What is a potentiometer?

A potentiometer is a variable resistor used to resist electrical current.

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Think of a potentiometer as a water valve, only for electricity, most of the same principals exist. For instance if you crack a valve open and get the desired flow, and the water pressure increases or the valve gets larger, the volume of water coming out of the valve will increase. In this analogy the water pressure is represented by the voltage and the size of the valve is the current.