CHAPTER IV.
DEFENSIVE TORPEDO WARFARE--_continued_.
_CLOSING the Electric Circuit._--In connection with the system of coast defence by means of electrical submarine mines, there are two distinct methods of effecting the closing of the electric circuit, and consequently, the firing battery being connected, the explosion of the mine or mines, which methods may be used separately, or in combination, and are as follows:--
1.--The self-acting method. 2.--The firing by judgment, or observation method.
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like a practicable form of self-acting apparatus; but within the last few years, the former has almost entirely superseded the latter method, except in very exceptional cases; this revolution being due to the vast improvements that have been, and still are being effected in the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:--
1.--Mathieson's inertia circuit closer. 2.--Mathieson's spiral spring circuit closer. 3.--Austrian self-acting circuit closer. 4.--McEvoy's mercury circuit closer. 5.--McEvoy's weight magneto circuit closer.
_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_, _m^{1}_ 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.
The arm _h_ is close to, but insulated from a metal plate _n_, which latter is permanently connected with the conducting wire leading from the firing battery, and thus while in a state of rest is electrically charged; beyond the arm _i_ is a spring _o_, which is connected with the earth, and in such a position that when the central portion is moved round, this spring _o_ comes in contact with the arm _i_, and the plate _n_ with the arm _h_ simultaneously, and the circuit is thus completed through earth to the battery, but the current of electricity does not pass through the fuze. The arms _k_, _l_ on the opposite sides of the cylinder, and consequently insulated one from the other, are connected with the fuze, and the arm _m_ is connected with the earth.
On a further pressure of the vessel on the buffer, the arm _i_ is pushed beyond the spring, and in contact therewith, and consequently the circuit by earth to the battery is broken, while the contact of the arm _h_ and plate _n_ is still retained, and the current is passed by the arm _k_ through the fuze to the arm _l_, and then to earth through the arm _m_, thus completing the electric circuit of the firing battery through the fuze, and to exploding the mine.
The spring acts as a circuit breaker, and by means of an intensity coil in connection with the firing battery, the current is only passed through the fuze when at the point of greatest intensity.
By detaching the firing battery, the channel defended by such submarine mines may be rendered safe.
_Fuze only in Circuit at Moment of Firing it._--One of the principal objects to be gained by the employment of such an arrangement for the closing of the electric circuit in connection with submarine mines, is the prevention of premature explosion from induction which might be caused by the proximity of any atmospheric electricity, the fuze in this system being entirely cut out of circuit until the moment when it is necessary to fire it.
The Austrians employed this form of circuit closing instrument during the war of 1866, and still continue to use it in connection with their coast defence by submarine mines.
_McEvoy's Mercury Circuit Closer._--At Fig. 59 is represented a longitudinal section of a circuit closer of this construction.
It is placed in the mine in such a manner that when undisturbed it maintains an approximately upright position.
It consists of a metal tube _a_ into which the cup _b_ of vulcanite, or other insulating material is fixed. The cup is contracted at some distance from the top by the perforated plug _c_, which is also of insulating material; _d_ is a metal pin fixed into the bottom of the cup _b_, it is connected with the wire _e_, which is insulated and passes to the battery; _f_ is a metal plug closing the tube _a_ and the cup _b_ at the top; _g_ is a wire attached to the plug _f_, and passing from it to an earth connection. The cup _b_ is filled with mercury up to the level of the plug _c_. By the contact of a passing vessel the instrument would be tilted sufficiently to cause the mercury to flow into contact with the metal plug _f_, thus completing the electric circuit and exploding the mine.
This form of circuit closer, though not generally adopted, would, on account of its being less liable to derangement by the motion of the waves, or by the explosion of an adjacent or counter mine, seem to fulfil the many requirements of a circuit closer for general service.
_McEvoy's Weight Magneto Circuit Closer._--This form of circuit closer, which is shown in section and plan at Figs. 60 and 61, is one of the most important improvements that has ever been effected in such apparatus, and bids fair to become universally adopted.
A heavy metal conical shaped weight _a_ (Fig. 60), hollowed out in its base and working in a ball and socket joint _b_, rests on a solid brass base _c_, and is so arranged that on the apparatus being struck, the weight _a_ will fall over, pivoting on one of its supports _d_, _d_; _e_ is a band of india rubber, encircling the weight _a_, for the purpose of preventing a jar on its falling against the sides of the brass cylinder _f_, which contains the weight _a_ and joint _b_. A brass rod _g_, connected to the ball and socket joint, passes through the base _c_, through a strong spiral spring _h_ (which latter rests on an adjusting screw _k_), through a piece of ebonite _l_, which supports the bobbins and core _m_, _m_^{1}; then between these bobbins _m_, _m_^{1} through an armature _n_, which is pivoted at _p_; and lastly through a slight spiral spring _o_, which is kept in position by the adjusting screw _i_.
The armature _n_ is fitted with a small piece of brass _r_, so arranged that when it (the armature) is in the position shown in Fig. 60, this piece of brass _r_ does not make contact with the two strips of metal, _s_, _s_, between which it, _r_, works; but when the armature _n_ is in contact with the cores of the bobbins _m_, _m_^{1}, then the piece of brass _r_ makes contact with the metal strips _s_ _s_, and so makes a short circuit for the electric current. An ordinary telephone _t_, Fig. 61, in which some small shot, bells, &c., are placed, is fixed to the top of the brass cylinder _f_.
_Action of Circuit Closer._--The action of this apparatus is as follows:--
On the mine carrying this form of circuit closer being struck by a passing vessel, the weight _a_ is caused to fall over towards the side of the brass cylinder _f_, thus allowing the strong spiral spring _h_ to act on the brass rod _g_ in an upward direction, by which means the armature _n_ is brought into contact with the soft iron cores of the bobbins _m_, _m_^{1}.
The connections of the wires are made as follows:--
The line wire _w_ is led through the base of the apparatus and connected to a piece of brass under the ebonite support _l_, in connection with one of the wires of the bobbin _m_, the other wire of which is attached to the metal strip _s_; the wires of the bobbin _m_^{1} are connected, the one to the metal strip _s__{1}, the other to a piece of brass under the ebonite support _l_; from this latter piece of brass a wire _w__{1} is led to the brass screw _x_. The wires _w__{2}, _w__{3}, from the fuzes are led, the one to the brass screw _x_, the other to a screw _y_, which forms through the metal of the apparatus the earth plate. One of the wires of the telephone _t_ is connected to the brass screw _x_, the other _w__{4} is connected to the piece of brass to which the line wire _w_ is also attached. While the circuit closer remains in a state of rest, the current from the signalling battery flows along the line wire _w_, up the telephone wire _w__{4}, through the telephone which has a high resistance, then by the wire _w__{2} through the fuzes, and to earth by the wire _w__{3}.
On the circuit closer being struck, by which cause the armature _n_ is brought up to the cores of the bobbins _m_, _m_^{1}, and the piece of brass _r_ in contact with the metal strips _s_, _s__{1}, the signalling current, instead of circulating through the high resistance of the telephone _t_, passes round the bobbin _m_, down the metal strip _s_, across the brass piece _r_, up the metal strip _s__{1}, round the bobbin _m__{1} (thus forming an electro magnet of _m_, _m__{1}), and by the wire _w_, direct through the fuzes to earth, and so explodes the torpedo. The effect of the telephone resistance being cut out, is to strengthen the signalling current, and enable it to work the shutter apparatus and so throw the firing battery in circuit and explode the mine.
The advantages of this circuit closing apparatus are:--
1.--Simplicity.
2.--Compactness.
3.--Increased certainty of action, due to the sustained contact of the armature _n_, on the apparatus being struck.
4.--Additional means of testing a system of electrical submarine mines, which is afforded by the telephone:--
When this form of circuit closer is put in action by a friendly vessel coming in contact with it, or when experiments are being made, the signalling current must be reversed, so that no doubt may exist as to the armature _n_ having dropped, on the apparatus coming to rest.
The telephone test indicates whether the circuit closer is in position or not, the shot, &c., within the telephone being shaken about by the movement of the buoyant circuit closer, the noise so created is readily distinguished by the receiving telephone at the station.
Another form of submarine mine is that known as the "Electro Mechanical" mine. The difference between this form and an ordinary mechanical mine is, that the exploding agent is electricity, and that it may be converted into an electro contact mine if desirable.
_Description of a Russian Electro._--The electro mechanical mine, used by the Russians during the late Turco-Russian war, is shown in elevation and section at Figs. 62 and 63.
_Mechanical Submarine Mine, used by them during the late Turco-Russian War._--_A_ is the conical shaped case; _B_ the loading hole; _C_ the base plug; _D_, _D_, &c., are five horns, screwed into the head of the case _A_; these are composed of a glass tube _A_, containing a chlorate of potash mixture, enclosed in a lead tube _B_, over which is screwed a brass safety cylinder _C_; when ready for action this latter tube _C_ is removed; directly beneath each of the horns _A_, on the inside of the case, as at _E_, is a thin brass cylinder, closed at one end by a piece of wood _d_, and containing several pieces of zinc and carbon, arranged in the form of a battery, the zinc and carbon wires _z_ and _x_ being led through the piece of wood _d_; _F_ is a copper cylinder containing the priming charge of gun-cotton _g_, and detonating fuse _f_; the terminals of the fuze are connected to two insulated wires, _w_ and _w__{1}, the former of which is led direct to the loading hole _B_, and attached on the inside to the five zinc connecting wires _z_, &c.; the latter is attached to one end of a safety arrangement _S_, the other end of which is connected to the wire _w__{2}, which is attached on the inside to the carbon wires _x_, &c.; the safety arrangement _S_ consists of an ebonite cylinder, containing a brass spiral spring fixed to one end of it, and pressing against a brass plate at the other, thus preserving a metallic connection between the wires _w__{1}, and _w__{2}; the mine is rendered inactive by pressing the spring down, and inserting a piece of ebonite between it and the plate.
_Its Action._--The action of this form of electro mechanical submarine mine is very simple; the brass safety cylinders _c_, _c_, &c., being removed on a vessel striking either of the horns, _D_, _D_, &c., the lead tube _b_ is bent, causing the glass tube _a_ to be broken, and the mixture contained therein to flow into the cylinder _E_, instantly generating a current of electricity in the zinc carbon battery, and exploding the mine.
_Mode of Converting into an Electro Contact or Observation Mine._--To convert this mine into an electro contact one, it is only necessary to connect the wires _w__{1} and _w__{2} to other wires leading from the shore; also by replacing the horns _D_, _D_ by solid brass screw plugs, the mine may be converted into an ordinary observation one. In this case the two wires _w_ and _w__{1} attached to the fuze _f_, terminals would have to be connected to the observation instruments on shore.
_Turkish Vessel sunk._--It was by means of one of these electro mechanical mines, that the Turkish gunboat _Suna_ was sunk at Soulina.
Firing by observation, that is to say, effecting the ignition of an electrical submarine mine at the precise moment of a hostile vessel being vertically over it, through the agency of one or two observers stationed at a very considerable distance from the mine, should, with the very perfect self-acting circuit closers that exist at the present time, be resorted to only in very exceptional cases, or in connection with the self-acting system.
There are two defects, which are common to all methods of firing submarine mines by observation, and these are:--
1.--At night time, or in foggy weather, it cannot be employed.
2.--It is necessary to employ at least two observers, at a considerable distance apart, who to effect a proper action at the right moment, must work in perfect unison. These defects alone are sufficient to explain the preference given to a self-acting method of closing the electric circuit at the precise moment of a vessel being in position over a mine by those governments who have adopted electrical submarine mines as a means of coast defence.
_Methods of Firing by Observation._--There are several methods of firing by observation, of which the following are the ones principally used:--
1.--By pickets or range stakes. 2.--By cross bearings. 3.--By intersectional arcs fitted with telescopes. 4.--The Prussian system.
