Fig. 32.
In [Fig. 30] is shown an external view of an electric tension fuse. It consists of a metal cap containing a detonating composition, upon the top of which is placed the priming to be ignited by the electric spark. The ends of two insulated wires project into this priming, which is fired by the passage of the spark from one of these wires to the other. The insulated wires are sufficiently long to reach a few inches beyond the bore-hole.
Sometimes the fuse is attached to the end of a stick, and the wires are affixed to the latter in the manner shown in [Fig. 31]. The rigidity of the stick allows the fuse to be readily pushed into the bore-hole. When the ground is not very wet, bare wires are, for cheapness, used, and the stick is in that case covered with oiled paper, or some other substance capable of resisting moisture. These “blasting sticks,” as they are called, are extensively used in Germany. When heavy tamping is employed, the stick is not suitable, by reason of the space which it occupies in the bore-hole.
A mode of insulating the wires, less expensive than the guttapercha shown in [Fig. 30], is illustrated in [Fig. 32]. In this case, the wires are cemented between strips of paper, and the whole is dipped into some resinous substance to protect it from water. These “ribbon” wires may be used in ground that is not very wet. They occupy little or no space in the bore-hole, and therefore are suitable for use with tamping.
To connect the fuses with the machine or the battery, two sets of wires are required when a single shot is fired, and three sets may be needed when two or more shots are fired simultaneously. Of these several sets of wires, the first consists of those which are attached to the fuses, and which, by reason of their being placed in the shot-hole, are called the “shot-hole wires.” Two shot-hole wires must be attached to each fuse, and they must be of such a length that, when the fuse has been placed in its proper position in the charge, the ends may project a few inches from the hole. These wires must also be “insulated,” that is, covered with a substance capable of preventing the escape of electricity.
The second set of wires consists of those which are employed to connect the charges one with another, and which, for this reason, are called “connecting wires.” In connecting the charges in single circuit, the end of one of the shot-hole wires of the first charge is left free, and the other wire is connected, by means of a piece of this connecting wire, to one of the shot-hole wires in the second hole; the other wire in this second hole is then connected, in the same manner, to one of the wires in the third hole; and so on till the last hole is reached, one shot-hole wire of which is left free, as in the first. Whenever the connecting wires can be kept from touching the rock, and also from coming into contact one with another—and in most cases this may be done—bare wire may be used, the cost of which is very little. But when this condition cannot be complied with, and, of course, when blasting in water, the connecting wires, like the shot-hole wires, must be insulated. When guttapercha shot-hole wires are used, it is best to have them sufficiently long to allow the ends projecting from one hole to reach those projecting from the next hole. This renders connecting wire unnecessary, and moreover saves one joint for each shot.
Fig. 33.
