Reed switches are activated by magnetism.

• A. True
• B. False
• C. Only at high temperature
• D. Only with external electric field

Answer: (A)

Reed switches are operated by magnetic fields; the two thin ferromagnetic reeds inside a sealed glass capsule are kept slightly separated. When a magnetic field from a nearby magnet is present, the reeds magnetize and attract each other, closing the contacts and completing the circuit. Remove the magnet and the reeds demagnetize, the springs push them apart, and the circuit opens again. Because the actuation comes from magnetism, not temperature or an electric field, statements about triggering only at high temperature or only with an external electric field don’t fit. This magnetic mechanism is what makes reed switches ideal for simple proximity sensing and door/window alarms.