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A rheostat is an adjustable resistor that regulates the flow of current in an electrical circuit. This functionality allows users to vary the resistance and thus control the current that flows through the circuit, enabling applications such as dimming lights or adjusting the speed of electric motors.

In contrast, a capacitor stores electrical energy and releases it when needed, a transformer changes the voltage of alternating current, and a diode allows current to flow in one direction only while blocking it in the opposite direction. Each of these components has a distinct role in electrical systems, but it is the rheostat that specifically provides the ability to adjust resistance and control current. This makes it critical in applications where variable current levels are necessary for effective operation.