Which device uses an electromagnet to control another circuit?

• A. Relay
• B. Resistor
• C. Capacitor
• D. Transistor

Answer: (A)

Relays use an electromagnet to control another circuit by energizing a coil that creates a magnetic field, pulling a switch contact to open or close the separate circuit. This lets a small control signal switch a larger current in a load circuit while keeping the control and load sides electrically isolated. A resistor is simply a passive component that limits current, not a switch. A capacitor stores and releases energy but doesn’t operate via a magnetic field to move contacts. A transistor is a semiconductor switch that controls current without using an electromagnet. So the device that uses an electromagnet to control another circuit is the relay.