5.—They can be raised for examination, or removed when no longer required, with ease and safety.
Such are some of the chief advantages of employing the agency of electricity to effect the ignition of the charge in a system of defence by submarine mines.
Defects of Electrical Submarine Mines.—The following are the chief defects connected with the use of electrical mines:—
1.—The number of wires that are required to be used with them.
2.—The necessity of employing specially trained men in their manipulation.
In time there seems little doubt but that the former obstacle will be to a considerable extent overcome, but the latter must always be a flaw in an otherwise perfect system of coast defence by submarine mines.
Rules to be observed in using Electrical Submarine Mines.—In connection with a system of electrical submarine mines the following rules should be carefully observed:—
1.—They should be moored in deep channels, that is to say, where the larger class of vessels would in attempting to force a passage be obliged to go.
Note.—Mechanical submarine mines should never be used under these circumstances, as the difficulties of mooring them and keeping them in position would be very considerable, also a vessel being sunk in a very deep channel would not necessarily block it, and as a mechanical mine cannot be replaced, a gap would be left in the defence.
2.—They should be placed in the narrowest parts of the channel.
Note.—The object of this rule is evident, fewer mines being required, and consequently in the case of electrical ones, a far lessnumber of wires are needed, which gives an increase of simplicity, and consequently more effectiveness. This point should be observed in connection with mechanical, as well as electrical submarine mines.
3.—They should where practicable be moored on the ground.
Note.—The advantages attendant on an observance of this rule are:—
a.—Increased vertical effect.
b.—Avoidance of mooring difficulties.
c.—Less liability of shifting from its original position.
d.—Less chance of its being discovered and rendered useless by an enemy.
e.—By far heavier charges may be conveniently employed.
4.—Where possible, no indication whatever should be given of the position of the mines by their circuit closers, or in the case of small buoyant ones, by the mines themselves.
Note.—In some instances this will be almost impracticable, as for example, where there is a very great rise and fall of tide. For instance, at Noel Bay in the Bay of Fundy, the rise is over fifty feet. Here, when circuit closers, or small buoyant mines are used, both of which ought never to be more than twenty feet below the surface, long before low water they would be found floating on the surface in full view. Many attempts have been made to overcome this difficulty, but as yet no really practicable means have been devised.
5.—The stations where the firing batteries, &c., are placed, should be in the defensive work likely to be held the longest, thus enabling the mines to be commanded up to the last moment.
6.—The electric cables should be laid in positions such that their discovery by the enemy would be extremely difficult, and almost impossible.
Note.—This may be to a certain extent effected by leading them from the mines to the firing and observing stations by circuitous routes, and by burying them in trenches.
7.—They should not be thrown away on boats.
Notes.—As they can in all cases be fired by will, even when circuit closers are used, this rule is easily observed. But to prevent an enemy's boats from rendering the mines useless, a line of small torpedoes might be placed in advance of the large ones, or the circuit closers themselves might be charged.
At night, or in foggy weather it will be necessary to employ guard-boats, electric lights, &c., to protect them against damage by an enemy's boats, &c.
In the foregoing pages of this chapter will be found the requirements and conditions essential to a perfect system of electrical submarine mines for the defence of a harbour, river, &c.; in the following pages a general description of the component parts of such defensive torpedoes, under the following heads—Form and Construction of Case; Electrical Fuzes; Electric Cables; Watertight Joints; Junction Boxes; and Mode of Mooring, will be considered.
Form and Construction of Torpedo Case.—The case of a submarine mine should be capable of fulfilling the following conditions:—
1. It must be able at great depths to withstand a great pressure of water, and remain perfectly watertight.
Note.—This in the case of a charge of gunpowder being an imperative necessity.
2. As a buoyant mine, it must be capable of affording a considerable excess of buoyancy, by which it may be rendered stationary when moored.
