Fig. 40 represents a section of a McEvoy temporary joint for single cored unarmoured cables, which seems to fulfil all the conditions necessary to a perfect joint of that description. This joint is, with the exception of there being two screw plugs instead of one, very similar to Beardslee's joint described at page 46; this alteration is a great improvement, remedying as it does the one defect of Beardslee's joint, viz., the liability of the cables to be drawn apart due to any great tension being brought on them.
A permanent joint in electrical submarine cables, which from its nature requires to be an exceptionally good one, is a somewhat difficult and troublesome operation, and also requires a considerable time to form a thoroughly reliable one.
Siemens's Methods of Jointing.—The following methods, and instructions for forming such joints, are those adopted by Messrs. Siemens Brothers in connection with their telegraph cables, and which will be found generally applicable to all insulated cables.
The Formation of a Joint in the Conductor of an Insulated Cable.—The conductor is either covered with a gutta percha or an india rubber dielectric. In both cases cut off the dielectric so as to bare the conductor-wire for a length of about three inches, taking care never to cut at right angles to the conductor-wire, for fear of injuring it with the cutting-knife or scissors.
Then clean the wires forming the strand with file-card and emery-paper, and solder them into a solid bar for a length of about one inch.
Having soldered the wires, forming the ends of the two lengths of conductors to be joined, into two solid rods, file each of them off in a slanting manner, so that they will form a scarf-joint when put together.
Place the two ends of strand in the two small vices on a stand which is supplied for the purpose, so that the two scarfed ends overlap each other, and bind them round with a piece of fine black iron wire, in the shape of a spiral, so as to keep the ends close together, then solder the two ends together by applying a hot soldering iron.
Then remove the iron binding wire and clean up the joint, filing off all unnecessary solder.
And make a band of four fine tinned copper wires, and bind them tightly side by side round the joint, covering the whole length of the scarf, and then solder the band and joint solidly together.
Then make another band of four fine tinned copper wires and bind them round the joint in the same manner as before, but extending about a quarter of an inch beyond each end of the other binding wire, the parts only of this second binding which project beyond the end of the first binding are to be soldered, so that the centre part remains loose and may keep up a connection between the two ends by forming a spiral between them in the event of the scarf giving way and the two ends of the conductor separating slightly.
This form of joint is called the "spring" joint.
The finished joint should be washed with spirit of wine and brushed, so as to take away all particles of soldering flux, and to avoid oxidation of the wire. The washed joint should then be dried with a piece of cloth and exposed to the flame of a spirit lamp to dry it thoroughly. A cable conductor ought never to be jointed with the help of soldering acid, but with that of resin, sal ammoniac, or borax only, so that any chance oxidation, and consequently destruction, of the conducting wire may be avoided.
There are other modes of jointing conductors, such as the twisting and scale joint, but the foregoing method will sufficiently explain this part of electric cable work.
The Formation of a Joint in an India rubber Insulated Cable.—In making a joint in any insulated cable, the very greatest care must be taken to keep the hands, tools, and materials clean and dry.
Remove the felt for about twelve inches from each end of the core by soaking it with mineral naphtha and then rubbing it off clean with the file-card. The cleaned surface sear with a red-hot iron, to burn off all remaining fibres of the felt. Wash these seared ends clean with naphtha.
Then cut off about four inches of the insulating material (taking care never to cut at right angles to the conducting wire for fear of injuring it) so as to leave enough of the conductor bare to join and solder in the manner described at page 47.
After the conductor is jointed and soldered, clean again the seared parts of the insulator with the glazed side of the squares of cloth moistened with mineral naphtha, so as to leave a clean adhesiveness only; taper again the insulating material down to the conductor for about two inches on each side of the conductor-joint with a pair of curved and very clean scissors.
The tapering must be completed in such slanting way that the different layers of the dielectric are so far exposed as to enable a secure laying on of the new jointing material.
India rubber core consists chiefly of three layers of insulating material: the first layer next to the strand is called the pure or brown; the second layer is the white or separating; the third layer is the light red or jacket rubber.
Coat the conductor with a pure (brown) rubber tape tightly laid on in a spiral form, commencing at the spot where the separator (white) ends, across the corresponding place on the opposite side of the joint and back again in a contrary direction. The ends are fastened down by pressing a clean, heated searing-iron or a heated knife on them. By doing so the band will stick; the remaining portions of the band to be cut off with the scissors.
