Engineering Wonders of the World
Volume III

358 ENGINEERING WONDERS OF THE WORLD. Fig. 2 .—STRANDING MACHINE. From C the wire is threaded through the die - plate G, where it is enveloped by the outer wires. The latter are worked on. bobbins, D, mounted on a horizontal turn-table revolving with the shaft C. These wires are conveyed from their indi- vidual bobbins through the two dies F and G in turn, where they meet tho centre wire, and are laid round it in more or less elongated spirals. The number of these bobbins obviously depends on the number of outer wires composing the strand. The so stranded wire is con- veyed by means of a pulley to a measuring drum, and thence on to a carrying reel, which, when fully loaded, is taken off the machine and replaced by another. from a steam or other available engine, the wire being stranded up in about 2-mile lengths, as a rule.* this country in crude lumps, which are thereupon subjected to a series of cleans- ing processes before application round the conducting wire. A highly satisfactory machine, devised by the late Mr. Matthew Gray, for applying the purified gutta-percha, is depicted in Fig. 3. With this apparatus several wires may be covered at once. They are hauled off their respective hanks through the die-box, con- taining dies in accordance with the thickness of the coating required, and thence through a long trough of intensely cold water so as to render the gutta-percha thoroughly hard be- fore reaching the collecting drum. The exact thickness of this insulating cover is, like the conductor, governed by electrical considera- tions for obtaining the required speed of sig- nailing through a given length.* It is also governed by mechanical considerations, a con- ductor of a certain size involving a thickness Water being a good conductor of electricity, the copper wire has to be covered with some substance which is a bad conducting or insulating The medium, to prevent much Dielectric. . , ... . of the transmitted cur- rent leaking to earth, instead of going to the farther end of the line. Gutta- percha is found to be peculiarly well adapted to the purpose, its insulating qualities improving immensely under the pressure and low temperature of ocean depths, f Gutta-percha is obtained from certain sapotaceous, wild - growing East Indian trees, from which it exudes when an incision is made in the bark. It arrives in * Full particulars regarding this process may be found in “Submarine Telegraphs: Their History, Construction, and Working,” by Charles Bright, F.R.S.E., A.M.Inst.C.K, M.I.Mech.E., M.I.E.E. London: Crosby Lockwood and Son. f India-rubber (somewhat similar as a gum) is occasionally adopted for certain tropical waters invaded by the teredo and such, other “ objects of the deep ” as have a penchant for the comparatively cheese-like gutta-percha. of insulation in proportion to that size in Fig. 3.—GUTTA-PERCHA COVERING MACHINE. Tho gum, placed between the upper sides of the two rollers D D. is drawn down between them in a thin sheet, and forced along to a die-box, B, by the Archimedean screw A. The entire machine is steam-heated — so as to keep the gutta - percha in a plastic condition—and is driven by steam or other available power. order to avoid buckling through due to great rigidity. This thickness may be anything from -065 to '139 of an inch, according to the length and required speed. The diameter of an ordinary insulated wire for submarine * Full details in regard to this are given in the author’s lecture to the Royal United Service Institution of April 17, 1907, as well as in “ Submarine Telegraphs.”