Extempore Fuzes.—It may be necessary in some cases, when a specially manufactured fuze is not attainable, to make a fuze on the spot. The following is a neat and simple method of constructing an extempore high tension fuze.
Fisher's Extempore Fuze.—This form of fuze was devised by Lieutenant now Captain Fisher, R.N. It consists of a small disc of gutta percha, through which the ends of two wires are inserted about 1/4 inch apart, their ends terminating in small copper plates formed by hammering down the wire. These flat ends should be fixed parallel, and in the first place in contact with one another, also should be level with the surface of the gutta percha. The other two extremities of the wires are then placed in circuit with a sensitive galvanometer and a test battery; the needle of the former deflects violently, there being a complete metallic circuit; the flat ends of the wires or poles of the fuze are then separated very carefully, until the needle just ceases to deflect. In the space thus formed, a little scraped charcoal is placed, and rammed in by a piece of wood. By the application of pressure, any degree of sensitiveness may be attained, merely observing the deflection of the galvanometer needle. Over the charcoal a little powdered resin is shaken, and pressed down, by which means the charcoal is fixed in position, and owing to the inflammability of the resin, the ignition of the gunpowder priming is ensured. The disc of gutta percha is then placed in an empty Snider ball cartridge, &c., and by the application of a little warm gutta percha applied externally, the holes where the projecting ends of the wires pass are closed, and the disc is fixed and insulated. The case is then filled with some mealed powder and fine grained powder, on the top of which is placed a little cotton wool, and the whole pressed down tightly with the finger, the open end of the case being then choked, as in Beardslee's fuze and Abel's extempore one. The apex is then covered with some warm gutta percha, and the whole of the fuze coated over with red sealing-wax dissolved in methylated spirits.
Insulated Electric Cables.—For the work of defence by electrical submarine mines, the wires along which the electric current flows have, on account of their being led underground and through the water, to be covered with some substance which shall prevent the current during its passage from escaping to earth, or in other words, they (the wires) must be insulated.
The substances in general use for such purposes are as follows:—
- 1.—Gutta percha.
- 2.—Ordinary india rubber.
- 3.—Hooper's material.
Gutta Percha.—This substance was used by Messrs. Siemens in the cables manufactured by them for the Austrian government in 1866, and is to some extent still employed, though Hooper's material or vulcanised india rubber, has been found to be more suitable. The dielectric, gutta percha, possesses the following advantages:—
a.—It can be put on the conducting wire, as an unbroken tube.
b.—It only absorbs 1 per cent. of water.
c.—It has the property of clinging to the metallic conductor, by which is meant, that should it (conductor) be cut through, and any strain be brought on the cable, there is a tendency on the part of the gutta percha to cling to the conducting wire, thereby not increasing the fault.
The defects of such an insulator are:—