_Intersection by Pickets or Range Stakes._--In narrow channels and at short distances, this system of ascertaining the relative position of a hostile vessel and a submarine mine may be used, provided that skilled and careful men are employed to work it. Two or more pickets or stakes are arranged in front of the firing station in such a manner that a vessel passing up the channel on the prolongation of these stakes will be over a mine. This arrangement should of course always be considered as an extempore one; it was used on several occasions by the Confederates during the American civil war.
_Firing by Cross Bearings._--The simplest method of so determining the relative position of a vessel and a submarine mine, and exploding it at the right moment, is that in which observers are placed on the prolongation of the mines. This mode is shown at Fig. 64, where _m__{1}, _m__{2}, _m__{3}, &c., and _n__{1}, _n__{2}, _n__{3}, &c., are the mines; _A_ and _B_, the points in prolongation of the mines where the observers are stationed; _D_ the firing battery, and _s_, and _s__{1} two hostile vessels.
At the stations _A_ and _B_ firing keys are placed, at the former one for each separate mine, perfectly distinct and insulated from each other, at the latter a single key. The pivot points of the series of keys at _A_ are connected by separate wires to one pole of the firing battery _D_, the other pole of which is connected by a single cored insulated cable to the pivot point of the key at _B_; the contact points of the series of keys at _A_ are connected by separate line wires as _A m__{1}, _A m__{2}, _A m__{3}, &c., to the different mines, while the contact point of the key at _B_ is put to earth. Thus it will be seen that, in the case of the row of mines, _m__{1}, _m__{2}, &c., unless the key at _B_, and the key at _A_, of either of those mines are both pressed down at the same instant, no current can pass, and therefore none of those mines can be exploded.
In the case of the vessel _S_, though at _C_, she is on the prolongation of the line _A m__{5}, _C_, and therefore the key of the mine _m__{5}, is pressed down at _A_, yet not being on the prolongation of the line _B_, _E_, the key at _B_ is not pressed down, therefore the firing battery is not thrown in circuit, or the mine _m__{5} exploded, but when the vessel _s_ reaches the position _N_, that is over the mine _m__{3}, she being on the prolongation of the lines _A m__{3}, and _B E_, the key (_m__{3}) at _A_, and the key at _B_ would both be pressed down, and therefore the mine _m__{3} exploded, and the ship destroyed. In the case of a vessel passing through an interval between any two mines at such a distance as to be out of the radius of destructive effect of either of the mines belonging to the first row (which is shown at _s__{1},) only the key at _B_ would be pressed down, and thus the vessel enabled to pass safely through, but only to come to grief at the second or third row of mines, provided they have been properly placed, and separate though similar arrangements as in the case of the line of mines, _m__{1}, _m__{2}, &c. have been made.
_Firing by a Preconcerted Signal._--At Fig. 65 is represented a somewhat similar, though a much simpler plan of the foregoing system, by employing a preconcerted signal at the station _B_ in the place of the firing key and insulated cable, as in the former case. The only material difference in the arrangement of these two methods, is that in the latter case the pole of the firing battery at _A_, which in the former case was connected to the firing key at _B_, is put direct to earth. As will be readily understood, this latter system requires great coolness and nerve on the part of the operator at _A_, who has not only to watch the vessel passing across his intersections, but also to be on the alert to receive the signal from the observer at _B_. Should it ever be necessary to adopt this latter system, it will be found advisable to employ two men at station _A_, one to watch station _B_, the other to attend to the firing key and intersections. A separate signal-flag for each line of mines, and also a separate firing arrangement, would be required. As in many cases it would not be practicable to have a station in such an advanced position as at _B_, in Figs. 64 and 65, on account of the danger of its being cut off by an enemy, another combination becomes necessary. In this instance the station _B_ is placed on the opposite side of the river, &c., to that on which the station _A_ is placed, and a series of firing keys, instead of a single one, is here used, necessitating a multiple cable between the stations _A_ and _B_, in the place of single cored cable; the manner of manipulating this method is very similar to that previously described.
_Firing by Intersectional Arcs fitted with Telescopes._--The foregoing methods of firing by cross bearings are replete with many serious defects, to remedy which, to a considerable extent, special arrangements have been devised, that is, the employment of intersectional arcs fitted with telescopes at the stations _A_ and _B_.
Figs. 66 and 67 show the arrangements of these arcs, the former being the one used at the firing station _A_, the latter at the converging station _B_. At each station one arc is provided for each row of mines placed in position. The firing arc Fig. 66 consists of a cast iron frame _a_, with three feet _b_, _b_, _b_, these being provided with levelling screws.
To ascertain when this frame is level, a circular spirit level is attached thereto, a telescope _d_ provided with one horizontal and three vertical cross wires, supported on Y's, admitting of vertical motion and attached to an upright _e_. A mill-headed screw _f_ enables the telescope _d_ to be raised or lowered; the telescope, which is rigidly connected to a vernier _g_, traversing over a graduated arc _h_, can be moved rapidly in a lateral direction by means of a rack and pinion arrangement _i_, and it can be clamped in any position by means of the screw _h_. Sights are fixed on the telescope in a vertical plane passing through its axis. To the outer rim of the frame of the arc, which is smooth, are secured the sights _l l_ (shown on a large scale at Fig. 68), to give the direction of the mines. These sights are provided each with a brass point of V form, _m_, and a binding screw, _n_, in metallic connection with each other, but insulated by means of an ebonite plate from the rest of the metal of the sight. One end of a short piece of insulated wire is attached to the binding screw _n_, and the other passes through a hole in the base of the sight and projects below it; _o_ is a brass tube rigidly connected to and moving with the upright carrying the telescope _d_, and projecting in front of this latter. A brass spring _p_ (see Fig. 69) is attached to, but insulated from the outer extremity of this tube, and is so arranged as to make contact with the V point _m_ on the sight, by means of a corresponding projection fitted to its under side. An insulated wire passing the tube _o_, the outer end of which is connected to a screw on the spring _p_, forms a metallic connection between this projection and the firing key.
At Fig. 68 is shown an enlarged view of the front of the sight; in addition to the V projection _m_, and binding screw _n_, it is fitted with a capstan-headed screw to bear against the inner rim of the frame, and a thin wire upright _t_ for giving the alignment of the mine, to which a disc is attached, on which the number of the mine is affixed.
When the distance between the station and the mine is only about one mile, an ordinary eyepiece is used in the place of the telescope _d_.
At Fig. 67 is represented the arc employed at the converging station, which with the exception of there being no tube _o_, and only one sight, is precisely similar in construction to the one used at the firing station, and which has been described.
_Application of the Intersectional Arc Method._--The application of the method of firing by observation, by means of intersectional arcs fitted with telescopes, is shown at Fig. 70. _C_, _D_, and _E_ are three of the larger kind of arcs, one being used for each row of mines at the firing station _A_. At the converging station _B_, one of the smaller arcs is used for each row of mines, as shown at _F_, _G_, and _H_. _S_, _S__{1}, _S__{2}, are the signalling apparatus, the _F_ terminals of which are connected to the sights _l_, _l_, _l_, Fig. 69, of arcs _C_, _D_, _E_. Firing keys _a_, _a_, _a_ at station _A_ are connected to each arc, and to three of the cores of the cable connecting the two stations _A_ and _B_, respectively. At the converging station _B_, three firing keys _b_, _b_, _b_ are connected to earth and to three cores of the connecting cable respectively. The remaining core of this cable is connected to the recording instruments _d_, _e_. The action of the arcs, &c., will be readily understood from the diagram at Fig. 70.
This arrangement does not interfere with the action of the circuit closer, as all that is effected by the observing arc circuit is to put the signalling battery current at the converging station _B_ to earth instead of at the circuit closer.
_Prussian System of Firing by Observation._--The principle on which this system is based, depends upon the proposition that if _c d_, in the triangle shown in Fig. 71, be always kept parallel to _H B_, then _A c_, _c d_, _d A_ bear exactly the same proportion to each other as _A B_, _B H_, _H A_ do to one another; so that by means of the small triangle _A d c_, the lengths of the sides of the large triangle _A B H_ can be obtained, and hence the position of the point _H_, the base _A B_ being of course known. In Fig. 71 at _A_ there is a slate table representing the roadstead, and upon it the exact position of every torpedo is laid down, corresponding to their position in the roadstead. At _A_ and _B_, 500 yards apart, telescopes having cross wires are placed; at _A_ a long narrow straight-edged strip of glass _A d_ is arranged to move in unison with the telescope at _A_; and by the application of dynamo electricity, a similarly constructed piece of glass _c d_ moves in exact unison with the telescope at _B_, and having its pivot at _C_; that is to say, _C d_ keeps parallel with _B H_, the line of sight of the observer at _B_.
Then if the observers at _A_ and _B_ have got a ship in their telescopes, the point of intersection _d_ of the two pieces of glass _A d_ and _C d_ gives the position of the ship on the slate table at _A_, and when this point _d_ comes over the position of any one mine on the slate, it is known that the ship is over that particular mine in the harbour, and she may be destroyed accordingly, by throwing the firing battery into circuit.
By the employment of electricity and a mirror, the great defect of this method, viz., the necessity of employing four people to manipulate it, would be remedied. The foregoing is a modification of Siemens's method of ascertaining distances at sea, &c.
_Rules observed in Planting Mines._--In placing a system of submarine mines in position, the following are some of the chief points to be attended to, this work depending in a great measure on local circumstances, and on the method that is to be adopted in exploding and mooring them:--
1.--The plan of defence must be carefully laid down on a chart, on a scale of not less than six inches to the mile, and on this plan are to be marked the sites of the observing stations, the positions of each mine, circuit closer, and junction box, with their corresponding numbers, and also of the electric cables.
2.--The position of each mine having been determined, should be marked off by buoys.
3.--The utmost care should be taken to lay the electric cables, so that they shall be as far as possible away from the mines in the vicinity of which it may be necessary to take them, so as to lessen the liability of injury to them, by the explosion of the latter.
4.--The electric cables should be laid parallel, and never be allowed to cross directly over each other, otherwise the operation of underrunning them will be much complicated, also a certain amount of slack should be allowed to facilitate in picking the cables up for repair, &c.
5.--Every manner of device is to be used to conceal the electric cables, such as laying dummies, making detours inland, &c.
6.--All marks indicating position of the mines to be removed, after the mines have been placed in position.
7.--The identity of each cable and mine to be very carefully preserved throughout, by means of a number.
8.--A number of electro contact mines should be placed in advance of the leading line of mines, at irregular intervals, to prevent the enemy, having once ascertained the position of one mine of a line, from knowing within limits the position of the others of that line.
In connection with a system of defence by electrical submarine mines, the following batteries are required:--
1.--Firing battery. 2.--Signalling, or shutter battery. 3.--Testing battery. 4.--Telegraph battery.
_Firing Battery._--The firing battery should be suited to the nature of the fuze employed, and should possess considerable excess of power to enable it to overcome accidental defects, such as increased resistance in the various connections, or defective insulation in the line wire, &c.
As platinum wire or low tension fuzes are now universally adopted as the mode of ignition for submarine mines, it will be only necessary to describe those electrical batteries which are most suitable as an exploding agent in connection with such fuzes; these are as follows:--
1.--Siemens's dynamo low tension machine. 2.--Von Ebner's Voltaic battery. 3.--Chromic acid or Bichromate Voltaic battery. 4.--Leclanché's Voltaic battery.
_Siemens's Low Tension Dynamo Electrical Machine._--This instrument consists of an electro magnet and an ordinary Siemens armature, which, by the turning of a handle, is caused to revolve between the poles of the electro magnet. The coils of the electro magnet are in circuit with the wire of the revolving armature, and during rotation the residual magnetism of the soft iron electro magnet cores at first excites weak currents which pass into the electro magnet coils, increasing the magnetism of the core, thus inducing still stronger currents in the armature wire. This accumulation by mutual action goes on until the limit of magnetic saturation of the iron cores of the electro magnets is reached.
By the automatic action of the machine, the powerful current so produced is sent into the leading wire or cable to the fuze to be exploded.