Note.—This is generally obtained by having an air space within the torpedo, thus requiring a much larger case in which the charge is enclosed than would otherwise be necessary, causing increased difficulties in transportation, mooring, and raising them for examination, &c.
3. When explosive agents which require a certain time for thorough combustion are used as the charge, such as gunpowder, picric powder, gun-cotton (not fired by detonation), &c., a much stronger case is necessary to obtain the full explosive effect than would be the case were detonated charges, under the same conditions, employed.
Note.—This is an extremely important point, for if a weak case is employed with a charge of gunpowder, &c., fired by a fuze primed with powder only, a portion of it on being fired would generate a sufficient quantity of gas to burst the case, thus blowing out the remainder of the charge before its ignition had been effected.
4. It should be of such a form that the complete ignition of the charge is obtained by the employment of the least number of fuzes possible to effect this result.
Note.—This point is especially to be observed when gunpowder is the explosive agent.
The various forms of defensive torpedo cases may be classed under the following heads:—
Spherical Shape.—This form of case is theoretically the very best one possible to devise, but on account of the difficulty of constructing it, and its comparative costliness, such a form may be put aside as being impracticable.
Cylindrical Shape.—Torpedoists in general have hitherto adopted the cylindrical form of case as being the best adaptable for both ground and buoyant mines containing a heavy charge.
The Confederates employed exclusively this shape for their electrical submarine mines, which were ground ones, and the Austrians in the war of "66" approved of this form of case for their electrical submarine mines, which were buoyant ones.Fig. 19and20represent respectively the American and Austrian mines.
In England the cylindrical shape has up to quite lately found most favour with her torpedoists for both buoyant and ground mines. AtFig. 21is represented a 100-lb. buoyant electrical mine, surrounded by a wooden jacket,e, and having its circuit closer,C, enclosed within it; and atFig. 22is shown a 250-lb. electrical mine, which may be used either as a buoyant or ground one.
For large ground mines, the best form of torpedo case seems to be that of the turtle mine, which is shown atFig. 9. A heavy charge may be contained in it; it forms its own anchor; and it would withstand an explosion of an adjacent mine without sustaining any injury. At present the cylindrical shape is the form generally used, though as far as retaining its position on the ground in a strong tide, it cannot be compared to the turtle form.
FORM OF CASE OF SUBMARINE MINES.Plate VI
FORM OF CASE OF SUBMARINE MINES.
The Conical Shape.—Hitherto this shape of submarine mine case was only used in connection with mechanical mines, but now it is the form considered most suitable for all buoyant mines, electrical or mechanical. AtFig. 23is shown the conical shaped mechanical mine, employed by the Confederates for use with sensitive fuzes. The conical form of torpedo case lately approved of by the English torpedoauthorities is somewhat similar to that one, the charge being contained in a kind of box hung from the top of the case, and the circuit closer is screwed into the bottom of the case; surrounding the upper part of the case is a thick buffer of wood, by which damage to the mine is prevented by the passage of friendly ships. This is altogether a very neat and serviceable form of torpedo case. This form of case is also more difficult to discover by dragging, and easier to retain in position.
Electrical Fuzes.—The fuzes employed in connection with electrical submarine mines may be divided into two classes:—
1. Platinum wire bridge fuzes.
Note.—That is where the evolution of heat is caused by a largequantityof the electric force flowing through a good conductor of large section, such as the copper core of electric cables, being suddenly checked by a very thin wire composed of a metal which compared with the conductor offers a very great resistance, such asplatinum.
2. High tension fuzes.
Note.—That is where the evolution of heat is caused by the electric spark, or by the electric discharge taking place through a substance which offers very great resistance to the passage of the electric force.
Platinum Wire Fuze.—This is the form of electrical fuze most commonly used, and which will most certainly supersede altogether the high tension fuze.
