Final answer:
A variable resistor differs from a fixed resistor due to its adjustable resistance, allowing for alteration of the current flowing through a circuit or for calibration in measurement applications such as null measurements.
Step-by-step explanation:
A variable resistor is different from a fixed resistor because it can be adjusted. This means that the resistance offered by the component can be changed as needed in a circuit. Variable resistors, such as potentiometers, are often used to adjust levels, such as volume on a radio, or to calibrate equipment through the precise setting of voltages, often as part of a null measurement setup where an exact voltage is needed to balance the circuit. In a circuit with a potentiometer, a voltage source is connected to the resistor, which is usually a long wire; a constant current is passed through it, resulting in a steady potential drop along the wire (an IR drop). By varying the contact point along the wire, a variable potential is achieved. This characteristic is utilized, for example, when taking voltage measurements without significantly altering the current in the circuit. The use of digital devices and understanding of the behavior of resistors in series and parallel also contribute to the understanding and application of resistors in circuits. In measuring circuits, the aim is to minimize the impact of measurement tools (voltmeters and ammeters) on the circuit they are measuring. This requires careful consideration of the internal resistance these devices introduce to the circuit.