Electric Blasting. – A method of electric blasting will be understood on reference to Fig. 129. The electric fuse A consists of two insulated copper wires, the bare ends of which are brought close together, and between them is placed an explosive composition called the priming, that is fired by the passage of the electric current; this composition ignites the small capsule of gunpowder, the flame from which ignites the main charge. If dynamite is used instead of gunpowder, the exploder is a copper cap, B (Fig. 129), containing an electric fuse similar to the above, but instead of gunpowder in the cap there is a charge of fulminate of mercury, the explosion of which makes the required flame and detonation combined, which causes the explosion of the dynamite. The copper wires (insulated) of the fuse are generally 4 or 5 feet long, or more for a deep blast-hole. If only one shot is discharged, each of the fuse wires is attached to a conductor corning from the battery. (This is on the pit-bank, and the conductors are taken down the shaft-side in waterproof covering.) If several shots are to be discharged at once, the right-hand wire of one fuse is connected to the left-hand wire of the next fuse (see Fig. 129). The left-hand wire of the last fuse in one direction is connected to one conducting cable from the battery, and the right-hand wire from the last fuse in the other direction is connected with the other conducting cable. This is called connecting "in series;" the electric current has to pass through all the fuses in one circuit. The conducting cable is generally a strand of twisted copper wire covered with some insulating material, which is protected with plaited hemp or with tape, often with both. Another mode of discharging (see Fig. 130) is to connect the right-hand wire of each fuse directly with the right-hand cable by means of a short connecting piece of wire, and to connect the left-hand wire of each fuse with the left-hand conducting cable. The current in this latter case, instead of passing in circuit through the fuses, is divided amongst them all. This is called connecting "in parallel." A magnetic exploder is often used for high-tension fuses – the kind hitherto most generally adopted. In this machine, by turning a handle, and by means of multiplying gear, an armature is made to turn rapidly between the two poles of a magnet, thus producing an electric current which passes in circuit through the wires; by pressing a button this circuit is broken, and another circuit, through the conducting wires and fuses, is completed; this heats the priming, and so causes an explosion.

With a properly arranged and sufficiently powerful magnetic battery, as many as a dozen shots may be discharged at once. With small batteries, such as are usually supplied, five or six shots only can be discharged with certainty. Lately, low-tension fuses have been adopted, and are recommended by high authorities (see Fig. 131). The external appearance of the fuse is much the same. The electric current is produced by chemical action, the electric generator being a galvanic battery. A small one suitable for shot-firing in coal weighs about 2.5 lbs. including a strong wooden case.

Messrs. Bickford, Smith, and Co. make a kind of fuse for simultaneous blasting; for this purpose a slow-combustion fuse, burning at the rate of say 3 feet a minute, is connected to a tin tube in which is a small explosive charge. Into this tube are fixed the fuses connected with each shot; these fuses are made to burn at a very high speed; the explosive charge in the tube ignites all these high-speed fuses at the same instant, and as owing to the high speed of combustion in the shot fuses a short difference in length has no appreciable effect on the time, there is practically an immediate and simultaneous discharge.