A threaded metal cylinder with a hole that is sealed with a metal of low melting point that flows away if a pre-determined, high temperature is reached. Invented by Richard Trevithick.
The hole is tapered so that the plug cannot be just blown out under high pressure. It is designed to melt when boiler is overheated, releasing a jet of steam into the firebox and alerting the crew.
BS 1123:2006 "Fusible plugs for steam boilers and compressed air applications"
Subjects: Mechanical Engineering