Lay on tightly the separating india rubber tape in the same manner, but beginning where the jacket or outer layer of rubber ends. One lap will be sufficient.
Complete the insulation by lapping on tightly two layers of red india rubber tape: the last lap must cover each end of the core to four inches on each side of the conductor-joint, or extend to the searing or tackiness, but not beyond it.
Lay on three tight bindings of the cloth tapes, all in the same direction, care being taken to avoid wrinkles. The ends of the cloth tapes are cemented down with a thin coating of india rubber cement.
Immerse the joint in the jointing-bath at 150° to 200° F. and gradually raise the heat so that in half an hour the temperature will be 320° F., at which temperature keep the joint for twenty minutes: then take it out and let it cool in the open air.
The Formation of a Joint in a Gutta percha Insulated Cable.—Having jointed the conducting wires in the manner described at page 47, clean and dry the joint well and cover the bare conductor with a thin layer of compound. This is best done by heating a small stick of compound to nearly its melting point, and rubbing it over the bare conductor, which has been previously heated with the flame of a spirit-lamp.
Heat the gutta percha covering of both ends gently until it is quite soft, without, however, causing it to bubble or burn. Draw, then, with the fingers, the gutta percha coverings of both ends down, tapering them off until they meet in the middle of the joint; heat them sufficiently to make them adhere together.
Apply a layer of compound on the tapered-off gutta percha in the same manner as described for coating the bare conductor, and cover it with a first coating of gutta percha sheet to about half the thickness necessary to finish the joint. This is done by heating a small sheet of gutta percha, of about one-eighth of an inch in thickness, until it is quite soft, and by pressing it in that state round the joint to the required size; the greatest care to be taken to expel all the air.
The projecting lips are then cut off with a pair of curved scissors. The seam thus produced is to be rubbed with a hot iron until it is completely closed and the joint well rounded off.
Apply another layer of compound and a second layer of gutta percha in exactly the same manner as described for the first layer; care, however, is to be taken to get the seam in this second layer of gutta percha not over, but as nearly as possible right opposite to, the seam in the layer underneath.
The whole to be worked as cylindrical as possible, and to a size not exceeding the original core. The joint, so far finished, is then to be cooled with water until the gutta percha is quite consolidated.
Another, the overlapping gutta percha joint, is made in the following manner:—
Cut off the two ends of the core, so that the gutta percha and the conductor-wire are flush. Warm the gutta percha for a distance of about three inches from each of the ends with the flame of a spirit lamp, and, when sufficiently soft, push it back until it forms an enlargement. The two ends of the conductor are then to be soldered according to instructions for making joint in conductors.
To have a perfectly clean surface of the two gutta percha enlargements, remove all impurities by the way of peeling them with a sharp knife. Warm gently both knobs and the copper joint, and cover the whole length of the bare wire with compound, planing it with a warm smoothing-iron.
Draw then with the fingers one of the warmed and softened knobs carefully up to the other knob or enlargement, leaving on its way a perfect tube of gutta percha upon the wire, decreasing gradually to the thickness of the copper strand towards the other knob. Any superfluous gutta percha is removed. This scarf is finished with a warm smoothing-iron, so as to unite it to the compound on the wire strand, and a thin layer of compound is also put over the scarf in the same manner as before.
The other knob is then warmed and drawn in the same way over the tube already formed, which is at the same time heated sufficiently to make the two adhere.
Apply a layer of compound on the second scarf of gutta percha, covering it in the same manner as described for coating the bare conductor, and cover it with a small sheet of gutta percha in the same manner as described above, so as to make the finished joint to the size of the core as manufactured.
Rules to be observed in forming Joints.—The following rules must be carefully observed in forming either a temporary or permanent joint:—
Great care is absolutely necessary in making junctions, as they are the principal sources of defect in the insulation of electrical submarine cables.
Junction Boxes.—When it is necessary to employ a multiple cable, a junction box is used to facilitate the connection of the several separate wires diverging from the extremities of such a cable. In one angle of such a box the multiple cable is introduced, while the separate cables make their exit on the opposite sides and pass to the different mines. Different views of a junction box are shown at Fig. 41, where A is a plan of the top or lid, B a plan of the bottom, with the lid off, C an elevation, and D a section of the box.