There are numerous advantages accruing from the use of platinum wire fuzes, the chief of which are here enumerated:—
a.—Great facilities for, and entire safety whilst testing the circuit.
b.—Extreme simplicity of manufacture.
c.—Non-liability to deteriorate.
d.—Perfect insulation of the electric cables used in connection with submarine mines not necessary.
English Service Platinum Wire Fuze.—The following is a description of the platinum wire fuze of the form adopted in the English service, a section of which is shown atFig. 24. It consists of a head of ebonitea, hollowed out, in which a metal mould is fixed, the wires which have been previously bared are inserted into holes in this mould, and firmly fixed thereto by means of a composition poured into the mould, whilst hot; this is shown atb. The two bared ends of the wires which projectbeyond the metal mould, asc,c, are connected by a bridge of platinum-silver wire ·0014" in diameter and weighing ·21 grs. per yard. This is effected as follows:—
A very fine shallow groove is made in the flat ends of the bare wiresc,c, and the platinum-silver wire is laid across in the incisions, and fixed there by means of solder. The length of the bridgedis ·25."
A tubee, made of tin, and soldered to a brass socketf, is fixed by means of cement to the ebonite heada; in this tube is placed the fulminate of mercury, the open end of the tubegbeing closed with a pellet of red lead and shellac varnish; around the bridge of the fuze is placed some loose gun-cotton.
McEvoy's Platinum Wire Fuze.—Another form of platinum wire fuze, which has been devised by Captain McEvoy, formerly of the Confederate Service, is shown atFig. 25. It consists of the heada, formed of a mixture of ground glass, or Portland cement, worked up with sulphur as a base: this mixture when hot is poured into a mould, in which the two insulated copper wires,b,b, have been previously placed; when cold, the mixture with the wires affixed is removed from the mould, and the platinum wire bridgecbeing secured to the bare ends of the copper wires, the whole is firmly fixed in a brass socketd, by means of cement; the spaceeis filled with loose dry gun-cotton, so as to surround the bridgec; a copper tubef, closed at one end, is partly filled with fulminate of mercury, and when the fuze is required for service, this tube is secured to the brass socketdby means of cement.
In this form of low tension fuze there is no liability whatever of any injury being caused to the bridge by the working of the wires in the head, or by damp even after lying in the water for a month or more. One peculiarity of this fuze is that the composition is run over the insulated wires without materially softening the dielectric, or affecting in the slightest degree the insulation of the wires.
High Tension Fuzes.—The high tension fuze was devised for use with electrical submarine mines, in the place of the platinum wire fuze, on account of the little knowledge possessed, in the early days of submarine warfare, in regard to the manipulation of Voltaic batteries.
Platinum wire requires a temperature of some 500° F. to heat it to incandescence, and therefore necessitates the use of a powerful Voltaic battery, both in intensity and power, to effect the ignition of gunpowder by this means at considerable distances.
The Grove and Bunsen pile were the only suitable form of Voltaic battery known at the period of the introduction of high tension fuzes, both of which possessed the defects of uncertainty and inconstancy, and also were by far too cumbersome and too difficult to keep in effective working order to be of any real practicable value.
High tension fuzes may be ignited by means of either an electro-magneto machine, an electro-dynamo machine, a frictional machine, or by a Voltaic battery, generating an electric current of high intensity. Various kinds of this form of electrical fuze have been designed, the principal of which are as follows:—
Statham's Fuze.—A section and elevation of this electric fuze are shown at Fig. 26;a,bis a gutta percha tube, with an opening cut in it, as shown in figure. The interior of this vulcanised gutta percha tube is coated with a thin layer of sulphide of copper, which coating is obtained by leaving a bare copper wire for some time in connection with the above-mentioned tube. The extremities of two insulated copper wiresc,c, considerably smaller than the conducting wires, are uncovered, scraped, and then inserted into the tubea,b, with an interval of ·15 inch between them. The wires are then bent as shown in the figure, and the priming placed between the terminals. The whole is covered with a gutta percha bag, which is filled with fine grained gunpowder. The priming substance is composed of fulminate of mercury worked up with gum water. The objection to this fuze, which was used by the Allies in their destruction of the Russian fortifications at Sebastopol, is the want of sensitiveness of sulphide of copper, and the consequent necessity of a very powerful firing battery.
