An example of a rheostat is a dimmer switch on a light.

• A. A dimmer switch on a light
• B. A light bulb
• C. A battery
• D. A fuse

Answer: (A)

A rheostat is a device that acts as a variable resistor to control current in a circuit. A dimmer switch on a light does exactly that: it changes resistance in series with the lamp, so the current through the bulb is reduced when you dim it, making the light dimmer. This behavior—varying resistance to regulate current—is what defines a rheostat. The other items don’t fit the role of a variable current-controlling element: a light bulb is the load, a battery is the power source, and a fuse provides protection.