The manner of using the junction box is as follows:—
The multiple cable is put in at a, and secured there by means of a nipping hook, shown at Fig. 42, which hook passes through the bottom of the junction and is made secure by means of a nut. The single core cables radiating from the junction box pass through the openings b, b, b on the sides, and angle opposite to where the multiple cable a enters. Each multiple cable is composed of seven cores, and each of these is connected by means of joints with the mine cables within the junction box, and each of these seven cables is secured by means of a nipper similar to, but smaller than, the one shown at Fig. 42, which are also secured by means of nuts, as in the case of the multiple cable nipping hook. When all the connections are made, the lid A is placed so as to rest on the studs c, c, c, and firmly secured by a bolt d, which is made water-tight by means of a washer and nut.
By means of the nipping hooks, which take any strain that may be brought on the cables, the connections within the box are ensured against injury by such a cause.
To enable the whole to be lifted together for the purposes of examination of the cables, &c., a buoyed rope is connected to the eye-bolt e. For this service a dummy circuit closer is the best form of buoy, it having great buoyancy and resembling in appearance an active circuit closer.
A junction box should be placed in such a position as to be easily attained, even in the presence of an enemy, and its buoy should, if possible, not be seen. It is also very essential that it should be in a safe and guarded position, for any injury to the junction box or multiple cable would be fatal to the group of mines in connection.
In the following cases, special junction boxes are used:—
Junction Box for Multiple Cables.—At Fig. 43 is represented a plan of lower half of this form of junction box. It consists of a pair of cast iron plates of precisely similar form to the one shown at Fig. 43, and so made as to be capable of being fastened tightly together by means of four bolts and nuts passing through the holes a, a. The grooves b, b at the two extremities are just large enough to grip the armoured cable firmly, when the upper and lower parts are screwed together. A larger space is provided in the hollow for the joint.
Junction Box for Single Cored Cables.—For this purpose a junction box similar to, but smaller than the one above described is employed.
T Junction Box.—This form of junction box is employed when the system of electrical contact mines on branches from a single cable is used. This system is dependent on the use of a platinum wire fuze in connection with a platinum wire bridge in each branch close to its junction with the main cable.
This form of junction box, which is shown at Fig. 44 is very similar to the one used for the connection of two multiple cables, only differing in its shape, which is that of a T. a is a disconnector, which will be described further on; b, b, b' are the armoured electric cables, b, b being the main, and b' the branch cable in connection with the forked joint formed within the T junction box; c, c, c are Turk's heads formed to prevent any strain being brought on the forked joint. This form of Turk's head is made by turning back the wires of the cable armouring, and frapping them round with spun yarn until the necessary size and shape is attained.
McEvoy's Turk's Head.—Another form of Turk's head, devised by Captain McEvoy, is shown at Fig. 45. It consists of two separate pieces of brass, a and b, the former screwing over the latter. The mode of using it is as follows:—
Slip the piece of brass b over the cable c, and turn back the wires of the cable d, d, &c., so that they lie against the shoulder of the brass piece b, then slip the other piece of brass a over the cable and screw it on the piece b, firmly jamming the turned back wires d, d, &c. This is a very neat and quick method of forming a Turk's head, and it should be invariably used in preference to the foregoing method, which is clumsy, and which takes some time to form.
The section of a disconnector is shown at Fig. 46. It consists of an iron cover, or dome a, which is provided with a screw fitting on to another screw on the ebonite body b of the apparatus. When the dome a is screwed tightly down on the washer i, the whole is made perfectly watertight. c, c are insulated terminals for connecting the cores of the branch and main cables after their armouring has been removed, as shown at Fig. 44. d, d are two copper conducting wires (No. 16 B. W. G.) passing through the centre of the ebonite body b, and projecting into the interior of the apparatus. These wires are held in position and insulated by means of a composition formed of a mixture of pitch, tallow, beeswax and gutta percha. This composition is put on whilst hot and allowed to cool gradually, when it becomes hard and durable. Great care is necessary to ensure the cavity within the ebonite body b being completely filled, as otherwise a leakage might occur, owing to the great pressure of water at depths where the disconnection would be generally used. f is a boxwood cover which is slipped on, and fits fairly tight to the ebonite body b; g is a piece of thin platinum wire, weighing 1·6 grains to the yard, and being 4/10 inch in length; h is an ebonite pin, which passes through two small holes in the boxwood cover f, into which it fits tightly, and in such a position as to be directly beneath the platinum wire bridge g, when the boxwood cover f is fixed on. The pin h is pushed through the holes in the cover f from the outside, so as to pass beneath the bridge g after the priming has been inserted, and the cover has been placed on.