Beardslee's Fuze.—This high tension fuze is shown atFig. 27. It consists of a cylindrical piece of soft wood a, which is about three-quarters of an inch in length and in diameter; two copper nails,b,b, are driven through this piece of wooda, in such a way that while the two heads come together as close as possible without absolutely touching, the pointed ends are some distance apart fromeach other, and project through the wooda; two insulated copper wires,c,c, are firmly soldered to these projecting ends, and a piece of soft wax,d, is pressed around the junction points. In a groove, across the heads of the copper nails, is placed a little black lead, to which is added a minute quantity of some substance, the nature of which is known only to Mr. Beardslee. Several folds of paper are wrapped round the wooden cylinder, forming a cylinder about 2-1/2 inches long, one end of which is tightly fastened round the insulated wires as ate. The other end of the cylinder is then filled with powder,f, and closed by a piece of twine. The whole fuze is then coated with black varnish. Though not highly sensitive, Beardslee's fuze is exceedingly efficient, and extremely simple.
Von Ebner's Fuze.—This form of fuze was devised by Colonel Von Ebner of the Austrian Engineers. A section and elevation of it is shown atFig. 28. It consists of an outer cylinder,a, of gutta percha, and an inner one of copper,b, which latter encloses a core formed of ground glass and sulphur,c, which core is cast round the two conducting wiresd,din such a way that they are completely insulated from one another. In the first instance the wire is in one continuous length, the openingebeing subsequently made, and carefully gauged, so as to ensure a uniform break, or interval in the conductor of each fuze. The priming composition, which consists of equal parts of sulphide of antimony and chlorate of potash, is placed in the hollowf, to which is added some powdered plumbago, for the purpose of increasing the conducting power of the composition. This mixture is put into the hollow,f, of the fuze under considerable pressure, the terminals being connected with a sensitive galvanometer, in circuit with a test battery, and the pressure applied so as to obtain, as far as possible, uniformity in the electrical resistance of each fuze.
The Austrians employed this form of high tension fuze in connection with a frictional machine for the electrical mines used in their defence of Venice, &c. during the war of 1866.
Abel's Fuze.—Mr. Abel devised a high tension fuze, which in 1858 was extensively experimented with; the Beardslee and Von Ebner fuze being based upon the principles applied for the first time in Abel's fuze.
ELECTRICAL FUZES.Plate VII
ELECTRICAL FUZES.
Many modifications of it have been from time to time devised by Mr. Abel; a section and elevation of the more recent form of his fuze is shown atFig. 29. It consists ofb,b, a body of beech wood, hollowedfor half its length, in which space the priming charge is placed; it is also perforated by three holes, one vertical for the reception of the capsule of sensitive mixture, the other two horizontal, in which the conducting wires are placed;a,aare two insulated copper wires, passing into the vertical hole, and resting on the sensitive mixture; in a cavity,d, of the body of the fuze is placed some mealed powder, which is fired by the ignition of the sensitive mixture on the passage of the electrical current.
The insulated wires used in connection with this fuze consist of two copper wires, about 2 inches long, and ·022 inch in diameter, enclosed in a covering of gutta percha ·13 inch in diameter, and separated about ·06 inch from each other.
At one end the wires are bared to 1·25 inch, at the other they are merely cut across by a very sharp pair of scissors. This end of the double covered wire is inserted into a paper cylinderc,c, which holds a small quantity of the priming mixture. This capped end of the wires is inserted into the wooden body of the fuze through the vertical holei, and projects ·15 inch into the cavityd. The bare ends of the double covered wires are pressed into small grooves in the head of the cylindere e, and each extremity is bent into one of the small channelsd' d', which are at right angles to the vertical perforation.d' d'are two small copper tubes driven into these channels over the wire ends, to keep the wires in position, and to form the opening into which the conducting wiresfare inserted and bent round, as ate'.
