Fig. 34.
Cables.
—The third set of wires required consists of those used to connect the charges with the machine or the battery. These wires, which are called the “cables,” consist each of three or more strands of copper wire well insulated with guttapercha, or better, indiarubber, the coating of these materials being protected from injury by a sheathing of tape or of galvanized iron wire underlaid with hemp. Two cables are needed to complete the circuit; the one which is attached to the positive pole of the machine, that is, the pole through which the electric current passes out, is distinguished as the “leading cable,” and the other, which is attached to the negative pole, that is, the pole through which the current returns to the machine, is described as the return cable. Sometimes both the leading and the return cables are contained within one covering. When a cable having a metallic sheathing is used, the sheathing may be made to serve as a return cable, care being taken to make good metallic contact with the wires that connect the sheathing to the fuses and to the terminal of the machine. The best kind of unprotected cable consists of a three-strand tinned copper wire, each 0·035 inch in diameter, insulated with three layers of indiarubber to 0·22 inch diameter, and taped with indiarubber-saturated cotton to 0·24 inch diameter, as shown in [Fig. 33]. The best protected cable consists of a similar strand of copper wire, covered with guttapercha and tarred jute, and sheathed with fifteen galvanized iron wires of 0·08 inch diameter each, to a total diameter of 0·48 inch, as shown in [Fig. 34].
Detonators.
—The new explosives of the nitro-cotton and nitro-glycerine class cannot be effectively fired by means of safety or other fuse alone. To bring about their instantaneous decomposition, it is necessary to produce in their midst the explosion of some other substance. The force of this initial explosion causes the charge of gun-cotton, or dynamite, as the case may be, to detonate. It has been found that the explosion of the fulminate of mercury brings about this result most effectively and with the greatest certainty; and this substance is therefore generally used for the purpose. The charge of fulminate is contained in a copper capsule about a quarter of an inch in diameter, and from 1 inch to 11⁄4 inch in length. These caps, with their charge of fulminate, which are now well known to users of the nitro-compounds, are called “detonators.” It is of the highest importance that these detonators should contain a sufficiently strong charge to produce detonation, for if too weak, not only is the whole force of the explosive not developed, but a large quantity of noxious gas is generated. Gun-cotton requires a much stronger charge of fulminate than dynamite.
Fig. 35.
In the electric fuses [illustrated], the metal case shown is the detonator, the fuse being placed inside above the fulminate. When safety fuse is used, the end is cut off clean and inserted into the cap, which is then pressed tightly upon the fuse by means of a pair of nippers, as shown in [Fig. 35]. When water tamping is used, and when, with ordinary tamping, the hole is very wet, a little white-lead or grease must be put round the edge of the cap as a protection. The electric fuses are always made waterproof; consequently, they are ready for use under all circumstances. When the safety fuse burns down into the cap, or when, in the other case, the priming of the electric fuse is fired, the fulminate explodes and causes the detonation of the charge in which it is placed.