When prepared for use, the platinum wire bridge g is surrounded by some loose gun-cotton priming, sufficient in quantity to blow off the boxwood cover f, without destroying the dome a; the cover f being blown off, carries the ebonite pin h with it, and through the platinum wire bridge g, thereby rupturing it, and breaking the continuity of the circuit. The object of so doing is to cut off the connection of an exploded mine, so that the full amount of the firing current is available for the other mines, and not suffered to be wasted by passing through the exposed wire of the broken circuit, which, were the disconnector not employed, would be the case.
When any particular mine of a system is struck, the current passes through the main cable b, the disconnector a (which is in connection with that mine), and branch cable b' to the fuze, and so explodes the mine, and destroys the platinum wire bridge g of the disconnector at practically the same instant. The effect of the latter operation would be to cut off and insulate the branch cable of the exploded mine, and so prevent any loss of the electrical current, when another mine of that system is required to be fired.
The platinum wire bridge g is 4/10 inch long, while that of the fuze is 3/10 inch, the object of this difference in length of the bridges being to ensure the former one g being fired, and thus the insulation made doubly sure. Many other forms of disconnectors have been devised, but none have proved in practice so effective as the one just described.
Mooring Electrical Submarine Mines.—This is one of the most difficult problems to be solved in connection with a system of submarine mines. The objects to be attained in mooring are as follows:—
Note.—From the comparatively small radius of destructive effect, of even heavily charged submarine mines, it will be understood how absolutely essential, in the case of mines fired by judgment, it is that this object should be attained.
The difficulties attendant upon the efficient mooring of submarine mines are immense, as will be understood when the action of gales of wind, and strong tides, which latter vary continually in their direction and in their rise and fall, are taken into consideration.
The foregoing remarks apply more particularly to a system of buoyant submarine mines, as those placed on the ground are comparatively easy to moor.
Several modes of mooring buoyant submarine mines have been suggested, the most practicable of which are as follows:—
Ladder Mooring.—This is a method of mooring, which in places where it may be necessary to place the anchors far apart will be found useful.
The circuit closer is connected to the mine by two ropes which lead thence to two anchors, the ropes being separated by wooden rounds, or spreaders, 1 to 3 feet long, by which the tendency to twisting is prevented.
The anchors are placed some 12 feet apart.
The only defect of the ladder mooring is the quantity of sea-weed, &c., that is liable to be lodged on the rounds, thus causing the circuit closer to be drawn out of its proper position.
Fore and aft Mooring.—This mode may be advantageously employed in a tideway where the current runs very strong, that is to say, five knots per hour, or more. It consists simply of two anchors, one of which is moored up, and the other down the stream.
Austrian Method of Mooring.—This method of mooring, adopted by the Austrians during the war of 1866, is shown at Fig. 47. It consists of a wooden triangular platform on which several heavy weights a, a, a are placed; the mine is attached to this platform by means of three wire ropes b, b, b, connected to the angles of the latter, and fastened to three chains, which by means of a catch holds the mine at the position required.
This catch consists of a pulley attached to the extremity of the wire rope of the platform, through which the mooring chain of the mine is passed, and fastened by a key at the required depth by means of a self-acting arrangement.
This key, which is of considerable weight, slips down as the mine is being hauled into position, but the moment the chain is slacked, two arms catch into a link of the chain, and so hold the mine in position. The weight of such a key is about 60 lbs. It is fitted with nuts, &c., to enable it to be taken to pieces.
This plan of mooring proved very effective in the harbours of the Adriatic, where there is hardly any tide or current to twist the mooring ropes, or otherwise disturb the mines. The Austrians have lately adopted the mushroom sinker in place of the wooden platform and weights, for their anchor.
Single Rope Mooring.—This simple method of mooring has after numerous exhaustive experiments been adopted as the most practicable and effective of all others. Whenever possible, a wire instead of hempen cable should be used to connect the mine and its circuit closer to the mooring anchor, as the former is less liable to twist, kink, or wear from friction than the latter.
A ground mine with circuit closer attached is represented at Fig. 48, where a is the wire mooring rope, b the electric cable leading from the mine to the circuit closer, C, and c the cable leading from the firing station to the mine; d is the oblong sinker attached to the mine, and e the tripping chain leading to the shore, to which the cable c is attached at intervals, so that by underrunning the electric cable, the tripping chain may be easily picked up, and the mine raised.
