A component in a fire detection system that signals the presence of a fire or overheat condition.

Beads of a thermistor material (a material, such as germanium, whose resistance changes with its temperature) support a wire that forms part of the fire detection circuit. The bead-supported wire is enclosed in a metal tube routed through the area to be protected.

If the tube is heated by a fire, the thermistor material changes from an insulator into a conductor and completes an electrical circuit between the central wire and the tube. This completed circuit initiates the fire-warning procedure by turning on the fire-warning light and sounding the fire-warning bell.