Final answer:
A two-lead variable resistor that adjusts current in a circuit is known as a potentiometer. It allows for precise adjustments in potential without altering circuit current by tapping at variable points along the resistor wire.
Step-by-step explanation:
A two-lead variable resistor used to adjust current in a circuit is called a potentiometer. A potentiometer operates by passing a constant current through a long wire resistor. This setup results in a stable but adjustable voltage drop (an IR drop) along the wire, allowing for a variable potential to be tapped at different points along the wire. The potentiometer can be adjusted until the galvanometer, which is connected in series with an unknown electromotive force (emf), reads zero. This allows the unknown emf to be proportional to the resistance of the wire segment, based on its length and resistivity. It's important for students and professionals working with circuits to understand that by using a potentiometer, they can control the voltage and current running through a component or circuit, enhancing the accuracy of potential measurements without altering the current in the circuit.