use of an operational amplifier with an inverting configuration. Unfortunately after months of working with op amps of various sensitivities, I still can't get a stable reading. The signal either rails or has no gain at all. I'm afraid my skill level for working with circuitry is what is lacking here. Does anyone have ideas for getting a reading without using an op amp? I admit I'm in over my head here, but I'm not ready to give up!

Bobbywhy
Gold Member
How have others solved the problem of measuring femptoamperes? To answer this I used Google, search term "measuring femptoampres" and found many examples of a variety of techniques used successfully. A few examples are:
http://www.tmworld.com/design/other/4381004/Femtoamp-fA-measurements
http://www.acminstruments.com/datasheets/FemtoAmp Datasheet.pdf

Since in the last example four operational amplifiers (LTC1222) are used to give a gain=200,000 it seems to me unlikely you can measure such small currents without using operational amplifiers.

Edit: By the way, Welcome to Physics Forums!

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berkeman
Mentor
I am trying to devise a way to test the electrical conductivity/resistivity of the atmosphere through rising altitudes. The platform for the experiment is a NASA weather balloon with the allowance of a 1 kg payload. I have started by creating a sort of leaky capacitor design which involves the collection of charge on two parallel plates spaced closely together. The problem is getting a measurable reading. The output signal at this point will be on the scale of femtoamps. This requires the use of an operational amplifier with an inverting configuration. Unfortunately after months of working with op amps of various sensitivities, I still can't get a stable reading. The signal either rails or has no gain at all. I'm afraid my skill level for working with circuitry is what is lacking here. Does anyone have ideas for getting a reading without using an op amp? I admit I'm in over my head here, but I'm not ready to give up!

Welcome to the PF.

In addition to the useful links posted by Bobbywhy, check out the Keithley website. They are the largest manufacturer of PicoAmmeters, I believe:

http://www.keithley.com/knowledgecenter

I've used their instruments here at work, and I believe they have a useful application hints manual somewhere on their website.

There are a lot of things to get right when measuring such small currents. You might consider doing your conductance measurement at some moderate AC frequency, to avoid having to deal with DC issues and leakages...

berkeman
Mentor
On the Keithley web page that I linked above, I think this is the handbook I was remembering:

"Low Level Measurements Handbook: Precision DC Current, Voltage, and Resistance Measurements (6th edition: 2004)"

The link to it is in the middle of the right border of the page.

mfb
Mentor
Keithley devices are good, but I think they all exceed the 1kg-restriction (Keithley 487 -> 5kg, for example). I think some current amplification will be necessary.
Did you try individual transistors instead of op amps?