The priming mixture of Abel's original fuze, which was the one used by the Confederates, was composed of 10 parts of subphosphide of copper, 45 parts of subsulphide of copper, and 15 parts of chlorate of potash. These ingredients reduced to a very fine state of division, and intimately mixed, in a mortar, with the addition of a little alcohol, are dried at a low temperature and preserved in bottles until required for use. The sensitive mixture used by Mr. Abel more recently for his submarine electrical high tension fuzes, is composed of an intimate mixture of graphite and fulminate of mercury. By the process of ramming, the electrical resistance of the fuze is regulated.
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 Lieutenantnow 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:—
Gutta Percha.—This substance was used by Messrs. Siemens in thecables 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:—
a.—Its liability to become hard and brittle when exposed to dry heat, and consequently it requires to be stored under water.
b.—It becomes comparatively a bad dielectric at 100° F.
c.—It becomes plastic at high temperatures, which causes the conducting wire to alter its position.
In some particulars ordinary india rubber is a better insulator than gutta percha, but this substance is equally inferior to Hooper's material, &c. The advantages possessed by this substance are:—
a.—It is not easily affected by a dry heat.
b.—It is a very excellent dielectric.
The defects of this mode of insulation are:—
a.—It must be put on the conducting wires in a series of jointed pieces.
b.—It does not cling to the conducting wire, so that if the electric cable be cut, and any strain be brought on it (cable), the previous fault is increased.
c.—It absorbs 25 per cent. of water.
Hooper's Material.—This insulating material consists of an inside coating of pure india rubber, then another similar coating in conjunction with oxide of zinc, which is termed the separator, and an outside coating of india rubber combined with sulphur. The use of the separator is to prevent any damage to the conducting wires by the action of the sulphur. The three coatings are then baked for some hours at a very high temperature, which fuses the whole into a solid mass, and vulcanises the outer coating. The properties of the pure india rubber which is in contact with the metallic conductor are thuspreserved, while any decay of the outer covering is prevented by the vulcanising process.
The advantages claimed by Mr. Hooper for this mode of insulating electric submarine cables, are:—
a.—High insulation.
b.—Flexibility.
c.—Capability of withstanding the bad effects of dry heat.
The qualifications essential to a perfect insulated electrical cable for use with submarine mines are as follows:—
1.—Capacity to bear a certain amount of strain without breaking.
2.—Perfect insulation, or at least as nearly so as it is possible to obtain, and composed of a substance capable of being readily stored, and kept for a considerable length of time without being injured.
3.—Pliability so that it may be wound on, or paid out from, a moderately sized drum without injury.
4.—Provided with an external covering capable of protecting the dielectric from injury when used in situations where there is a rocky or shingly bottom, &c.
The insulated wire of a submarine cable is technically spoken of as itscore.
By acableis meant to be understood any piece of covered wire.
Several forms of submarine electrical cables have been devised, all of which more or less possess the qualifications enumerated above. The following are some of the most effective:—
Siemens's Cable.—This form of cable is represented atFig. 30. It consists of a stranda, which is composed of three or more copper wires formed by laying up the several single copper wires spirally, several layers of gutta percha, or india rubber,b, two coverings of hemp, saturated with Stockholm tar,candd, and several plies of copper tapee, wound on, so that each strip overlaps the preceding one, as shown atFig. 30. The conductivity of the copper employed for the strand is equal to at least 90 per cent. of that of pure copper.
This exterior covering of copper tape is a patent of Messrs. SiemensBrothers, and when once laid down, the cable so covered is very efficiently protected, and of course it is little affected by the action of the sea water. This mode of protection has one great defect, viz., that in the event of a kink occurring in paying out the line, and at the same time a sharp strain being applied, the copper tape at that point is extremely likely to destroy the insulation by being drawn in such a way as to cut through the dielectric. On this account great care must be observed in handling this form of cable.
