Which of the following is an application of a variable resistor?

• A. Radio volume controls
• B. Rotary dimmer switch
• C. Motor speed control
• D. Television controls

Answer: (B)

A variable resistor changes how much current flows in a circuit by altering its resistance, which lets you control a parameter like brightness or load power. The rotary dimmer switch is a classic example: turning the knob moves a contact along a resistive element, changing the resistance in series with a lighting load. As resistance decreases, more current reaches the lamp and it becomes brighter; as it increases, the lamp dims. This direct use of a changing resistance to regulate power to a load makes it the clearest application of a variable resistor.

Radio volume controls do use a potentiometer to set signal level, which is also a form of a variable resistor, but the primary function there is adjusting an audio signal rather than controlling power to a load. Motor speed control can involve varying resistance, but in practice this is inefficient for motors and newer methods like PWM are common. Television controls may include potentiometers for specific settings, but they’re not as direct an application of a variable resistor to control a load as the dimmer switch.