At Fig. 49 is shown a buoyant mine. The only difference in the mooring of this and the one before described, is that instead of resting on its anchor on the ground, it is moored at a certain distance above its anchor d, to which it is secured by a chain e.
Fig. 50 represents an electro contact mine. M is the mine with circuit closer enclosed, a the wire mooring rope, d the mushroom anchor, and b the electric cable leading from the mine to the disconnector D.
The mushroom sinker or anchor, which is undoubtedly the most effective of all other forms of mooring anchors used for the purposes of anchoring submarine mines, is shown at e, Fig. 49; the legs are added for use on rocky or hard bottoms, under which circumstances the weight of the anchor should also be increased.
For ground mines the form of sinker shown at d, Fig. 48 is employed; it is of an oblong shape, and hollowed out in the centre to allow of its being lashed close up to the mine.
Large blocks of stones with their bases slightly hollowed are useful as extempore moorings, so also is the one shown at Fig. 51, which consists of a strong heavy wooden shaft a, with a number of wooden arms b, b attached to its base; this form of extempore sinker was considered very efficient by the American authorities.
The wooden weighted platform, which was described at page 56, is also a very useful form of extempore sinker.
For dead weight moorings, pigs of ballast, heavy stones, &c., may be used.
The weight of the anchor or sinker for mooring submarine mines is a very important consideration. It will depend on the amount of buoyancy of the mine, on the strength of current, and on the nature of the bottom, also whether the mines are to be hauled down to, or moored with the anchor.
Stotherd uses the following formula:
P is the pressure exerted by any given current on the same buoyant mine;
W the weight of sinker necessary to overcome the tendency of the mine to move. In still water P becomes nothing, and therefore W equal to 2 B, that is, in still water double the buoyancy of a mine is a sufficient weight for its anchor.
The value of P may be found from the formula P = 4·085 × V2, where V is the velocity of the current in miles per hour.
From this equation P will be found in terms of pressure in pounds per square foot of flat surface, which is nearly double that on the curved surface of a cylinder.
In regard to the amount of buoyancy of a submarine mine, it has been found by actual practice that in the case of a mine moored in still water it should certainly be not less than the weight of the charge, whilst if subjected to the lateral pressure due to a current, it should be not less than three times the pressure exerted by the current.
It is always necessary to allow an excess of buoyancy over the calculated amount to counteract any leakage, or other disturbing cause which might otherwise materially affect the efficiency of the mine.
There are two modes of placing a mine in position; either by attaching the anchor, with the cable necessary for the depth of water, to the mine, and lowering both together, or by placing the anchor first, and then hauling the mine down to it, and by means of a catch, fastening it at the required depth.
The first mode is exceedingly simple, but except under very favourable circumstances cannot be relied on when firing by observation is the means adopted to explode a system of submarine mines. The second plan is practically easy to carry out, and by it a mine may be placed more accurately. To enable either of the above methods to be properly carried out, specially fitted steamboats, &c., are requisite.
At Fig. 52 is represented a 42 feet launch fitted for laying down a submarine mine by the first of the two modes enumerated above.
a is the mine; b is the electric cable carried from the drum c to the charge, and connected for use; d is the circuit closer, which is attached to the mine by its electric cable and mooring rope; f is the mushroom sinker attached by means of its mooring chain to the mine, it is suspended by a slip rope g, which passes over a small crutch fitted with a sheave h; i is a hollow iron derrick, and k the tackle and fall for lifting mine into boat; this derrick is formed of an iron tube about 3 inches diameter, 3/8 inch thick, and 10 feet 6 inches long; it is attached to an iron tube mast of similar diameter and thickness to the derrick, but 12 feet 3 inches long, an iron chain 6 feet 6 inches long and 5/8 inch diameter, connects the derrick to the mast; m is a leading sheave to keep the cable clear whilst it is being paid out; l is a crab, for working the tackle k, &c., and c is the drum on which the electric cable is wound.
In connection with the defence of a harbour by a system of electrical submarine mines of large size, it will be necessary to employ a service of steamtugs, steamboats, mooring-barges, &c., specially fitted for such work. One of the great advantages of the hauling down method of placing mines in position, is, that the anchors, with the cables connected thereto, may be carefully and accurately got into position during the time of peace, and the mines themselves, which should be kept in store ready fitted for immediate use, need not be placed in position until they are actually required. The drums used for reeling a multiple cable on, are capable of holding half a nautical mile in length. That used for a single core armoured cable is similar to but smaller than the aforesaid drum, and is capable of stowing one nautical mile of such a cable. For transportation wooden drums are ordinarily used.