In this apparatus the electric current passes continuously through the line wire until a sufficiently powerful current is generated to heat or fuze the bridge of the fuze, and so ignite the gun-cotton priming. The coils of the armature and electro magnets are wound with wire of large diameter, to a total resistance of 8 to 10 Siemens units, or 7·6 to 9·5 ohms, in about 2,000 windings.
With a platinum wire weighing 1·65 grains per yard, 6-1/2 inches can be fuzed on short circuit, and 14 inches can be heated to redness.
The total weight of this machine, which is manufactured by Messrs. Siemens Brothers, is about 60 lbs.
_Advantages of Siemens's Dynamo Electrical Machine._--The advantages of such a machine over Voltaic apparatus are:--
1.--The absence of chemical agents.
2.--There is less liability to get out of order.
3.--No special knowledge is required to work them, or to keep them in order.
4.--Greater durability.
The great defect of this and all similar machines is that the electric force has to be developed by turning a handle for a certain time before it is possible to generate a current sufficiently powerful to ignite a fuze, which defect, in connection with a system of defence by self-acting submarine mines, particularly at night, renders them inferior to Voltaic batteries, as under such circumstances, an apparatus is required that will cause an electric current to flow at any moment when the circuit is completed.
The application of steam power would to a certain extent remedy the above-mentioned defect, but the cost of such a method, compared to that of a Voltaic arrangement, would be far too great to allow of its superseding the latter arrangement.
_Von Ebner's Voltaic Battery._--This form of Voltaic battery, which may be considered as a modification of that known as Smee's, was designed by Baron von Ebner, colonel of the Austrian imperial corps of engineers, for use in connection with the Austrian system of submarine defence, by self-acting electrical mines.
A section of one of these cells is shown at Fig. 72. It consists of a glass vessel _a_, to contain the diluted sulphuric acid, within which is suspended a plate _b_ of platinised lead, which is bent round into a cylindrical form to fit close around the inner surface of the glass vessel. In the centre of this latter is hung a porcelain perforated cup _c_, containing some cut-up zinc and mercury to keep it (the zinc) amalgamated. The top of each cell is furnished with a porcelain cover, through which the wires attached to the positive and negative poles of the cell project.
Due to the large quantity of liquid contained in the cell, the tendency to alter its internal resistance is retarded; also by the arrangement of the porcelain cup, above detailed, the consumption of zinc and mercury, which in an ordinary Voltaic battery is very considerable, is materially diminished.
_Chromic Acid or Bichromate Battery._--This form of battery is very similar to Grove's, the difference being that, in the place of the nitric acid as the exciting liquid, either chromic acid, or a solution of bichromate of potash, sulphuric acid and water is substituted.
A form of this battery, as designed by Dr. Hertz, is used in connection with the German system of torpedo defence.
_Leclanché Voltaic Battery._--This form of Voltaic battery was invented by M. Leclanché, some twelve years ago. At Fig. 73 is shown a cell of this battery in its original form. The positive pole _a_ consists of a plate of graphite in a porous pot _b_, and surrounded by a mixture of peroxide of manganese and graphite. The negative pole _c_ is a rod or pencil of amalgamated zinc. The whole is enclosed in an outer vessel of glass _d_ containing a solution of sal ammoniac.
A modified form of the Leclanché cell as used in a firing battery is shown at Fig. 74. It consists of an ebonite trough or outer vessel _a_ about 16" long, 9" deep, and 2-3/4" wide. The negative pole or zinc plate _b_ is of similar shape to the trough _a_, but with its base removed, and does not fit the trough exactly, the space between it and the trough being left to ensure the former being completely surrounded by the sal ammoniac solution; the positive pole, or carbon element, consists of four gas carbon plates _c_ attached together at their head by means of lead, and enclosed in a flannel bag, in which they are firmly embedded in the peroxide of manganese mixture; the positive element is of such a shape that it fits loosely between the sides, and is nearly of the same height as the zinc plate.
The object of such a form of cell was to obtain an electric current of large _quantity_, with as few cells as possible, by which means the loss of power which might occur from the employment of a great number of small cells is avoided.
_Advantages of a Leclanché Firing Battery._--The advantages of the Leclanché firing battery are:--
1.--The absence of chemical action when the battery circuit is not complete, and consequently there is no waste of material.
2.--Requires little or no looking after.
3.--It may be kept ready for action in store without in any way deteriorating.
4.--It is comparatively very cheap.
These advantages combine to make a Leclanché battery the most suitable of any other form of electrical battery for use as the exploding agent for electrical submarine mines, and it is now universally used for such purposes.
_Signalling Battery._--The signalling battery should be so constituted as to be capable of working the electro magnet of the shutter apparatus effectually when the circuit is closed direct to earth, and yet not so powerful as by the continuous passage of the current generated by it to fire the fuze in the mine. In the case of a platinum wire fuze being in the circuit, plenty of power may be given to the battery without fear of a premature explosion from this cause, but in the case of a high tension fuze it is necessary to be very careful in order to guard against such a contingency.
As in the case of a signalling or shutter battery, the electric current will be continually flowing, it is necessary to employ a constant battery, or one that requires least trouble and expense to maintain it in working order, and it is for this reason that a modified form of Daniell battery has been adopted to work the shutter apparatus.
_Daniell Signalling Battery._--At Fig. 75 is shown the manner of arranging a Daniell cell. A glass or porcelain vessel _a_ contains a saturated solution of sulphate of copper, in which is immersed a copper cylinder _b_ open at both ends and perforated by holes; at the upper part of this cylinder there is an annular shelf _d_, also perforated by holes, and below the level of the liquid; this is for the purpose of supporting crystals of sulphate of copper for the replacing of that decomposed as the electrical action proceeds. Inside the cylinder _b_ is a thin porous vessel _c_ of unglazed earthenware; this contains either water, or a solution of common salt, or dilute sulphuric acid, in which is placed the cylinder of amalgamated zinc _e_. Two strips of copper _p_ and _n_, fixed by binding screws to the copper and to the zinc, serve for connecting the elements in series, or otherwise.
For the purposes of testing, either the Leclanché or Daniell battery specially arranged, or the Menotti battery, which is really a modification of the Daniell, may be used.
_Description of a Menotti Cell._--A Menotti cell, shown at Fig. 76, consists of a copper cup containing some crystals of sulphate of copper and covered with a fearnought diaphragm _a_, placed at the bottom of an ebonite cell _b_; over this cup is put some sawdust, and resting on top of this is a disc of zinc _c_ on another piece of fearnought. The upper portion of the zinc and its connection with the insulated wire are carefully insulated. Fresh water poured on the sawdust renders the battery active.
_Description of a Menotti Test Battery._--Fig. 77 represents a plan of the top of such a test battery with a 20-ohm galvanometer attached thereto. The connections are made as follows:--
One of the wires _w_ of the object to be tested is attached to the terminal _f_, which is also connected by an insulated wire to the copper cup _a_; the other main wire _w__{1} is attached to the terminal _g_ of the galvanometer; _h_, the other terminal of the galvanometer, is connected by a short piece of wire _k_ to the terminal _l_ of the contact key _m_; and the contact point _n_ is in connection with the zinc plate _c_; thus the current from the battery flows along the wire _w_ through the object to be tested, back along the wire _w__{1}, through the coils of the galvanometer, along the wire _k_ to the contact key _m_, and if this is pressed down to the zinc plate _c_, so completing the circuit.
To steady the needle of the galvanometer a bar magnet is used, which is inserted in the space _r_. The whole of the apparatus is enclosed in a leathern case fitted with a cover and strap.
This is a very compact and simple form of test battery, and will be found extremely useful in boats, &c., when placing mines in position.
_Telegraph Battery._--For the purposes of telegraphing between torpedo stations, &c., a form of Leclanché battery, known as No. 3 commercial pattern, is generally used.
_Voltaic Batteries._--The following points in connection with the use of voltaic batteries, which are taken from Beechey's 'Electro Telegraphy,' should be carefully observed:--
1.--Each cell of a battery should be carefully insulated.
2.--The floors and tables in the battery room should be kept scrupulously clean and dry, so as to prevent the least leakage or escape of the current.
3.--The plates of a battery should be clean.
4.--Porous cells should be examined, and cracked ones replaced.
5.--No sulphate of zinc or dirt should be allowed to collect at the lips of the cells.
In the case of a Daniell battery--
1.--The solutions should be inspected daily, and crystals of sulphate of copper added as required.
2.--The zinc plate must not touch the porous cell, or copper will be deposited on it (the zinc).
3.--The battery should be charged with sulphate of zinc from the first.
4.--The copper solution must be watched and prevented from rising over the edge of the porous jar, the tendency of such solutions being to mix with each other by an action termed _osmosis_.
These being in addition to foregoing general directions for Voltaic batteries.
_Defects in a Voltaic Battery on its Current becoming Deficient._--On the electric current of a Voltaic battery becoming deficient, the following defects should be looked for:--
1.--Solutions exhausted; for instance, sulphate of copper in a Daniell's entirely or nearly gone, leaving a colourless solution.
2.--Terminals or connections between the cells corroded, so that instead of metallic contact there are oxides of almost insulating resistance intervening in the circuit.
3.--Cells empty, or nearly so.
4.--Filaments of deposited metals stretching from electrode (pole) to electrode (pole).
Also intermittent currents are sometimes produced by loose wires or a broken electrode, which alternately makes and breaks contact when shaken. Inconstant currents are also sometimes produced when batteries are shaken. The motion shakes the gases off the electrodes, thus increasing temporarily the electro-motive force of the battery.
_Firing Keys and Shutter Apparatus._--The following is a description of the various firing keys and shutter signalling apparatus, which is used in connection with a system of electrical submarine mines. By means of the former the firing or other batteries may be thrown into circuit at will, whilst by means of the latter the firing battery is thrown in circuit without the aid of an operator, and a signal at the same instant given, indicating that a certain mine of the system has been struck.
_Description of a Series of Firing Keys._--At Fig. 78 is shown a plan and section of a series of firing keys as arranged for firing several mines by observation.
It consists of a strong wooden frame _a_, of a convenient form for the purpose of attaching it to the firing table by screws through the holes _b_, _b_. On this frame a series of keys _c_, _c_, _c_ are fixed at convenient intervals. These consist of a strong brass spring firmly screwed to a series of brass plates _d_, _d_, _d_ on the front of the wooden box _a_. From these latter short copper wires pass through the woodwork, and of such a length that, when required, the mine wires may be easily attached by means of binding screws, as shown at _f_. The inner end of each key is fitted with an ebonite knob (which is shown at _c_ in the section) to insulate the hand of the operator when using the key. On the frame, and directly under each of the ebonite knobs, are arranged a series of metallic points _g_, _g_, _g_, so placed that on either of the keys _c_ being pressed down, a perfect contact is made between it and its respective metallic point; _h_, _h_, _h_ are copper wires leading from the metallic points _g_, _g_, _g_ through the box, and of such a length that binding screws _f_, _f_, _f_ can be easily attached to them when necessary.
A single firing key of an improved form is shown at Fig. 79. It consists of a strong wooden box _a a_, weighted at the bottom with lead in order to steady the key on the table, &c., on which it may be placed; on the inside of the bottom of the box is fixed a piece of ebonite, by which means the metallic point _b_, and the terminal of the firing key _c_, are insulated from each other; _d d'_ are two terminals at the end of the box, to which the circuit wires are attached, one of these terminals is connected in metallic circuit to the firing key at _c_, the other one to the metallic point _b_; a wooden cover _h_, fitted with a catch _k_, protects the connections of the wires; by means of a plate, and catch _e e_, the key can be rendered inactive, thus preventing the danger of a premature closing of the electric circuit; by means of a spring _s_ a break is always established between the key and the metallic point. It is immaterial to which of the two terminals _d d'_ either wire is connected.
_The Morse Firing Key._--This form of key is so well known in connection with the Morse telegraph, that it is not necessary to describe it.
