=175. Fuses and Detonators.=—The explosion of gunpowder and other deflagrating explosives is caused by the direct application of a flame led to the charge by a powder fuse, or they may be fired by a blasting cap which is itself exploded by the heat from a fuse or an electric spark. The powder fuse is a cord made up of a train of powder securely wrapped in a number of thicknesses of woven cotton or linen threads and usually made waterproof. Ordinary fuse burns at about 2 feet per minute but there may be wide variations from this rate due to the quality of the fuse, moisture, temperature, or pressure. Moisture tends to retard the rate, pressure to increase it. Instantaneous fuse will burn at about 120 feet per second. It is distinguished from the ordinary safety fuse both by eye and touch due to the rough red braid with which it is covered. It is used in firing a number of charges simultaneously. Powder fuses are lighted by the application of a flame or smoldering torch to the freshly cut or opened end exposing the powder grains. Cordeau Bickford is lead tubing filled with triton, in which the flame travels at about 17,000 feet per second. This is also used for igniting charges simultaneously. The detonation of an explosive is caused by the shock or heat of the explosion of a more sensitive substance which has been exploded by a powder fuse or electric spark. The common method of detonating explosive charges is by the firing of a blasting cap. These caps are copper cylinders, closed at one end, about 1½ inches long and ¼ to ⅜ of an inch in diameter, or larger. They contain a mixture of about 85 per cent fulminate of mercury and 15 per cent potassium chlorate held in place by a wad of shellac, collodion, or paper. The strength of detonators is based on the weight of fulminate of mercury and is designated as shown in Table 63. TABLE 63 STRENGTH OF BLASTING CAPS ────────────────────────────────────────┬────────────────────────────── Blasting Cap, Commercial Grade │ Grains Fulminate of Mercury ────────────────────────────────────────┼────────────────────────────── 3X or Triple │ 8.3 4X or Quadruple │ 10.0 5X or Quintuple │ 12.3 6X or Sextuple │ 15.4 7X or Number 20 │ 23.1 8X or Number 30 │ 30.9 Single strength │ 12.3 Double strength │ 15.4 Triple strength │ 23.1 Quadruple strength │ 30.9 ────────────────────────────────────────┴──────────────────────────────