In practice precautions must be taken to prevent the copper tape covering from being brought into contact with any iron, for were such to happen, electrical action would at once ensue, causing the iron to corrode with enormous rapidity.
In some of Siemens's cables, vulcanised india rubber replaces the gutta percha insulation. Iron covered cables, either galvanised or plain, are manufactured as well as the copper tape covered ones by that firm.
Hooper's Cable.—This form of cable is represented atFig. 31. It consists of a metal conducting wire, generally copper,a, covered with an alloy to protect it from chemical action, the insulating substanceb, known as Hooper's material, previously described atpage 39, a covering of tarred hempc, and an outer covering of iron wires (No. 11 B. W. G.), each of which is separately covered with tarred hemp and wound on spirally,d.
Gray's cable is very similar to the one just described, the chief difference in it as compared with Hooper's being the absence of the separator.
Silvertown Cables.—The following is a description of the core of an electrical submarine cable, which is used by the English government, and is supposed to contain all the advantages of the foregoing, and none of their defects. It consists of a strand conductor of four copper wires (No. 20 B. W. G.) of quality not less than 92 per cent. of pure copper, and possessing an electrical resistance of not more than 14 ohms per nautical mile. This strand is tinned and insulated with vulcanised india rubber to a diameter of ·24 inch, and then covered with a layer of felt, and the whole subjected to a temperature of 300° F. under steam pressure. This forms the core of the various kinds of cables employed in connection with a system of defence by electrical submarine mines, which are enumerated as follows:—
1.—Single core armoured cable.
2.—Multiple cable.
3.—Circuit closer cable.
4.—Single core unarmoured cable.
5.—Special cables for firing by cross bearings.
Single Core Armoured Cable.—This form of cable is used in connection with each mine of a group or system, and also to connect forts, &c. across an arm of the sea. Over the core, which has been fully described, is laid a spiral covering of tanned, picked Russian hemp, over this are laid ten galvanised iron wires (No. 13 B. W. G.), each one of which is covered with a similar hemp, which is laid in an opposite spiral to the former similar covering, with a twist of one revolution in about thirteen inches; in order to prevent these wires from gaping when the cable is kinked, a further covering of two servings of hemp passed spirally in opposite directions is laid, and the whole passed through a hot composition of a tar and pitch mixture. Exterior diameter of this cable is 7/8 inch. Its weight in air is 27-50/112 cwt., and in water 14-40/112 cwt. per nautical mile. The breaking strain of a cable thus manufactured is 62-1/2 cwt., and its cost about £47 per nautical mile. A diagram of this cable is shown atFig. 32.
Multiple Cable.—This form of cable is employed in cases where it is necessary to carry a large number of cables into the firing station, &c. It consists of seven single cores formed into a strand, over which a padding of hemp fibres is laid longitudinally, and over this again is laid an armouring of sixteen (No. 9 B. W. G.) galvanised iron wires, each one of which is covered with a layer of tarred tape put on spirally with a twist of one revolution in 15 inches. The exterior covering consists of two layers of hemp and composition, which is laid on with a short twist, and in opposite directions. The external diameter of this cable is 1-1/4 inch. Its weight in air and water is 78-25/112 cwt., and 45-32/112 cwt. respectively per nautical mile. Its breaking strain is 135 cwt., and cost about £357 per nautical mile. This form of cable is used in connection with a junction box, from which the single armoured cables leading to the different mines radiate, and is shown atFig. 33.
Circuit Closer Cable.—This cable, which connects the mine and circuit closer, has been found to be subjected to exceptional wear and tear, and therefore requires a special form of exterior protection. The core of this cable is the same as the one described atpage 41, also it iscovered with a similar padding of hemp, but instead of the iron wires as in the case of the multiple cable, &c., nine strands, each of which is composed of fourteen No. 22 Bessemer Steel Wires, are wound on, each such strand being covered with hemp, which is put on with a twist of one revolution in every 7-1/2 inches, the external covering being the same as in other cables.
