safety-plug

Definitions

noun In steam-boilers, a bolt having its center filled with a fusible metal, screwed into the top of the fire-box, so that when the water becomes too low the increased temperature melts out the metal, and thus admits steam into the fire-box or furnace to put the fire out. Also called fusible plug.
noun A screw-plug of fusible metal used for the same purpose in steam-heating boilers carrying pressures of from 5 to 10 pounds.
noun A form of spring-valve screwed into a barrel containing fermenting liquids to allow the gas to escape if the pressure becomes too great.

Sorry, no etymologies found.

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In testing the Navy time-fuse by the watch, or michronometer, the safety-plug must be removed: otherwise the fuze will burn longer than the time for which it is marked.

Ordnance Instructions for the United States Navy. 1866. Fourth edition. United States. Navy Dept. Bureau of Ordnance
The safety-plug should be removed when the Navy time-fuse is used in rifled cannon, as recent experiments show that it is a probable cause of premature explosions of shells.

Ordnance Instructions for the United States Navy. 1866. Fourth edition. United States. Navy Dept. Bureau of Ordnance
A safety-plug at the lower extremity prevents the communication of fire to the powder in the shell, in the event of the accidental ignition of the fuze after being uncapped.

Ordnance Instructions for the United States Navy. 1866. Fourth edition. United States. Navy Dept. Bureau of Ordnance