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I am using a 6485 - to measure the current generated when a liquid is passed through a packed column of nanoparticles. The column itself acts as a current source (typically in the nA-uA range) and has a resistance of the order of megaohms. I connect the - *across* the column (electrodes are in contact at the entry and exit) and get current readings that make sense but I am not sure if they are absolute or relative. My question is what current am I measuring? It seems I am using the - in the arrangement I'd use for a voltmeter. How should I connect the - to a high resistance current source? Or, what is the equivalent circuit for the way I have the meter hooked up?
I'm not sure what you mean by "absolute or relative". From what you described I think you have the - connected properly. Instead of your device you could imagine connecting a simple battery (one that could only produce a very small current) to the ammeter in the fashion you have described and measuring the current it could provide to a short circuit. If you connected a voltmeter to the same battery to measure it's open circuit voltage, it would be connected exactly the same. The difference would be that the voltmeter would have a very high impedance and the ammeter would have very low impedance.
Many thanks for your prompt reply. I'll describe my system in more detail to help explain the absolute vs. relative comment.
An insulating tube T is filled with a porous, compressed plug of nanoparticles P. A liquid L is pumped through the tube. In the presence of the liquid, the surface of the nanoparticles becomes electrically charged and an equal counter charge develops in the liquid. In effect, a capacitor is formed with the solid/immobile particles being one plate and the liquid the other. When the liquid is pumped, the liquid "plate" moves relative to the solid/immobile "plate" and, by definition, generates an electrical current. This so-called "streaming current" is what I am interested in measuring. It allows me to see how the surface of the particles changes due to changes in the liquid (e.g., due to detergents). The liquid has a finite conductivity and so the current flow generates an electrical potential ("streaming potential"). This is why I describe this as a current generator with high resistance - perhaps there's a more correct analogy(?)
I have positioned porous electrodes E at the entry and exit of the tube in contact with the porous plug of nanoparticles and connected to the meter A. I get sensible numbers (the right order of magnitude and they change in the expected way when I modify the liquid etc) but I'm not convinced I am measuring the current being generated or just a fraction of it (i.e., absolute vs relative - or proportional). For conventional applications, connecting the ammeter in this fashion would create a short. But in my case, the current is being continually generated - it cannot be shorted as such. So I'm really not sure quite what the meter is reading and how, if at all, it affects the system it is measuring.
I appreciate your guidance.
I'm afraid I'm not going to be of much help with respect to what you are actually measuring. This would appear to be more of an issue with your structure than with the meter.
As far as connecting the meter, in the manor you described, it does not actually create a dead short. An ideal ammeter would create a dead short, but real ammeter's have a specification called 'voltage burden'. This is the voltage drop across the meter when measuring current and, while quite low, it does exist.
In my last post I mentioned the idea of short circuit current and open circuit voltage. After posting that message I thought it might be worthwhile to mention that these two tests are commonly done on photovoltaic cells and the ammeter and voltmeter are connected to the device in same way for each test.
Your reference to photovoltaic cells is very interesting. It led me to dig a bit deeper and I found an obscure paper that explicitly stated that my kind of measurement should be made under short-circuit conditions, i.e. with the ammeter across the column in the way I have it configured.
So thank you - I now have faith that I am making the measurement correctly!
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