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4-wire (Kelvin) resistance measurement


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Cross-posted on NI here.

I just witnessed a 4-wire resistance measurement across the barrel of a metalized connector made with a standard set of alligator clip-like probes.


One operator makes this particular measurement with the clips only slightly opened, and the resistance exceeds the test limits. This triggered the Manufacturing Engineer to get involved and his habit is to make this measurement with the clips almost fully opened. The resistance, when doing it this way, is about 1/3 of the first operators method.

I tried to imagine why this might occur, but I'm stumped. I thought about the length of the current paths through the material and how the corresponding change in distance would increase the resistance when the probes are closer together, but this doesn't explain the 3x factor. This would also imply that turning one clip 180 degrees should result in a change in the opposite direction, but this isn't observed. I'll keep guessing.

Have you seen this? Can you explain it?

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