A fusible plug is a threaded metal cylinder 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 that flows away if a pre-determined, high temperature is reached. The device was invented in 1803 by Richard Trevithick, the proponent of high-pressure (as opposed to atmospheric) steam engines, in consequence of an explosion in one of his new boilers. Experiments in the 19th century proved its value as a safety precaution against low water levels in steam engine boilers, but later applications extended its use to other closed vessels, such as air conditioning systems and tanks for transporting corrosive or liquefied petroleum gasses.
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