During the early days of submarine defensive warfare, the latter method alone was used, owing to the absence of anything like a practicable form of self-acting apparatus; but within the last few years, the former has almost entirely superseded the latter method, except in very exceptional cases; this revolution being due to the vast improvements that have been, and still are being effected in the system of firing electrical submarine mines automatically.
Use of Circuit Closers.—Electrical submarine mines may by means of an apparatus, termed a circuit closer, be rendered self-acting; that is to say, by the action of a vessel coming in contact with such an apparatus, which may be either within the mine itself, or within a buoy attached to the mine, the electric circuit is closed, and the mine in connection with the circuit closer so struck, exploded. The essential feature of such a mode of closing the electric circuit is, that electrical submarine mines may be rendered either active or harmless, at the will of the operator, which is effected by the putting in, or taking out of a plug, by which means the firing current is either thrown in, or out of the circuit.
Circuit closers.—Many different forms of circuit closers have been devised, among which the following seem the most suitable and are those generally used:—
Mathieson's Circuit Closer.—This form of circuit closer has been adopted by the English government in connection with their system of defence by electrical submarine mines.
The details of this apparatus are shown at Pl. xiii.
Fig. 53, a is a gun-metal dome screwed on to a metal base b, its foot resting on a gutta percha washer c, so as to exclude any water; d is a cap screwed on to the top of the dome, and made watertight by the leather washer e; f is a guard cap screwed into the cap d, this is to keep the spindle of the circuit closer steady during transport, and would be removed when the apparatus is prepared for service; g is the ebonite base plug through which pass the insulated wires E and L; h is an hexagonal collar, working in the metal base plate b, by means of which, and the brass collar i, and the leather washer k, the base plug is secured, and water is excluded from the interior of the circuit closer; l, l, l are brass columns supporting a circular ebonite piece m; n is a metal bridge screwed on to the base plate b, into which is screwed the spindle p, both of which are prevented from moving after being screwed up by the set screws r and s.
The spindle p carries a leaden ball t, which is supported upon the rest v, and is secured in position by the screw nut w; x is an india rubber ring, the object of which is to prevent any damage being done to the spindle should the ball when set in action by a heavy blow from a passing vessel be brought into contact with the dome; 2 is a brass disc attached to the spindle carrying an ebonite disc 4, connected to it by screws; 6 is a brass contact ring also fixed to the ebonite disc 4, provided with a screw 8, for the attachment of one of the base plug wires, and with platinised projections 3, 3, 3, Fig. 56. The contact ring 6 is completely insulated from the spindle and brass disc 2. Three contact springs 5, are attached to the circular ebonite piece m, and the faces opposite to the platinised projections of the disc 2 are also platinised. 7 shows the contact screws of the connecting pieces, which serve also as adjusting screws to regulate the sensitiveness of the apparatus, the points of which as well as their bearings on the springs are platinised.
The springs are connected together by means of the wires 9, Fig. 55, one end of which is secured to the connecting piece by the screw 10, and the other passes through to the top of the ebonite piece, and is attached to the top of the spring next in succession to that to which it is fixed below.
One terminal of a coil of 1000 ohms resistance (which is used for testing purposes) is attached to the line L, terminal of the ebonite base plug, which latter is also connected to the screw 8, on the circumference of the contact ring 6; the other terminal of the resistance coil is connected to the earth, E terminal of the base plug.
A bare copper wire of No. 16 B. W. G. connects the top of the last contact spring with the set screw s; a piece of similar wire jointed to it is passed round one of the brass collars and connected to the screw r. As a precaution against bad contact, the contact springs are connected together by bare wires A, B, C. This completes the connections for the signalling circuit, the earth being formed by the body of the instrument; D is a hole left in the metal base for the passage of the insulating wire which connects the earth plate to the earth E terminal of the base plug.
Testing Current.—For testing purposes the current from the test battery arrives by the line wire L, and passes thence through the resistance coil to earth by means of the wire E, which is attached to a zinc earth plate placed in a recess in the jacket of the circuit closer.
