The electrically operated detection device that uses a bi-metallic strip is called what?

• A. Heat detector
• B. Ionization detector
• C. Photoelectric detector
• D. Smoke detector

Answer: (A)

The key idea is that a bi-metallic strip acts as a thermal switch. Two metals with different coefficients of expansion are bonded together; when heat causes one metal to expand more than the other, the strip bends. That bending moves a contact and closes the electrical circuit, triggering the alarm. This is the operating principle of a heat detector, which detects heat or a rapid temperature rise and converts it into an electrical signal to raise the alert.

Ionization detectors rely on ionized air in a chamber and smoke particles disturb the current, which is a different mechanism. Photoelectric detectors use light and smoke scattering to trigger when particles interrupt or scatter the beam. Smoke detector is a broader category that can include heat detectors, but the device described with the bi-metallic strip is specifically a heat detector.