You might think the probes in the picture are just funny looking alligator clips. But if you watch [tomtektest’s] recent video, you’ll learn they are really Kelvin probes. Kelvin probes are a special type of probe for making accurate resistance measurements using four wires and, in fact, the probe’s jaws are electrically isolated from each other.
We liked [Tom’s] advice from his old instructor: you aren’t really ever measuring a resistance. You are measuring a voltage and a current. With a four-wire measurement, one pair of wires carries current to the device under test and the other pair of wires measure the voltage drop.
If you wonder why that’s better than two probes, it all comes down to resistance in the test probes. Pulling supply current through the probe wires — which have some resistance — causes a voltage drop that affects the measurement. While the sense wire pair will also have resistance, the sensing current can be very small which means there will be correspondingly less error in the measurement.
We’ve seen these probes built from scratch, too. You do need a meter that will do the actual four-wire measurement, although a power supply and a voltmeter will do the job, too. If you don’t want to probe in real life, you can always do it in virtual reality. (Well, a circuit simulator, anyway.)
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