It is usually employed in torpedo work in connection with a testing and firing table.
_The Shutter Apparatus._--The shutter signalling and firing apparatus was devised to enable the firing battery current to be thrown in circuit without the aid of a personal operator, the signalling current (which is always kept in circuit) at the same instant ringing a bell, by which is known the particular mine that has been struck.
At Fig. 80 is represented a diagram of such an apparatus. _a_ is an armature working on a pivot between the two horns of an electro magnet _b b_, and held in position by a spiral spring _c_; the latter is in connection with a regulating screw, by which more or less pressure may be brought to bear in an opposite direction to that of the attractive action of the electro magnet. A stud _i_ regulates the distance to which the armature may be drawn back; _d_ is a shutter on which a reference number for each mine should be indicated, attached to a lever pivoted at the point _e_, the inner arm of which is just long enough to catch under the point of the armature _a_; when a current of sufficient strength is passed through the coils _b b_ of the electro magnet, the armature _a_ is attracted, releasing the lever attached to the shutter _d_, which by its own weight falls into the position shown by the dotted lines. _f_ and _g_ are two mercury cups, the former being in connection with the signalling current, and the latter with the firing current. When the lever is horizontal and the shutter drawn up and ready for action, the circuit of the signalling battery _s_ is completed through the mercury cup _f_, along an arm _h_ of the lever to the pivot _e_, and thence to the mine by the line wire _w_. When the circuit closer is struck by a passing vessel, and consequently the shutter thrown into the position shown by the dotted lines, another arm _k_, a prolongation of the lever, falls into the mercury cup _g_, which latter is in connection with the firing battery _F_. The armature _a_ is prevented from coming into actual contact with the horns of the electro magnet by two small studs. The object of this is to prevent any effect of residual magnetism which might otherwise interfere with the rapidity of action of the armature when released and drawn back by the spring _c_.
_The object of employing Mercury Cups._--Mercury cups were devised in the place of the springs used in connection with the original design of a shutter apparatus, for the reason that electrical circuits dependent on the pressure of springs are always liable to interruption from dirt or oxide intervening between the points of contact.
_Shutter Apparatus used with a Circuit Breaker._--When the circuit breaking system is used with the shutter signalling apparatus, the action of the armature in releasing the lever must be reversed; that is to say, that when the current is passing and the armature _a_ attracted to the electro magnet _b b_, the shutter _d_ must be held up, and when the current ceases, and the armature _a_ drawn back by the spring _c_, the lever must be released, and the shutter allowed to fall. This is effected by altering the end of the lever, so that it hooks into, instead of abutting against the armature _a_.
To each shutter apparatus an electric bell is fitted, by which notice is given when a circuit closer has been struck. For general service, a box containing seven such shutter signalling and firing apparatus has been adopted, a plan of which is represented at Figs. 81, 82 and 83. The connections of the different circuits are as follows:--
The insulated wire of the upper bobbin of the electro magnet is connected to the spring of the armature; the pivot of the lever is connected with the right-hand terminal _B_, or main line connection on the top of the box; the insulated wire from the lower bobbin is connected to the middle brass plate _k_ in the front ledge of the apparatus, the circuit from _B_ to _k_ being thus completed. The front adjoining brass plate _A_, provided with a terminal, is connected with the negative pole of the signalling battery, the positive pole being put to earth.
On a brass plug being put in the hole _l_, the signalling current will flow to the plate _k_, thence through the lower and upper bobbin to the spring of the armature, along the latter to the shutter lever, and from the pivot through the main line wire to the mine. The innermost brass plates _H H_ are all connected in the same metallic circuit, and to them are attached by means of the binding screw _D_ the test battery and galvanometer. Thus on the brass plug being removed from _l_, and placed in _m_, the signalling battery is cut out of circuit, and the test battery thrown in. In this way the condition of each individual mine may be ascertained while the connections of the remaining mines are left undisturbed. The positive pole of the firing battery (the negative being to earth) is connected to the terminal _S_ at the right-hand corner of the lower ledge of the box; the plate to which the terminal _S_ is fixed is divided at _G_, the left-hand portion being connected to a bar which runs horizontally the whole length of the box, and in metallic connection with each mercury cup _g_, Fig. 80. A brass plug is placed in the hole _G_, and when from any cause the lever drops, the firing battery will be thrown into circuit, and the mine to which the lever that has fallen is attached will be exploded.
_Shutter Instrument and Observing Telescope._--Each mine is given a number, which is put on the disc of the shutter instrument connected to it, and also on the corresponding tablet _C_. From the brass plate in connection with the spring _c_, Fig. 80, a wire is taken to the terminal _f_, Fig. 81, on top of the box. From this terminal a wire is led to the connections of the observing telescope, and thus the mines can be fired by judgment if required, without the aid of the circuit closer.
The signal battery current is always circulating, even when the system is in a state of rest, but in consequence of the resistance placed in this circuit, which may be either a resistance coil in the circuit, added to the resistance of the fuzes, when high tension fuzes are used, or only the former resistance in the case of low tension fuzes, this current is too feeble to form an electro magnet; directly, however, a circuit closer is struck, this resistance is cut out, and thus the signal battery current becomes sufficiently powerful to work the electro magnet of that particular mine.
The circuit of the signal battery, and that to the observing telescope, are broken the instant the lever commences to fall.
To enable the apparatus to be used on the circuit breaking system, a spare lever _E_ is provided for that purpose with each box.
The object to be gained by a system of testing is to ascertain the condition of the electrical submarine mines placed in the defence of a harbour, &c., and should there exist any fault, not only to detect its exact position and cause, but also its magnitude, so that it may be at once determined whether it is necessary to remedy the fault, or whether the electrical apparatus is sufficiently powerful to overcome the defect.
_Tests._--There are two distinct kinds of tests, viz.:--
1.--Mechanical tests. 2.--Electrical tests.
Mechanical tests are applied to ascertain that the mechanical arrangements of the shutter apparatus, circuit closers, and all similar appliances work efficiently and easily; that the several parts of the mine case when put together for service are thoroughly watertight; that the chains, wire cables, and ropes in connection with the mooring apparatus are of sufficient strength to perform the work required of them; that the weights of the anchors, or sinkers, are such as to keep the mines in position after submersion; and that the case of the mine be sufficiently strong to enable it to bear the external pressure due to the depth at which it may be submerged for a considerable time without any leakage.
The foregoing tests of the mine case and moorings would of course be performed during the process of manufacture, but to prevent any chance of failure they should be repeated before being employed on actual service.
_Electrical Tests._--Electrical tests are those which are applied to the several component parts of the system, to ascertain that the electrical conditions necessary to a successful result exist.
The importance of being able to carry out the above in its entirety is understood when it is remembered that a submarine mine becomes practically valueless unless it acts efficiently at the single instant of time that it would be required so to do.
_List of Instruments used in Testing._--The following are some of the instruments that are employed in connection with a system of electrical tests:--
1.--Thomson's electrometer. 2.--Thomson's reflecting galvanometer. 3.--Astatic galvanometer. 4.--Differential galvanometer. 5.--Detector galvanometer. 6.--Three coil galvanometer. 7.--Thermo galvanometer. 8.--Siemens's universal galvanometer. 9.--A shunt. 10.--Commutator. 11.--Rheostat. 12.--Resistance coils. 13.--Wheatstone's balance.
Electrometers indicate the presence of a statical charge of electricity, by showing the force of attraction or repulsion between two conducting bodies placed near together. This force depending in the first place on the quantity of electricity with which the conducting bodies are charged, ultimately depends on the difference of potential between them; an electrometer is therefore strictly an instrument for measuring difference of potential.[J]
Sir William Thomson's quadrant electrometer is the most perfect form of electrometer yet constructed, and the one usually employed in cable testing. It consists of a very thin flat aluminium needle spread out into two wings, and hung by a wire from an insulated stem inside a Leyden jar, which contains a cupful of strong sulphuric acid, the outer surface of which forms the inner coating of the Leyden jar. A wire stretched by a weight connects the aforesaid needle with this inner coating. A mirror, rigidly attached to this needle by a rod, serves to indicate the deflection of the needle by reflecting the image of a flame on to a scale. The needle hangs inside four quadrants, which are insulated by glass stems: each pair of opposite quadrants are in electrical connection. Above and below the quadrants two tubes, at the same potential as the needle, serve to screen it and the wires in connection with it from all induction except that produced by the four quadrants. Suppose the needle charged to a high negative potential (-), then if the quadrants are symmetrically placed, it will deflect neither to the right nor to the left, so long as the near quadrants are at the same potential. If one of these be positive relatively to the other, the end of the needle under them will be repelled from the negative quadrant to the positive one, and at the same time the other end of the needle will be repelled from in the opposite direction. This motion will be indicated by the motion of the spot of light reflected by the mirror, and the number of divisions which the spot of light traverses on the scale measures in an arbitrary unit the difference of potential between the + and - quadrants.
The reflecting electrometer being a very delicate instrument, requires careful handling, and should only be used by a practised electrician. Its use would therefore be restricted to important stations, and special tests of a delicate nature.
_Thomson's Reflecting Galvanometer._--A galvanometer is an instrument intended to detect the presence of a current and measure its magnitude.
The most sensitive galvanometer as yet constructed is the reflecting galvanometer of Sir William Thomson, a diagram of which is shown at Fig. 84.
A small piece of magnetised steel watch spring, 3/8ths of an inch long, is fastened with shellac on the back of a little round concave mirror, and of about the size of a fourpenny piece. This is suspended by a piece of unspun silk thread in the centre of a coil of many hundred turns of fine copper wire insulated with silk, and well protected between the turns with varnish. The two ends of the coils are soldered to terminal screws _a_, _b_, so that any conducting wire can be joined up to it as required. The little mirror hangs in the middle of its coil, with the magnet lying horizontally. By means of a lamp _L_ placed behind the screen, the light of which passes through a slit _M_, and is thrown on the face of the mirror, a spot of light is reflected on the scale _N_.
When a current passes through the coil, the little magnet is deflected, and since the magnet is attached to the mirror, which is very light, both are deflected as forming one body, and the spot of light moves accordingly along the scale _N_.
A powerful steel magnet _S_ is placed above the coil, and can be moved up or down, whereby the directive force of the earth may be increased or weakened. This magnet _S_ is used to steady the spot of light, which otherwise would shake about, and there would be no certainty about the measurement. A second magnet _T_ is placed perpendicular to the magnetic meridian, to adjust the zero of the instrument, i.e., to bring back the spot of light to a fiducial mark at the centre of the scale when no current is passing.
This instrument should only be used at important stations, and when special tests of a delicate nature are required to be applied.
_Astatic Galvanometer._--An astatic galvanometer is that in connection with which an astatic needle is employed, by the use of which the sensitiveness of a galvanometer is greatly increased.
An astatic needle is a combination of magnetised needles _with their poles turned opposite ways_.
At Fig. 85 a diagram of such an instrument is shown. Two magnets _D_ and _C_ are joined, with the north pole of one over the south pole of the other, forming one suspended system. In the ordinary form of astatic galvanometer the needles _D_ and _C_ are about two inches long, and are each covered by a coil, these latter being so joined that the current must circulate in opposite directions round the two so as to deflect both magnets similarly. The deflection of the needles _D_ and _C_ is observed by means of a pointer or glass needle _A_, _B_, rigidly connected with the astatic system by a prolongation of the brass rod connecting the needles _D_ and _C_. The coils are flat and of the shape indicated in Fig. 85, and are also made in two halves, placed side by side with just sufficient space between them to allow the rod to hang freely.
This form of galvanometer, though less delicate than the preceding one, is still a very sensitive one, and should only be applied in the case of fine and delicate tests.
_Differential Galvanometer._--A differential galvanometer consists of a magnetic needle surrounded by two separate coils of equal length and material carefully insulated from each other and wound in opposite directions. In using it one circuit acts against the other. If a current of equal strength were passing through each there would be no deflection of the needle, because the influence in both directions is equal. If one current were stronger than the other, the needle would be deflected by the stronger.
