A device consisting of a glass envelope and two ferromagnetic reeds that responds to magnetic fields is called which?

• A. Hall effect sensor
• B. Magnetic reed switch
• C. Proximity sensor
• D. Temperature switch

Answer: (B)

The key idea is a magnetic reed switch. It consists of two ferromagnetic reeds sealed inside a glass envelope. When a magnetic field is nearby, the reeds are drawn together and make contact, closing the circuit; when the field is removed, they separate and the circuit opens. The glass envelope provides a hermetic seal and protects the reeds from contaminants, allowing reliable operation in various environments. This behavior—mechanical contact triggered by a magnetic field—is distinct from a Hall effect sensor (which uses a semiconductor and outputs a voltage proportional to field), a proximity sensor (a broad category using non-contact detection by different principles), or a temperature switch (which responds to temperature changes rather than magnetic fields). Real-world use includes door/window sensors where a magnet and the reed switch enable simple on/off signaling.