This form of armouring for an electric cable possesses the qualifications of pliability, lightness, and great tensile strength. Its weight in air is 52-106/112 cwt., and in water 28-4/112 cwt. per nautical mile. Its breaking strain 65 cwt., and cost about £127 per nautical mile.
Single Core Unarmoured Cable.—This form of cable is used in a system of defence by submarine mines to connect the detached works of a maritime fortress, &c., for the purpose of telegraphing.
It consists of the ordinary service core, over which are laid two servings of tarred hemp, put on spirally. The weight of this cable in air is 4-13/112 cwt., and in water 1-36/112 cwt. per nautical mile; its breaking strain is 7-1/2 cwt., and its cost per nautical mile is about £35.
Special Cables.—In firing electrical submarine mines by means of cross bearings, a special cable is employed. As a general rule there would be three lines of mines placed to converge on one of the stations.
Each of these lines would be provided with a conducting wire in connection with the firing arrangements, while one line of wire in connection with the firing station would be required for telegraphing. For the purpose in question a four cored cable is used.
Land Service Cable.—The cable employed for this service consists of a core formed similar to that of the multiple cable, described atpage 41; over which is laid a padding of hemp, and finally two servings of tarred hemp laid spirally in opposite directions are wound on. Its weight in air is 16 cwt., and in water 4-50/112 cwt. per nautical mile. Its breaking strain 17-1/2 cwt., and cost per nautical mile about £137.
Sea Service Cable.—This consists of a similar core to the land service cable, and padding of hemp, over which is laid an armouring of fifteen No. 13 galvanised iron wires, each one being covered with tarred tape, and finally the ordinary servings of tarred hemp. Its weight in air is 49-101/112 cwt., and in water 25-109/112 cwt. per nautical mile. Its breaking strain 65-100/112 cwt., and cost per nautical mile about £202.
When frictional electricity is used to fire high tension fuzes, it hasbeen found by experiment that if several lines of insulated cables are laid in the same trench for a few hundred yards, the inductive effect of the electrical charge generated by a frictional machine is so great that its discharge through one cable is sufficient not only to fire the fuze in immediate connection with it, but by induction every other fuze in connection with the remaining wires laid in the trench. And this effect equally occurs when the electric cables are some feet apart, provided they run parallel for a few hundred yards, and whether the shore ends of the cables, the fuzes in connection with which are not intended to be fired, are insulated, or put directly to earth, the connections beyond the fuzes being to earth, or even insulated, provided a very few yards of conductor exist beyond the fuze.
The length of wire which it is necessary to use between the mine and its circuit closer would be quite sufficient for the purpose of effecting ignition by induction. With platinum wire fuses there is no danger whatever of the above happening, nor in the case of high tension fuzes is there so much danger of ignition by induction, when a constant instead of a frictional electric battery is used to generate the current.
Another mode of protecting an insulated cable is to place it, as it were, in the core of a hempen cable. In forming the rope on the cable, great care is necessary to prevent any serious amount of torsion, or tension coming on the insulated wire, either of which would most assuredly result in injury to the cable. This form of cable might in connection with obstructions, &c., be of great use, as on account of its closely resembling an ordinary rope, it would be very unlikely to excite suspicion, and so would most probably be cut, the result of which, by previous arrangement, would be an explosion of a mine, or by means of a galvanometer, &c., an indication that the obstructions, &c., were being interfered with.
Jointing Electrical Cables.—This is a very important point in connection with a system of defence or offence by electrical torpedoes. In many instances it will be found necessary to join either two lengths of cable, or an insulated wire and a cable, together, in both of which cases, great care must be used in making the joints, so that the insulation and the continuity of the circuit may be perfect.