This form of galvanometer will be found extremely useful in connection with a system of electrical tests.
Latimer Clark's double shunt differential galvanometer is the instrument best adapted for submarine mine tests.
_Detector Galvanometer._--A detector galvanometer is usually made with a vertical needle, and is employed to detect and roughly estimate the strength of a current where no particular accuracy is required.
It consists of a magnetic needle pivoted in the centre of a coil of insulated wire, and having an index needle attached to move with it, the latter appearing on a dial, divided into 360 equal arcs or portions: a diagram of such an instrument is shown at Fig. 86.
This instrument should be of small size and portable form, and as sensitive as it is possible to make it, under such conditions.
_Three Coil Galvanometer._--The three coil galvanometer is provided with a vertical needle, and is in other respects very similar in appearance to the detector galvanometer before described. It is formed with three coils of 2, 10, and 1000 ohms resistance; each coil is connected with a brass plate on the top of the box which encloses the whole, and may be switched into circuit by means of a plug at will. The object of the three resistances is to suit the different resistances that may occur, with a perfect, or imperfect state of the electrical combination in connection with each mine. A diagram of this instrument is shown at Fig. 87, the dotted portions are inside the case.
_Thermo Galvanometer._--A thermo galvanometer is an instrument used to ascertain the power of a firing battery which is employed to ignite platinum wire or low tension fuzes.
The form of thermo galvanometer generally used in connection with a test table, is arranged as follows:--
Two ebonite studs, fitted with brass connecting screws, are fixed to the lid of a box containing some resistance coils, and placed in circuit with them; these studs, placed about ·3 of an inch apart, are arranged to receive a piece of platinum wire which is stretched from one stud to the other; the firing battery being placed in circuit with the platinum wire, and the resistance coils, its working power would then be tested by the fusion of the wire through a given electrical resistance, as indicated by the resistance coils put in circuit.
Another form of thermo galvanometer, which is very compact and portable, is shown at Fig. 88. It consists of a wooden box _a_, with a cover of ebonite _b_, within the box is placed a resistance coil _c_; _d_ and _e_ are two ebonite standards ·3" apart, the former of which is connected by a copper wire with the terminal _f_, the latter to the terminal _g_; the terminal _h_ is similarly connected to the contact piece _k_, and the terminal _l_ to the firing key _m_, at _n_; the resistance coil _c_ is connected to the terminal _g_ and to the copper wire _n_; the platinum wire (of which several lengths are used, according to the resistance of the coil _c_) is placed between the standards _d_ and _e_. To test a battery, it is only necessary to connect it to the terminals _f_ and _h_, when by pressing down the key _m_ the power of the battery, according as to its fusing or not the platinum wires, will be ascertained; the use of the terminals _g_ and _l_ is to cut out the resistance, which is effected by connecting them by means of a copper wire.
_Siemens's Universal Galvanometer._--Siemens's universal galvanometer is an instrument combining in itself all the arrangements necessary for the following operations:--
1.--For measuring electrical resistances. 2.--For comparing electromotive forces. 3.--For measuring the intensity of a current.
The instrument which is shown in elevation and plan at Pl. xxiii., Figs. 1 and 2 respectively, consists of a sensitive galvanometer which can be turned in a horizontal plane, combined with a resistance bridge (the wire of which bridge instead of being straight is stretched round part of a circle). The galvanometer has an astatic needle, suspended by a cocoon fibre, and a flat bobbin frame wound with fine wire. The needle swings above a cardboard dial divided in degrees; as however, when using the instrument the deflection of the needle is never read off, but the needle instead always brought to zero, two ivory limiting pins are placed at about 20 degrees on each side of zero.
The galvanometer is fixed on a graduated slate disc, round which the platinum wire is stretched. Underneath the slate disc three resistance coils of the value of 10, 100, and 1000 Siemens' units are wound on a hollow wooden block, which protrudes at one side, and on the projection carries the terminals for the reception of the leading wires from the battery and unknown resistance. The adoption of three different resistance coils enables the measuring of large as well as small resistances with sufficient accuracy.
The whole instrument is mounted on a wooden disc, which is supported by three levelling screws, so that it may be turned round its axle. On the same axle a lever is placed which bears at its end an upright arm, carrying a contact roller. This roller is pressed against the platinum wire round the edge of the slate disc by means of a spring acting on the upright arm, and forms the junction between the _A_ and _B_ resistances of a Wheatstone's bridge, which resistances are formed by the platinum wire on either side of the contact roller, one of the three resistance coils forming the third resistance of the bridge. _G_ is the galvanometer, _k_ a milled head from which the needles are suspended, and by turning _k_ they can be raised or lowered, _m_ is the head of a screw which arrests or frees the needle when in motion. _h__{1}, _h__{2}, _h__{3}, _h__{4}, are the terminals of the respective ends of the three resistance coils, viz., 10, 100, and 1000 units, which are wound on the wooden block _C_; these terminals may be connected to each other by means of stoppers, and therefore one or more of the resistances may be brought into circuit as desired, and to the ends of these terminals the wires of the artificial resistances are connected as shown on diagrams Pl. xxiv., Figs. 1, 2, 3_a_ and 3_b_; _f_ is the graduated slate disc, round which the platinum wire is stretched in a slight groove at the edge of the disc, and is inserted in such manner that about half its diameter protrudes beyond the slate. The ends of the platinum wire are soldered to two brass terminals _l_ and _l_^{1}, which are placed at the angles formed by the sides of the gap in the slate disc, and which form the junctures, as in the ordinary resistance bridge, between _A_, _n_, and the galvanometer on one side, and _B_, _X_, and the galvanometer on the other side, of the parallelogram. The terminal _l_ is permanently connected by a thick copper wire or metal strip to terminal _h__{1}, and the other terminal _l_^{1} is connected in a similar manner to terminal III.
Slate is adopted for the material of which to make the disc _f_, because it is found by experience to be the material which is the least sensitive to variations in the weather or temperature.
The slate disc is graduated on its upper edge through an arc of 300 degrees, zero being in the centre, and the graduations figured up to 150 on each side at the terminals _l_ and _l_^{1} of the bridge wire.
In the centre of the circular plate _E_ of polished wood, supported upon three levelling screws _b_, _b_, _b_, a metal boss is inserted, in which turns the vertical pin _a_ which carries the instrument. This pin, being well fitted to the boss, supports the instrument firmly, but at the same time allows it to be turned freely round its vertical axis without losing its horizontal position when once obtained.
On the arm _D D_, which turns on the pin _a_, and somewhat behind the handle _g_, there is a small upright brass arm _d_ turning between two screw points _r_, and carrying in a gap at its upper end a small platinum jockey pulley _e_ turning on a vertical axis. This pulley forms the movable contact point along the bridge wire, against which it is kept firmly pressed by means of a spring acting on the arm _d_. The arm _D D_, which is insulated from the other parts of the apparatus, is permanently connected with the terminal I. On the top of _d_ a pointer _Z_ or a vernier is fixed, which laps over the upper edge of the slate disc and points to the graduations.
To the pin _a_ is attached a circular disc of polished wood _C_, about one inch thick, and having a groove turned in its edge for the reception of the insulated wires composing the resistances. The disc _C_ has a projection _c_, which carries the five insulated terminals marked I., II., III., IV., V., as shown on Figs. 1 and 2, Pl. xxiii. Terminals III. and IV. can be connected by a plug, II. and V. by the contact key _K_. Terminal I. is in connection with the lever _D D_.
Figs. 3 and 4, Pl. xxiii. show the shunt box supplied with the galvanometer if specially desired; the copper connecting arms _a_, _a_ are screwed to the terminals II. and IV. By inserting a plug at _c_ (Fig. 4, Pl. xxiii.), the galvanometer is put out of circuit altogether, whilst by plugging either of the other holes shunts of the value of 1/9, 1/99, or 1/999, are introduced into the circuit, and the effect upon the galvanometer is reduced to 1/10, 1/100, 1/1000, respectively of what it would have been without the insertion of the shunt.
Figs. 5 and 6, Pl. xxiii., show a battery commutator allowing to bring into the circuit four different amounts of battery power. It is placed in the battery circuit whenever consecutive tests with different batteries are desired to be made, it being only necessary to change the place of the stopper in the battery commutator, the terminal screw _a_ of the battery commutator being connected to terminal V. of the galvanometer, and the screws _b_, _b_, _b_, _b_ to various sections of the battery: see diagram of connections, Fig. 4, Pl. xxiv.
The application of the universal galvanometer will be clear from the diagrams on Pl ii.; instructions, however, for its practical use are added further on, and also tables for use when measuring conducting resistances.
As will be seen from diagram, Fig. 1, Pl. xxiv., the proportion between the unknown resistance X, and the artificial resistance _n_ is, when the deflection is read off on the side of the slate disc marked _A_:
X : _n_ = 150 + _a_ : 150 - _a_
or, X = ((150 + _a_) / (150 - _a_)) × _n_.
but if read off on the _B_ side of the disc--
X = ((150 - _a_) / (150 + _a_)) × _n_.
The values of these two fractions, for every half degree, will be found in the columns headed _A_ and _B_ of the table in the Appendix.
_Measuring Electrical Resistances._--For this purpose the instrument is arranged as a Wheatstone's balance. The connections are made as shown at Pl. xxiv., Figs. 1 and 5, where _X_ is the unknown resistance.
_a._--The needle _i_ is to be brought to the zero point of the small cardboard scale by turning the galvanometer _G_ round its vertical axis, taking care that the needle moves with perfect freedom.
_b._--The pointer or vernier _Z_ is to be brought, by means of the handle _g_, to the zero point of the large scale on the slate disc.
_c._--A plug is to be inserted between the terminals marked III. and IV.
_d._--The holes 10, 100, and 1000 are, two of them, to be plugged, and one left open, according to the extent of the unknown resistance to be measured; either 10 or 100 must be left open if the resistance is small, and 1000 if it is large.
_e._--The two ends of the unknown resistance are to be connected to terminals II. and IV.
_f._--The two poles of some galvanic battery are to be connected to terminals I. and V.
When the above-mentioned connections have been made, and on depressing the key _K_, the battery current is sent into the combination and deflects the needle, say, to the right-hand or _B_ side of the instrument, the pointer or vernier _Z_ must then be pushed, by means of the handle _g_, to the _B_ side of the instrument. If this is found to increase the deflection of the needle _i_, the pointer _Z_ should be pushed to the other or _A_ side of the instrument beyond the zero point of the large scale until the needle remains stationary when the key _K_ is depressed.
The number indicated by the vernier _Z_ should be read off carefully, and notice taken whether it is on the _A_ or _B_ side of the large scale. This number must then be referred to the galvanometer table,[K] when the figure opposite to the number, multiplied by the resistance unplugged, is the resistance of _X_. The value of the resistance to be determined will be thus found by a single operation.
Supposing the reading to be 50 on the _A_ side of the large scale, the resistance _n_ unplugged having been 100 units, we get according to the before-mentioned law of resistance bridge the following proportion (see Fig. 5, Pl. xxiv.):--
X : 100 = 150 + 50 : 150 - 50
X = ((150 + 50) / (150 - 50)) × 100
X = 200 units.
For measuring very small resistances a single cell will be found sufficient; but for large resistances more should be used, say, 15 to 20. If very accurate measurements of small resistances are to be taken, the screw at the end of the moving arm _D D_ should receive one battery wire, terminal V. receiving the other.
_Comparing Electromotive Forces._--For this purpose Professor E. du Bois-Reymond's modification of Poggendorff's compensation method is used.
The connections are made as shown at Pl. xxiv., Figs. 2 and 6.
For comparing two electromotive forces _E__{1} and _E__{2}, a third electromotor of higher electromotive force _E__{0} is used, and two separate tests taken.
The manipulations _a_ and _b_ are to be the same as before.
_c._--The hole between III. and IV. to be left unplugged.
