Consider an extreme example, where the solution to measure is in a glass tube, the input electrodes are in the solution at the ends of the tubes, and the measuring electrodes are on the outside of the glass tube. The glass acts as the insulator of a capacitor, and if you tried to do a measurement at DC, you’d get no signal. At low frequencies, the impedance of the capacitor may be large compared to your voltmeter, making the measurements much smaller than they should be, but once the frequency is high enough that the capacitance provides less than say 1% of the voltmeter impedance, you can make pretty accurate measurements of the voltage inside the tube.

]]>When i started to think about this, i saw the model you have above, a capacitor in parallel with a resistor and in series with the water resistance and since we are using an AC source, and considering at, not very high or very low frequencies, i would be measuring an impedance and not just the water resistance. That’s why i faced to the 4 pole solution and my question is: with 4 pole solution when i’m measuring the relation voltage/current it will give the real resistor, not the impedance, correct? So i wouldn’t need to worry about measuring the phase shift or measuring at very high frequencies to create a shortcut in the capacitor. Am I right ?

I understand you are approaching through for other purposes, but I looked to your articles with great interest and it seems you have valuable knowledge about this subject.

Thank you very much.

]]>Looking at the 4-point method in more detail, I see I’ve gotten it somewhat wrong, as it assumes a uniform sheet of material with 4 point contacts in a line, and the theory relies on how the electric field behaves in a (usually thick) sheet of material. If I were still a mathematician, I’d probably be able to remember how to do conformal mapping to handle the 2D case, but even then the 3D case for thin films would probably have required numerical approximation.

The current between the two middle points is not identical to the current between the end points, but is a constant factor times the current, dependent on the geometry. The basic idea, of measuring the voltage from electrodes that don’t have current through them, is still the same.

Although I’ve cast the problem here as a measurement of salinity, what I’m really more interested in teaching students is the impedance of electrodes—so that they understand why so many of the biosensors use Ag/AgCl electrodes and not stainless steel.

Maybe I should a section in the book about 4-probe measurement, though.

]]>Thank you, Henrique Matos.

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