ELECTRIC CABLES, EXTEMPORE CABLE JOINTS.Plate VIII
ELECTRIC CABLES, EXTEMPORE CABLE JOINTS.
Many species of junctions have been from time to time devised, the most practical and generally employed of which are:—
India rubber Tube Joint.—This form of joint is a very useful one for extempore purposes, being easily and quickly made, and being very effective. AtFig. 34is shown a sketch of such a junction. About 1·5 inches of the copper conductor of the two insulated cables are laid bare and connected together by means of Nicoll's metallic joint, as shown atFig. 36, or by turning one of the conductors round the other, their ends being carefully pressed down by means of pliers, to prevent any chance of the india rubber tube being pierced; over the splice thus formed serve some twine, and over the whole put a coating of india rubber cement, grease, &c., then draw the vulcanised india rubber tube, which has been previously placed on one of the insulated cables, over the splicea, as shown atb, and secure it firmly by means of twine,c,c, and then to prevent any strain being brought on the joint, form a half-crown as shown inFig. 35atA.
In forming the splice, it is very important that the metallic ends should be perfectly clean. The danger to this mode of jointing of the piercing of the tube by the ends of the conductors is entirely removed by employing the Nicoll metallic joint, which is formed as follows:—
Nicoll Metallic Joint.—One of the conducting wires, asa,Fig. 36, is formed into a spiral twist by means of a very simple instrument, and the other wireb, which is left straight, is inserted into the spiral, the whole being placed on an anvil, and pressed closely and securely together by a single blow of a hammer.
Mathieson's Joint.—This somewhat complicated, though very effective mode of jointing, which is adopted in the English torpedo service, is shown atFig. 37, in elevation and section. It consists of two ebonite cylindersa,a, through which the cables to be connected are passed. Within these cylinders an ebonite tubeb,bis placed, the ends of which are wedge-shaped, and which press against two vulcanite ringsc,c; in the interior of this tubeb,bis the metallic jointdof the two cables. The centre of the tubeb,bis of square section, and fits into a hollow of similar form in the cylindersa,a, the object of this being toprevent any twisting of the wires during the process of screwing up, which would be liable to injure the metallic jointd.
The manner of making this joint will be easily understood from the figure. With this, as with all other temporary joints, it is advisable to form a half-crown in the cable, including the joint.
Beardslee's Joint.—This form of temporary joint when used with strand conductors, which are composed of a number of small wires, has been found to be exceedingly useful and effective, the only defect of such a joint being the liability of straightening the wires of the conductors should a direct strain be brought upon the wire extremities.Fig. 38represents a section of this joint; it consists of an ebonite cylindera, one end of which is solid, and the other open and fitted with a screw thread, into which is screwed a plugb; through both the plugb, and the solid end of the cylindera, perforations are made just large enough to admit the insulated wiresc,c; about half an inch of the extremities of these wires are bared and cleaned, and then passed, the one through the plugb, a disc of vulcanised india rubberd, and a metal disce, and the end of the strand conductor turned back on the face of this metal disc, the other through the perforation in the solid end of the cylindera, then through similar discsdande, and the end of the strand conductor treated in the same manner as the former one; then by means of the screw plugb, the two metallic discsb,b, and consequently the bare extremities of the strand conductors are brought into close metallic contact.
McEvoy's Joint for Iron Wire covered Cable.—This form of joint is shown in section atFig. 39. Two brass capsa,aare slipped over the ends of the cables required to be joined, then the iron wire and other coverings of the cables down to the insulating substance are removed, the former being bent back close against the bottom of the capsa,a, as shown inFig. 39atb,b; the cores of the cables are then joined by an india rubber temporary jointc, which has been described atpage 45: the whole is then placed in the body of the joint, and the brass capsa,ascrewed up, jamming the bent back iron wires against a solid piece of brassd,d, by which means a firm and perfect joint is made in the cables.