_d._--Plugs to be inserted in 10, 100 and 1000.
_e._--The two poles of the electromotor of an electromotive force _E__{0} are to be connected to the terminals III. and V.
_f._--The poles of the battery whose electromotive force _E__{1} is to be compared are connected to terminals I. and IV. in such a manner that the similar poles of the two electromotors are joined to terminals I. and III., and to IV. and V. respectively.
When depressing the key _K_ the galvanometer needle will be deflected and can be brought back to zero by turning the pointer _Z_ either to the right or to the left. Should for instance the pointer have to be brought to 30° on the _A_ side we have the following equation--
E_{1} = E_{0} × ((150 - 30) / ( 300 + _n_)) (1),
where _n_ is the resistance of the battery _E__{0}.
The electromotor _E__{2} is now to be inserted in the place of _E__{1}, and the galvanometer needle, when it deflects, again brought back to zero by moving the pointer _Z_. If for instance the pointer has to be pushed to 40° on the _B_ side to obtain equilibrium we have--
E_{2} = E_{0} × ((150 + 40) / ( 300 + _n_)) (2).
By eliminating _n_ from equations 1 and 2 we have
E_{1} : E_{2} = (150 - 30) / (150 + 40) = 12 : 19 (3).
The two electromotive forces are in the same proportion as the two observed distances of the pointer _Z_ from 150° on the _A_ side of the instrument.
_For measuring the Intensity of a Current._--For this purpose the instrument is simply used as a sine galvanometer. The connections are made as shown at Pl. xxiv., Figs. 3_a_ and 7.
The manipulations _a_, _b_, _c_, and _d_ same as in the second case.
_e._--Connect one pole of a battery to terminal II. and put the other pole to earth.
_f._--Connect the line to terminal IV.
The galvanometer is then to be turned in the same direction as the needle is deflected until the needle coincides with the zero point. Whilst this is being done the large scale on the slate disc will move under the pointer _Z_, which must be left stationary; the sine of the angle indicated by _Z_ will thus give the value proportionate to the strength of the current. Should the shunt box be required, it has to be connected with terminals II. and IV.
Fig. 4 shows the same connections as Fig. 7, but without the shunt box, and with the battery commutator. Fig. 3_{a} shows diagram of the same connections but with the key _K_, and Fig. 3_{b} the same without the key.
_A Shunt._--A "Shunt" is a second path offered to a current traversing a given circuit, or portion of a circuit, so as to diminish the amount of the current flowing through that portion of the circuit. In the diagram shown at Fig. 89 the shunt diminishes the amount of the current flowing along the circuit between _A_ and _B_.
If only 1/Nth of the current is to pass along the circuit between _A_ and _B_ (of resistance _R_) then the resistance of the shunt must equal R/(N - 1).
By the aid of shunts it is quite possible to make use of very sensitive instruments to measure powerful currents.
_Commutators or Switch Plates._--A commutator or switch plate is an apparatus by which the direction of currents may be changed at will, or by which they may be opened or closed. Bertin's commutator, which is represented at Fig. 90, consists of a small base of hard wood on which is an ebonite plate, this by means of the handle _m_ is turned about a central axis between two stops _c_ and _c'_. On the disc are fixed two copper plates, one of which _o_ is always positive, being connected by the axis and by a plate (+) with the binding screw _P_, which receives the positive electrode of the battery; the other copper plate _i_, _e_, bent in the form of a horse-shoe, is connected by friction below the disc with a plate (-), which plate is connected with the negative electrode _N_. On the opposite side of the board are two binding screws _b_, and _b'_, to which are attached two elastic metal plates _r_, and _r'_.
On the disc being turned as shown in the figure, the current coming by the binding screw _P_ passes into the piece _o_, the plate _r_, and finally the binding screw _b_, which by means of a copper wire leads the current to the apparatus in connection with _b_; then returning to the binding screw _b'_, the current reaches the plate _r'_, the piece _i_, _e_, and so to the battery by the binding screw _N_.
If the disc is turned so that the handle _m_ is half way between _c_ and _c'_, the pieces _o_ and _i_, _e_, being no longer in contact with the plates _r_ and _r'_, the current will not pass. If _m_ is turned as far as _c_, the plate _o_ will then touch _r'_, and the current pass to _b'_, and return by _b_, thus reversing its direction.
"Peg" switches are also often used; they are arranged so that the removal or insertion of a brass peg or plug cuts out, or completes a circuit.
_Rheostat._--A rheostat is an instrument used for the comparison of resistances.
Wheatstone's rheostat, which is shown in elevation at Fig. 91, consists of two cylinders _A_ and _B_, one of brass and the other of non-conducting material, so arranged that a copper wire can be wound off the one on to the other by turning a handle _C_. The surface of the non-conducting cylinder _B_ has a screw thread cut in it for its whole length, in which the turns of the copper wire lie, so that its successive convolutions are well insulated from each other. Two binding screws _D_, _D'_ connected with the ends of the copper wire are provided, to which the circuit wires are connected. A scale is attached at _E_, by means of which the number of convolutions on _B_ can be read off; and parts of a revolution are indicated on a circle at one end. The handle _C_ can be shifted from one cylinder to the other.
Supposing the rheostat introduced into a circuit, and the whole of the copper wire wrapped on the metal cylinder _A_, then, on account of the large section of this metal cylinder, its resistance may be entirely neglected, but for every convolution of the wire on the non-conducting cylinder =B=, a specific resistance is introduced into the circuit. The amount of resistance can thus be varied as gradually as desired by winding on and off the cylinder _B_. This instrument is often used in connection with the thermo galvanometer.
_Resistance Box._--The general arrangement of a resistance box is shown in the diagram Fig. 92.
Between two terminal binding screws _T_ and _T__{1} secured on a vulcanite slab are fixed a series of brass junction pieces _a_, _b_, _c_, _d_; each of these is connected by a resistance coil to its neighbour, as shown at 1, 2, 3, and 4. A number of brass conical plugs with insulating handles of vulcanite are provided, which can be inserted between any two successive junction pieces, as between _T_ and _a_, or _a_ and _b_.
With all the plugs inserted, the electrical current will flow direct from _T_ to _T__{1}, the large metallic junction pieces directly connected by the plugs would offer no sensible resistance; but if all the plugs were removed, then the current would flow through each of the coils 1, 2, 3, and 4, and the resistance in the circuit would be the sum of the resistances of those four coils. With the plugs arranged as in the figure, the current would flow through coil 4 only, and the resistance in the circuit would be equal to the resistance of that coil.
_Wheatstone's Balance._--The electrical conductivity of a body is determined by ascertaining the ratio between the resistance of a certain length of the conductor in question, having a given section, to that of a known length of a known section of some substance taken as a standard.
For this purpose Wheatstone's bridge in connection with a box of resistance coils is the most convenient method.
At Fig. 94 is shown Wheatstone's balance (Post-office pattern), and at Fig. 93 the apparatus is reduced into the form of a parallelogram, which is the usual diagram of Wheatstone's bridge. The theory of the bridge is as follows:
Four conductors _A B_, _B C_, _A D_, and _D C_ are joined at _A_ and _C_ to the poles of a battery _Z_; the resistance between _A_ and _B_ is _R_; that between _A_ and _D_ is _r_; that between _D_ and _C_ is _R__{1}; and that between _B_ and _C_ is _x_, the unknown resistance to be measured. A convenient constant ratio is chosen for _R__{1} and _r_, such as equality 1 to 10, 1 to 100, or 1 to 1000; and then _R__{1} is adjusted until no current flows through the galvanometer _G_; when this is the case we have R : _r_=R_{1} : _x_, or _x_ = (_r_/R) × R_{1}; so that if _r_ = R/100, _x_ will be equal to R_{1}/100.
Two keys _a_ and _b_ are inserted; the current is wholly cut off the four conductors until contact is made at _a_; and then after the currents in the four conductors have come to their permanent condition, contact is made at _b_ to test whether any current flows through the galvanometer. The three resistances _R_, _R__{1} and _r_ and the resistance of the galvanometer should be small if _x_ is small, and great if _x_ is great.
The conductors _A B_ and _A D_ of the bridge are each formed of three resistance coils having a resistance of 10, 100, and 1000 ohms respectively, inserted between the terminals _B_ and _D_ of the balance, Fig. 94.
The conductor _D C_ is formed of a set of resistance coils from 1 up to 4000 ohms, amounting altogether to 11,110 ohms, inserted between the terminals _D_ and _C_ of the balance; in the balance, a brass plug being inserted between the terminals _D_ and _D__{1}, they may be considered as one terminal _D_. The conductor _B C_ is the wire to be tested, and is connected to the terminals _B_ and _C_ of the balance.
_Measurement of Resistances._--When a resistance is to be measured that is within the range of the coils in _R__{1}, _R_ and _r_ are made equal. The needle of the galvanometer will move in a different direction, either to the right or to the left, according as the resistance in _R__{1} is greater or less than the line wire _x_. The needle remains at zero only when the resistance in _R__{1} is equal to that in _x_. For _r_ : _R_ :: _R__{1} : _x_.
When the resistance of _x_ is greater than that of _R__{1}, as in an insulation test, the resistance in _r_ is made _less_ than that in _R_, in order that _r_ and _R_ may have such a proportion one to the other as will enable the coils in _R__{1} to balance a resistance in _x_, greater than their own, that is to say, greater than 11,100 ohms; thus _r_ : _R_ :: _R__{1} : _x_, or 10 : 1000 :: 10,000 : 1,000,000, the resistance in the line to be tested would be 1,000,000 ohms, supposing the values of _r_, _R_ and _R__{1} to be respectively 10, 1000, and 10,000 ohms.
When the resistance to be tested is less than that of the least coil in _R__{1} (1 ohm), then the resistance in _r_ is made greater than in _R_. Thus _r_ : _R_ :: _R__{1} : _x_, or 100 : 10 :: 2 : 0·2; the resistance of the line to be tested would in this case be 1/20 of an ohm.
_Manipulation._--In all cases the key in connection with the battery should first be depressed, then the galvanometer key, making very short contacts by the latter, just sufficient to show the direction of the deflection, until the coils in _R__{1} are nearly adjusted, otherwise considerable time will be lost in making a series of tests, owing to the swing given to the needle, which will take some little time before it again remains steady at zero. When once the coils in _R__{1} are adjusted, and a balance obtained, it should be ascertained whether the needle will remain steady when contact is made and broken.
_Test Tables._--In connection with a system of testing electrical submarine mines, for the sake of convenience and simplicity it is necessary to use a table (termed a "Test Table"), on which all the apparatus used for the purpose of testing are fixed. Several forms of tables have been designed for such a purpose. At Fig. 95 is shown the method of arranging such a table.[L]
_A_ is an astatic galvanometer placed between two switch plates, _B_ and _C_; ten other similar switch plates, 1, 2, 3, 4, _D_, 5, 6, 7, _E_, and 8, are arranged in front of the galvanometer _A_; _F_, _G_, and _H_ are three terminal plates; _K_ is a box of resistance coils used in connection with the thermo galvanometer _M_; _L_ is a firing key, and _N_ a battery commutator; _O_ is a three-coil galvanometer; _R_ is a Wheatstone balance (Post-office pattern).
The ten switch plates, 1, 2, 3, 4, _D_, &c., are used for the connection of any particular line to be tested, as well as for the earth connections and instruments employed in that operation.
_"Sea Cell" Tests._--The arrangement shown in the figure is that required in connection with the sea cell test, and Mr. Brown's method of keeping certain earth plates in a bucket instead of in the sea.
If two plates of suitable metal to form a Voltaic battery are placed in salt water and connected by a metallic conductor, a battery is at once formed capable of producing considerable deflection on a moderately delicate galvanometer. Testing by this arrangement has been termed the "sea cell" test.
