Which device is activated by magnetism?

• A. Ultrasonic sensor
• B. Reed switch
• C. Capacitor proximity sensor
• D. Proximity switch

Answer: (B)

The device that reacts to a magnetic field is the reed switch. Inside a reed switch are two small ferromagnetic metal reeds sealed in a glass envelope. When a magnet comes close, the magnetic field magnetizes the reeds and makes them attract and touch, closing the circuit. No physical contact is needed for it to change state, just the presence of a magnetic field.

Ultrasonic sensors operate by sending out sound waves and measuring echoes, so their action is based on acoustics, not magnetism. Capacitor proximity sensors detect changes in capacitance caused by nearby objects and their dielectric properties, which is an electric-field effect, not magnetic. A proximity switch is a broad category that includes several sensing methods, but the magnetism-driven behavior described here is characteristic of a reed switch.