Action of the Circuit.—The action of the apparatus is as follows:—
Closer.—On the circuit closer being struck, the weight of the lead ball t causes the steel rod p to be deflected and brings the brass ring 6 in contact with one of the springs 5; the signalling current which up to this moment has been passing through the 1000 ohms coil to earth, then passes to the contact ring 6 (avoiding the resistance coil) thence to the spring which is in contact with it, and from there by means of the wire connections to the set screws s and r, and so to earth through the metal body of the apparatus; the effect of the resistance coil being thus eliminated, is to strengthen the signalling current, and thus enable it to work the shutter apparatus, by which means the firing current is thrown into circuit and the mine exploded.
Circuit Breaker.—By altering the mode of connecting the wires, the above apparatus may be used as a circuit breaker, that is to say, the signal may be given, and the mine exploded by the cessation of a passing current, instead of by the closing of the electric circuit. This system was specially designed for use with platinum wire fuzes, but is rarely used.
Circuit Closer of Electro Contact Mines.—When the inertia circuit closer is employed in connection with electro contact mines, the circular ebonite piece m is replaced by a similar shaped piece of brass, and which is in metallic connection through the brass pillars l, l, l with the mass of the metal of the apparatus which forms the earth plate.
The insulated wire of the base plug is connected to one pole of a platinum wire fuze, the other pole of which is connected by another wire to the outer metal rim of the disc of the spindle. As long as the circuit closer remains undisturbed, a break will remain in the circuit, which is due to the ebonite insulation between the spindle and the outer metal rim of the disc; but the moment the apparatus is struck, which causes the spindle to vibrate, the outer metal rim will come in contact with one of the springs completing the circuit, through the circular metal portion and the pillars of the circuit closer to earth.
Adjustment of Circuit Closer.—The sensitiveness of Mathieson's inertia circuit closer is determined by the distance between the disc 4 and the springs 5, 5, 5, which is regulated by means of the adjusting screws 7, 7, 7, which press against the inner faces of the springs. Owing to the great weight of the leaden ball, when by any cause the circuit closer is inclined for a length of time, a permanent set is given to the spindle, thereby destroying the adjustment of the instrument.
Improvements in the Inertia Circuit Closer.—To remedy this very serious defect, a cylinder of india rubber is substituted for the leaden ball; a circuit closer so fitted is also less affected by the action of counter mines, which is a very important advantage.
Mathieson's Spiral Spring Circuit Closer.—A sectional elevation of this form of circuit closer is shown at Fig. 57. It consists of a brass base a, provided with a grooved flange for carrying a gutta percha washer, and it has also an hexagonal projection for the purpose of screwing the circuit closer into the gun-metal mouth of its air-tight cylinder, or buoy; b is a brass dome enclosing the apparatus for the purpose of protecting it from injury, and also by means of india rubber washers to prevent an ingress of water, should the circuit closer case become injured, and leak; c is a brass collar to which the brass contact springs i, i are attached, and which are regulated by the set screws j, j; a brass spiral spring d carries a metal rod e, which supports a brass ball f, surrounded by an india rubber band h. A contact disc g is secured to the base of the spindle e, but insulated from it by an ebonite boss; k is an ebonite base plug with two channels in it, through which the wires m, m1 pass.
An Improvement on the Inertia Circuit Closer.—This instrument is a vast improvement on the inertia apparatus previously described, being more simple and more certain in its action, a desideratum in all circuit closers; but notwithstanding, up to the present time Mathieson's inertia apparatus has been used by our government, to the exclusion of all other instruments of a similar nature, some of which were proved to be far superior when subjected to the crucial test of actual practice.
Austrian Self-acting Circuit Closer.—This form of circuit closing apparatus, which is purely a self-acting one, that is to say, a mine so fitted cannot be fired at will, is shown at Fig. 58.
It consists of several buffers a, a, a, which by means of strong springs are held in position, their heads projecting outside the torpedo case b; on being pressed in by the contact of a passing vessel, the ends of these buffers would be forced against a ratchet wheel c, which is also kept in position by means of a spring. Several strong pieces of wood d, d within the case keep the buffers and their attached arms in the proper direction, and also afford rigidity to the torpedo case. The brass ratchet wheel c being put in motion carries round with it a central arrangement e, the lower part of which is shown at Fig. 58, A.
This portion consists of a cylinder of brass f divided into two parts insulated one from the other by a piece of ebonite g; on one side of this cylinder there are three arms of brass, h, i, and k, and on the other there are two arms, l and m, all of which are insulated from each other.