Final answer:
A potentiometer is a variable resistor that is used to measure voltages in high-power circuits by obtaining variable potential along a wire with a consistent current passing through it.
Step-by-step explanation:
A potentiometer is a variable resistor used primarily in high-power circuits and serves as a null measurement device for measuring potentials, such as voltages. Typically, it consists of a long wire connected to a voltage source, and as current passes through it, a steady potential drop occurs along the wire due to its resistance. By making contact at different points along the wire, a variable potential can be obtained, which is useful in determining unknown electromotive forces (emfs) when connected in series with a galvanometer and adjusted until no current flows through the galvanometer, indicating a null reading.