Fuses, in military explosives, initiate the function of a made up charge or device. They are made up of some kind of burning material. Depending on their chemical design some will even work under water.
Safety Fuse or Fuse Wire was procured by the military from civilian manufacturers who supplied the mining industry, such as I.C.I. Ltd.
It was produced in slow and fast burning versions. The amount of time that it took for the fuse wire to burn down was measured in seconds per feet. According to the size of the explosive charge obviously the person firing it would want to be a certain distance away and so the time was a safety factor element allowing someone to get away, or be a certain distance away before the charge fired.
More often than not the fuse is fitted into a detonator (blasting cap) which detonates and causes a chain reaction explosion in the explosive material itself.
Safety fuse coating is usually black, commercial versions can be orange. Other military types are highly coloured and known as cords.
The Bickford or Safety Fuse has a black powder core with a coating of ashphaltum or other waterproofing agent and a tough textile outer wrapper. It generally burns at 60 to 90 seconds per foot.