_Arranging Earth Plates._--Mr. Brown's, Assistant-Chemist to the War Department, method of arranging the earth plates is as follows:--
A series of earth plates, such as copper, carbon, tin, zinc, &c., are placed in a bucket filled with sea water, and which is placed in the testing room. The water in the bucket is put in connection with the water of the sea by means of a conducting wire, terminating at one end with a zinc plate in the bucket, and at the other with a zinc plate in the sea. By this means the tests made with the different earth plates in the bucket are identical with those made with corresponding earths placed absolutely in the sea, and therefore these latter may be done away with, the sea cell tests being entirely carried out by means of the bucket earth plates.
In addition to the bucket earth plates there will be several other earth plates in connection with the testing room, these being placed in the sea, such as the zinc earth for the firing battery, the zinc earth for the signalling battery, &c.
_Connections of Switch Plates._--The switch plate _D_ is used for the connection of any particular mine cable which it may be required to test. The switch plate _E_ is connected with a zinc earth plate used for testing the firing battery. This must always be in the sea. The switch plate 1 is in connection with a zinc earth in the bucket; 2 is attached to a copper earth plate in the bucket; 3 is attached to a carbon earth plate in the bucket; 4 to a tin earth plate in the bucket; 5 is used for connection with the zinc signalling earth connection in the sea; 6 is attached to a copper earth plate used for the sea cell test, or any other purpose required, in the sea; 7 is attached to a zinc earth plate in the sea; and 8 is a common zinc earth in the sea.
The terminal plates _G_ and _H_ are used for the connection, for testing purposes of the negative and positive poles, of the firing battery, and _F_ is connected with a zinc earth in the sea, for a similar purpose. These plates are in connection with the resistance coils _K_ and the thermo galvanometer _M_, employed for testing the firing battery, the circuit being closed by the firing key _L_. Other ways of using these plates may of course be adopted if desired. The resistance coils _K_ range from 0·5 to 100 ohms, and are composed of wire adapted for the passage of a quantity current. A reversing key is generally used in connection with a testing battery and the three-coil galvanometer _O_. This reversing key would consist of two bridges completely insulated from each other, the upper one attached to the negative, the lower one to the positive pole of the test battery. In their normal position both keys press against the upper bridge, and until one or other of the keys is pressed down no current will pass, the direction of the current being altered by pressing down a different key. The point of each key is provided with a terminal and connected, the one to a zinc earth through the switch plate 8, the other to one terminal of the three-coil galvanometer when the tests are to be applied.
The Wheatstone balance _R_ is used in finding the resistances of electrical cables, balancing fuzes, &c. By means of a commutator, _N_, the necessary number of cells for any particular test may be thrown in circuit when required.
_Test of Platinum Wire Fuze for Conductivity._--The platinum wire fuze may be tested electrically as follows:--
If placed in circuit with a few cells of a Daniell or Leclanché battery and a detector galvanometer, before the platinum wire bridge of the fuze is fixed, there should be no deflection of the needle, for no metallic circuit exists; if it did, such would be fatal to the efficiency of the fuze. If similarly placed in circuit after the bridge has been fixed, a considerable deflection of the needle should result, such deflection being due to the current passing through the metallic bridge, which to be efficient ought to be the sole medium through which the circuit is completed.
_Test of Resistance of Platinum Wire Fuze._--The electrical resistance of a platinum wire fuze is ascertained by means of the Wheatstone's balance _R_ and galvanometer _A_, Fig. 95. The terminals of the fuze are connected to the binding screws of the balance, the commutator _N_ and galvanometer _A_ being connected up in circuit. The resistance of the coils is then adjusted by taking out plugs until the needle of the galvanometer _A_ is brought to zero, when the sum of the resistances indicated by the unplugged coils will be equal to that of the fuze. The resistance of a platinum wire fuze might also be ascertained by means of a differential galvanometer instead of a Wheatstone balance.
The electrical resistance of 3/10" of fine platinum wire, weighing 1·9 grains to the yard, is 3/10 of an ohm nearly (Schaw).
_Testing High Tension Fuzes._--High tension fuzes require very delicate and careful management in testing them, due to the high electrical resistance of such fuzes, which ranges from 1500 to 2000 ohms, combined with the danger of premature explosion when testing even with a small number of battery cells. Very sensitive galvanometers, such as the reflecting galvanometer, should if possible be used, otherwise the mode of making the tests for conductivity and resistance of a high-tension fuze is similar to that already given for a platinum wire fuze.
Detonating fuzes should always be placed in an iron case during the process of testing.
_Insulation Test for Electrical Cables._--To test an electrical cable for insulation, it should first be put in a tank of water, or in the sea, and allowed to soak for at least forty-eight hours. The object of this is to allow the water to penetrate the outer protection of hemp and iron wires, &c., and to search out and get into any weak places there may be in the insulation under the armouring. At Fig. 96 is shown the method of performing this test. _A_ is a tank holding the electrical cable, which has been in soak for forty-eight hours; _B_ is an astatic galvanometer; _C_, _Z_ a Leclanché or Daniell battery of great power; and _C_ is an ordinary firing key. One end of the electric cable _D_ is connected to the galvanometer _B_ through the firing key _C_; the other end of the cable is very carefully insulated; one pole of the battery is connected to the galvanometer _B_, the other is put to earth in the tank at _F_; should the insulation be perfect, no deflection of the needle should follow on the key being pressed down. A very slight deflection might be observed on a moderately sensitive galvanometer, due to the current passing through the insulation; its whole length being immersed, the surface through which such a current would pass would be large, and the sum of the infinitesimally small quantities escaping over the whole length, would in the aggregate be sufficient to deflect the needle to a small extent in completing the circuit of the battery. Should any considerable deflection occur, it would indicate a defect or leak in the insulation of the cable, the extent of which would be roughly measured by the amount of such deflection.
By using a reflecting galvanometer a very much more delicate test would be obtained, but for the comparatively short lengths of electric cables used in connection with submarine mines, such accuracy is hardly necessary.
To test an electric cable for conductivity, it would be only necessary to expose the metallic conductor _G_, and put it in the water of the tank. If the conductivity were good, then the whole of the current would pass through the cable and the needle of the galvanometer would be violently deflected. If the continuity were broken, no deflection would be observed.
_Defects observed in the Conductivity of the Cable._--To ascertain the position of a defect in the insulation of a cable, as indicated by the tests above described, it would be only necessary to keep a continuous current flowing through the cable, and gradually take it out of the tank. If the fault existed at a single point, the deflection of the needle would be suddenly reduced at the moment of that point of the cable being lifted out of the water, and therefore its position would be determined with considerable accuracy. Should several defects exist as each was lifted out, a sudden reduction of the deflection would occur.
_Discharge Test._--The conductor of an electrical cable may be broken without destroying the insulation, and on applying the foregoing tests, good insulation would be indicated, but no conductivity, and no information would be given as to the position of the fault. Under such circumstances the following test must be applied:--
Put one pole of a very powerful battery to earth, and charge one end of the defective cable, then immediately discharge it through a reflecting galvanometer, and note the extreme limit of the swing of the needle, then, charge the other end of the cable in a similar manner, and discharge it through the same galvanometer, noting as before the swing of the needle. This should be done three or four times, and the average of the deflections taken. Then the position of the fault would be indicated by the proportion between the average deflections in each case, and the cable might safely be cut at that point. Should the precise position of the fault not be discovered in thus cutting the cable, each section should be tested again for conductivity, and that in which a fault was still found to exist should be again tested by the discharge as before.
_Test of Electrical Resistance of Cable._--This is effected by balancing it against the Wheatstone balance, in a similar manner to that explained for a fuze. The electrical resistance of the conductor of a cable affords a very correct indication of the quality of the metal of which it is composed. For a very delicate test the reflecting galvanometer should be used.
_Electrical Test of Insulated Joints._--Insulated joints and connections, whether of a permanent or temporary nature, should be tested electrically, in a precisely similar manner to that explained for electric cables.
They should be soaked for forty-eight hours, and then tested for insulation, conductivity, and electrical resistance.
In testing permanent joints special tests are carried out, which are described by Mr. Culley in his 'Handbook of Practical Telegraphy.'
Voltaic batteries should be subjected to the following tests:--
1.--For potential. 2.--For internal resistance. 3.--For electromotive force.
For the purpose of testing the potential of a battery, one pole should be put to earth, and with the other one pair of the quadrants of a Thomson's reflecting galvanometer should be charged; when this is done, a certain deflection of the spot of light will occur, and the amount of such deflection, as compared with that produced by a standard cell applied to the instrument in a similar manner, would give the relative value of the potential of the battery.
The following method of determining the internal resistance of a battery is that recommended by Mr. Latimer Clark in his book on electrical measurements.
The instrument employed is a double shunt differential galvanometer, a diagram of which is shown at Fig. 97. Connect the battery and a set of resistance coils in circuit between the terminals _A_ and _D_, and insert plugs in the resistance coils so that they give no resistance; insert plugs at _A_ and _C_, and also both the shunt plugs at _A_ and _D_. The current will now flow through one half of the galvanometer circuit only, being, however, reduced to 1/100 of its amount by the shunt _D_; the deflection of the needle must be carefully read. The plug _A_ must now be removed to _B_, which causes the battery current to flow through both halves of the galvanometer (each being shunted). The circuit will now be as shown in the figure, and the needle will of course be deflected somewhat more than before. Now unplug the resistance coils which are in circuit with the battery until the deflection of the needle is reduced to its original amount, and the resistances unplugged will be equal to the internal resistance of the battery.
The following is another method of ascertaining the internal resistance of a battery cell.
A circuit is formed, consisting of the battery cell, a rheostat, and a galvanometer, and the strength _C_ is noted on the galvanometer. A second cell is then joined with the first, so as to form one of double the size, and therefore half the resistance, and then by adding a length _l_ of the rheostat, the strength is brought to what it originally was, _C_.
Then if _E_ is the electromotive force, and R the resistance of cell, _r_ the resistance of the galvanometer, and other parts of the circuit, the strength _C_ in the one case is C = E / (R + _r_), and in the other = E / ((1/2)R + _r_ + _l_), and since the strength in both cases is the same, R = 2_l_, i.e., the internal resistance of the cell is equal to twice the resistance corresponding to the length _l_ of the rheostat wire.
The comparative electromotive force of a battery may be determined by means of a double shunt differential galvanometer in the following method, as recommended by Mr. Latimer Clark.
"This can only be done relatively in terms of some other standard battery. First determine the resistance of the standard and of the other cells to be measured; then insert the shunt plugs at _A_ and _D_, Fig. 97, and also at _C_ and _B_, and join up the standard cell in circuit with a resistance coil to the terminals _A_ and _D_, and unplug the resistance coils until a convenient deflection is obtained, say 15°; note the sum of the resistances in circuit, including that of the battery galvanometer, resistance coil and connecting wires; now change the battery for another, and by unplugging the resistance coils bring the needle again to the same deflection, 15°; having again found the total resistance in the circuit, the relative electromotive force will be directly proportional to these resistances."
The electromotive force of a battery may also be measured statically by means of Thomson's quadrant electrometer, the poles of the battery being connected with the two chief electrodes of the instrument, in which arrangement no current will pass, and the electromotive force will be directly indicated by the difference of potential observed.
In the case of a quantity battery, that is, a battery capable of fusing a fine platinum wire, its electromotive force and internal resistance may be determined by means of the resistance coils _K_, and thermo galvanometer _M_, shown at Fig. 95.
_Tests after Submersion._--After an electrical submarine mine has been placed in position, it should be immediately tested to ascertain that all is right, and similar tests should be applied at intervals to ascertain that the charge remains dry; that the insulation and conductivity of the electric cable remains the same; and that its electrical resistance indicates a state of efficiency.
The nature of the tests applied to determine these points will depend upon the nature of the combination in which the mine is arranged.
The manner of applying the "sea cell" test, by which is ascertained the condition of a system of electrical submarine mines, will be readily understood from the following examples.
The arrangements for testing to ascertain whether a charge is dry, or wet, is shown at Fig. 98.
