Electronic – Why can’t I measure resistance when there is current on the potentiometer


Probably a very basic question but I have no idea why this happens.

I have a 1K potentiometer. When I measure the resistance over its legs when it is not connected to anything, the results are as expected and alter expectedly when I move the knob.

However, when I connect the pot to a 9V battery and try to measure the pot's resistance, I don't get any readings.

What is the reason for this behavior?

Best Answer

Because your multimeter can't measure resistance. So it applies a known current, measures the resulting voltage, and computes the resistance from that. 1

So when you're applying an external current to the potentiometer you are upsetting the meter's procedure, and the resulting voltage is probably outside the measurement range.

1 Unless it's really old. In which case it applies a fixed voltage, measures the current, and lets you read the resistance off an inverse scale.

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