200 



ELECTRICITY FOR BLASTING 



current across the interruption in the metallic circuit (i.e. across tho small distance 

 between the terminals of the wire in the fuze). It must be borne in mind that 

 the striking-distance, or the space between tho terminals across which tho current 

 from even a powerful magneto-electric machine will leap, is very small. With tho 

 large lever-magnet tho spark could only be produced when the wires were almost in 

 contact. Since, however, it is indispensable to the proper insulation of tho wires in 

 tho fuze-arrangement, that tho terminals should bo at least one-sixteenth of an inch 

 apart, it will be readily understood how essential to success, in operations with these 

 machines, it is that the priming material should possess considerable conducting 

 power. Hence the necessity of increasing the conducting power of the mixture of 

 sub-phosphide of copper and chlorate of potas.sa : a result which, it has been already 

 stated, was obtained in the first instance by the employment of finely-levigated coke, 

 and afterwards by the substitution of sub-sulphide of copper for that substance. 

 Many experiments were of course required to determine the proportions in which it 

 was advisable to employ the conducting constituent, so as to facilitate the passage 

 of the current through the mass as far as possible, without interfering too much with 

 the sensitiveness of the explosive mixture, or producing an almost perfectly continuous 

 connection between the two poles in the fuze, and thus promoting the passage of tho 

 current so greatly as to prevent the ignition of the composition. 



The fuzes contrived by me for use with magneto-electric apparatus are of two 

 kinds, the one being adapted for mining purposes and the other for firing cannon. 

 The fuze for mining purposes, fig. 783, consists of : 



a. A. head for receiving the wires which connect the fuze with the magnet and the 

 earth, fig. 784. 



b. Of the insulated wires, with the terminals of which the priming material is in 

 close contact. 



c. Of a small cartridge or charge of powder, enclosing the terminals, upon which 

 the sensitive composition rests. 



The wooden fuze-head contains three perforations (a a, bb, cc, fig. 784) ; the one, 



"83 



785 



784 



passing downwards through the centre, receives about two inches of double insulated 

 wire, d. The other two perforations, which are parallel to each other on each side of 

 the central one, and at right angles to it, serve for the reception of the circuit- wires. 

 The arrangement for securing the connection of these with tho insulated wires in tho 

 fuzes is as follows : 



The piece of double-covered wire above referred to, is originally of a sufficient 

 length to allow of the gutta-percha being removed from about 1 inch of the wires. 

 These bare ends of the fine wires, which are made to protrude from the top of tho 

 fuze-head , fig, 784, are then pressed into slight grooves in the wood provided for 

 their protection, and the extremity of each is passed into one of the horizontal per- 

 forations in the head, in which position it is afterwards fixed by tho introduction into 

 tho hole of a tightly-fitting piece of copper tube, so that tho wire is firmly wedged 

 betweeen the wood and the exterior of this tube, and is thus at the same time brought 

 into close contact with a comparatively largo surface of metal. It will be seen that it 

 is only necessary to fix one of the circuit-wires into each of those tubes in the opposite 

 sides of tho fuze-head, in order to ensure u sufficient and perfectly distinct connection 

 of each one of them with one of the insulated wires in the fuze. 



The phosphide-of-copper fuze for firing cannon, fig. 785, differs somewhat in con- 

 struction from the mining fuze. The head is somewhat longer, and of such a form 

 that the double-covered wires are completely enclosed in it, tho lower extremity of its 



