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I sort of don’t get this either. If your testing equipment is introducing resistance can you not “tare” or calibrate your measurements by measuring the equipments current flow in a direct circuit between probes to determine its base resistance then introduce your sample and measure the difference? The resistance in a serial circuit is additive, no?

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The parasitic resistance will be orders of magnitude larger than your sample resistance, any detector capable of detecting the sample resistance would be completely swamped out by it.

They are measuring voltage, not resistance (the resistance is computed). It's impossible to measure resistance without current and some voltage across the resistance, and while the parasitic effects are small the resistance we're talking about (and hence the voltage across that resistance) is so small that second order effects introduced by the probe wires can have a real effect, for instance if the probe wires have a higher resistance that makes them more susceptible to electric fields, which in turn would show up as a voltage. So this is anything but trivial.

If you pick your reference current properly this effect should be extremely small. It will have a little bit of effect but nothing that you should normally have to worry about.

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