Re: What is it?
If you pass an unknown current through a resistor, you will get a voltage drop.
If you simultaneously pass a reference current through the same resistor in the opposite direction, and adjust that reference current until you get zero voltage drop across the resistor, the reference current will exactly match the unknown current. You can now measure the reference current, and get the value of the unknown current without introducing any voltage drop in the unknown current path - ie. a Zero Resistance Ammeter.
You could set the reference current either with a feedback loop, or manually set with the multi turn pot.
Stuart
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