Which device is a two-terminal adjustable resistor used to vary current in a circuit?

• A. Rheostat
• B. Antenna
• C. Ground
• D. Battery

Answer: (A)

A rheostat is the device designed for this role. It’s a resistive element with a movable contact that lets you tap off different portions of the resistance. When you connect it in a circuit, you have two terminals: one end of the resistor and the movable contact. Turning or sliding the control changes the effective resistance, which, by Ohm’s law (I = V/R), directly changes the current flowing through the circuit.

This is why it’s used to vary current: increasing the resistance lowers the current, while decreasing the resistance raises it. Antenna, ground, and battery don’t function as adjustable resistors: an antenna handles RF signals, ground is a reference node, and a battery provides energy, not a variable resistance.