Fusible Plug

Fusible Plug is a threaded metal plug, usually of bronze, brass or gunmetal, with a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point, usually lead or tin. It is screwed into the crown sheet (the top plate) of a steam engine firebox, typically extending about an inch into the water space above. Its purpose is to act as a last-resort safety device in the event of the water level falling dangerously low: when the top of the plug is out of the water it overheats, the lead core melts away and the resulting noisy release of steam into the firebox serves to warn the operators of the danger before the top of the firebox itself runs completely dry.