Final answer:
A potentiometer is a variable resistor with three terminals and a contact that can slide, used for adjusting voltage or current and for precise voltage measurements in circuits.
Step-by-step explanation:
A variable resistor with three terminals and a sliding contact is known as a potentiometer. It can be used for various applications including volume controls in audio equipment, as a position transducer in a joystick, or for fine voltage adjustments and calibrations in circuits. The potentiometer operates by adjusting the position of a sliding contact across a uniform resistance. As the contact moves, the resistance between the terminals changes, which can be used to control the voltage or current in a part of the circuit.
In electrical engineering, using a potentiometer as a null measurement device is a method for measuring voltage or potential difference. The device works by connecting a known voltage across a resistor, creating a steady potential gradient along it. By adjusting the contact point along the resistor, you can obtain a variable potential, which is especially useful when measuring unknown voltages with high precision since no current flows through the measurement device (such as a galvanometer) at the point of null potential.