_z_ is a plate of zinc introduced in the circuit within the charge, and between the fuze and the shore; another earth plate of carbon _x_ is connected with the electric cable beyond the fuze, forming the ordinary earth connection of the system at that point; and at home a copper earth plate _c_ is used.
First, in the case of a dry charge with the insulation and conductivity of the cable, good; under these circumstances there would be formed a sea cell between the earth plates _x_, and _c_, which would produce a certain deflection of the needle of a galvanometer _g_, which is placed in the circuit, and in a certain direction.
Secondly, in the case of a charge becoming wet, through leakage, with the insulation and conductivity of the cable, good; under these circumstances, a sea cell would be formed between the plates _c_ and _z_, causing a different deflection of the needle in amount and in direction, by which it would be at once indicated that the charge had become wet.
_"Sea cell" Test for Insulation._--Again, in the case of the insulation of the electric cable being damaged to such an extent as to expose the copper conductor. Under these circumstances there would be formed a sea cell between the copper earth plate _c_, and the exposed copper conductor of the cable, by which a certain definite deflection of the galvanometer would be observed, which deflection would differ in character from that produced by the copper carbon sea cell, when the insulation of the cable was good, and the system in working order, and therefore it would indicate that some change in the electrical conditions of the system had occurred. The fact that a leak existed in the insulation would be proved by changing the earth plate at home from copper to zinc, carbon, tin, &c.
In the case of no deflection being produced on the galvanometer, on applying the sea cell test, a want of continuity, or inefficient connections would be indicated.
The foregoing afford examples of the vast utility of the "sea cell" in connection with a system of electrical tests for submarine mines, numerous variations of which may be effected by employing a series of earth plates, of different metals, at the home end of the circuit, in connection with a carbon and zinc earth plate at the other end. And the mode of manipulating these tests may, by means of numerous switch plates, as shown at Fig. 95, be made extremely simple and efficient.
_Armstrong's System of Electrical Testing._--A very simple method of testing electrical submarine mines, with which low tension fuzes are used, has been devised by Captain Armstrong, R.E., and is shown at Fig. 99. _a_ is the electric cable leading from the shore; _b_ the cable attached to a polarised relay _c_, and connecting the charge through the fuze _f_ to the earth; _b'_ the cable, attached to another polarised relay _c'_, and connecting the mine with the circuit closer; the polarised relay _c_, in the mine, is arranged to be worked by a positive current, that is to say, the wire surrounding the core is so wound as to increase the polarity of the electro magnet, near the armature _d_, when a positive current is passed through it, and to diminish the polarity when a negative current is passed through the wire surrounding the core; the polarised relay _c'_ within the circuit closer is arranged to be worked by a negative current, the coil being so wound as to produce an influence exactly the reverse of _c_.
Then, a positive current passing along the line wire _a_, the armature _d_ in the charge will be attracted, while _d'_ will remain unaffected; again, if a negative current be circulated, the armature _d'_ within the circuit closer will be attracted, while the armature _d_ will remain unaffected. Two insulated wires forked together are wound round each electro magnet, one a thin wire (_g_ and _g'_) having a considerable resistance, about 1000 ohms, being connected direct to the earth plates _e_ and _e'_, and the other a thick wire (_h_ and _h'_) offering a very small resistance, and so arranged that when the armature is attracted, they may be in contact with and complete the circuit through the armature to earth.
The thin wire coils are so arranged that a certain number of Leclanché cells (ten or twelve, as may be desired) will make the electro magnets act, while with fewer cells the current would be too weak, and would therefore pass through them to earth without affecting the armature.
By means of the three-coil galvanometer, a table of the deflections, obtained by the foregoing system of testing, should be carefully recorded, when the circuit is known to be in good working order, so that any defect in the circuit would be at once indicated on the application of the various tests, by the results so obtained differing from those originally recorded. When a system of submarine mines is placed in position for the purposes of practice and experiment, every trouble should be taken to endeavour to fix the exact position of any defect that may exist, also to ascertain its magnitude, &c., but in time of war, should a defect exist in the system, no time must be lost in such operations, but the mine at once lifted, and the fault repaired, or a fresh one laid in its place, unless the presence of an enemy or other imperative cause should prevent such work being done.
_Austrian Testing Table._--The following is a description of the Austrian testing table, and their mode of making electrical tests with it, in connection with their system of self-acting electrical submarine mines.
Its design is shown at Fig. 100; _c z_ represents the battery with one pole to earth at _e_, and the other in connection with an intensity coil _a_, through which the current passes to the contact plate _b_. When it is desired to put the system of mines in connection with the table, in a state of preparation to be fired by the contact of a vessel, a plug is inserted between the contact plates _b_ and _f_, and the current passes through the galvanometer _g_, and electrically charges the conducting wires connecting the mines with the battery, through the several binding screws on the contact plates, numbering 1, 2, 3, &c. The fact that the charge has been fired is also at once indicated on the galvanometer _g_.
_Test to discover an Exploded Charge._--It then becomes necessary to ascertain which particular mine of the system has been exploded; for this purpose a separate circuit in connection with a single cell _d_ is employed. This cell is in connection through a galvanometer _g'_ (a more sensitive instrument than the galvanometer _g_) with the pivot of the key _h_, and rheotome _R_, which latter is connected, as shown by the dotted lines, with each individual mine of the system attached to the contact plates numbered 1, 2, 3, &c. The handle of the rheotome is moved round, to each number in succession and directly it is placed in contact with that corresponding to the exploding mine, the electrical circuit is completed through the exposed end of the fractured wire, and this is indicated by the galvanometer _g'_. During the testing process the firing battery _c z_ must be disconnected; this is done by raising one of the bridges _i i_ with which each group of ten mines is provided.
_Insulation Test._--The rheotome and testing galvanometer _g'_ are also used to test the insulation of the electric cables connecting the mines to the testing table. This is done in precisely the same manner as testing for an exploded mine: the handle of the rheotome is turned round, and each cable connected in succession with the testing circuit as before; should the galvanometer _g'_ remain stationary, the insulation is good; but should a defect of insulation exist, the current passing through it would act on and deflect the galvanometer, indicating the particular line in which it exists, and, roughly, its extent in proportion to the deflection shown; should the fault be considerable, the defective cable should be at once detached, as the current lost through it might so diminish the working power of the firing battery, as to prevent it exploding any of the fuzes attached to the group in connection with it. By the above arrangement, the insulation of each line can be tested at any moment required.
In making the delicate test for insulation, which should invariably be done at leisure, and, if possible, when an enemy's vessels are not in the vicinity of the mines, a large number of Daniell's or other cells of suitable form should always be used. To do this, it would only be necessary to connect such a battery in place of a single cell permanently arranged, as described, in the testing circuit, and to proceed with the details of the operation as before. As the cable would, in actual work, always be charged with the full power of a firing battery, the value of its insulation to resist an electrical charge at such a high potential would be an important point to determine. The fuzes being entirely out of the circuit till the moment of the action arrives, no danger of a premature explosion need be apprehended; if a fuze were in such a position as to be fired prematurely, it would be exploded, in connection with the firing circuit, independently of the operation of testing the insulation of the cables.
_To render a Channel Safe._--In order to render the channel safe for a friendly vessel, it is only necessary to remove the plug from between the contact plates _b_ and _f_; this disconnects the firing battery from the circuit.
_Defence of Harbours by Booms, &c._--Booms or cables supported by rafts may also be employed in the defence of harbours, or rivers, either by themselves, or in combination with submarine mines; in the latter case, the booms, &c., may be moored either in advance of the mines, or in rear of the front row, this last method of mooring them being the most effective one.
There are a great variety of forms in which a boom may be constructed. The qualities essential for a good and practicable boom are:--
1.--Great strength. 2.--Great power of resistance. 3.--Convenience in handling. 4.--Easy to manipulate. 5.--Its materials easily procurable.
_Construction of a Boom._--The general construction of a boom consists of a main cable, buoyed up at intervals by floats. The main cable may be either wire, chain, or rope, the former being very much superior for this purpose to chain or rope. The floats consist of balks of timber built round the main cable and bound together by means of iron hoops &c. A space is left between each float, by which a certain amount of flexibility in the boom is obtained, without which it would be of comparatively little use, as it might be easily overrun.
It must be borne in mind, in constructing all such booms, that the smaller the proportion of timber used in forming the floats to the cable, consistent with buoyancy, the stronger will be the structure.
A very important feature in connection with such a mode of defence is the manner of mooring it; for if it be moored so as to be unyielding, then its sole power of resisting a vessel charging it is the actual strength of the materials composing the structure, but if it be moored so that it is capable of yielding to a sudden blow, this force will be to some extent absorbed, and resistance of the defence greatly increased.
The raft employed to support the main cable should be moored by means of very heavy chains (without anchors) in the direction of the attack, and with ordinary anchors and cables on the other side.
As a rule, the booms should be moored obliquely to the direction of the current, where there is any, as the tendency of the current to overrun the boom when so placed will be less, and also a ship ramming it must place herself athwart the current to attack the boom at right angles.
_Clearing a Passage through the Torpedo Defences of an Enemy._--The subject of clearing a passage through the torpedo defences of an enemy is one fraught with innumerable difficulties, on account of the varied nature and impracticability of obtaining accurate and _certain_ information of such defences, and thus it is impossible to lay down any fixed rule or plan for carrying out such an operation.
In fact, it will be only under the most favourable circumstances that such a service will be successfully accomplished, that is to say, in the case of a harbour or river defended by submarine mines but unsupported by guns, or guard boats, or where the electric light is used.
Numerous methods have been devised from time to time to effect the destruction of an enemy's submarine defences, among which are the following:--
1.--Projecting frames, &c., from the bows of a vessel. 2.--Creeping and sweeping by boats. 3.--Countermining.
_Projecting Frames, &c., from the Bows of a Vessel._--This method was adopted by the Federals during the American civil war of 1861-5, and in many instances it was the means of saving their ships when proceeding up rivers which had been torpedoed by the Confederates, though notwithstanding this precaution several vessels were sunk. The submarine mines against which this mode of defence was used, were in nine cases out of ten mechanical ones, and therefore the framework defence afforded a better means of protection then, than would be the case now that electrical ground mines and circuit closers are used, as the framework would catch the circuit closer only, and the vessel would probably be over the mine when the explosion took place. The Americans moor their circuit closers in rear of their mines, so that a vessel fitted with a bow frame or not, coming in contact with the former must be right over the charge at the instant of explosion.
Against ground electrical mines fired at will, the bow net, &c., is no protection whatever, still under certain circumstances it would be found extremely useful.
_Sweeping for Submarine Mines._--This method of clearing a channel of submarine mines could not possibly be carried out under artillery fire, but in waters not so defended it would prove of some value.
Where only buoyant mines, or ground mines with circuit closers are to be cleared away, two or more boats dragging a hawser between them would be sufficient to discover them, and so lead to their destruction; but where dummy mines and inverted creepers are moored in addition, another method of sweeping must be resorted to, viz., that of bringing an explosive charge of gun-cotton to act on the obstruction grappled, and thus destroy it. This is effected by lashing a charge to each end of the sweep, so that whatever is grappled may slide along it, until caught by hooks, which are attached for this purpose to the centre of the charge. On grappling an obstruction, the two boats drop their anchors, one hauling in, the other veering out the sweep, until the charge is hooked by the obstruction; this being effected, the boats move out of range, and the charge is fired.
_Creeping for Electrical Cables, &c._--Creeping is the method employed for picking up the electric cables of the enemy's submarine mines, and is effected by boats towing an ordinary grappling iron, or specially prepared creeper on the ground.
In both sweeping and creeping it would be found necessary to employ a diver, who would ascertain the nature of the grappled obstructions which could not be easily raised by the boats.
The Lay torpedo boat, which is fully described in the chapter on offensive torpedoes, is capable of being used for the foregoing purposes.
_Countermining._--Countermining, that is, the destruction of submarine mines by the explosion of other mines dropped close to them, will under certain conditions prove of great use in clearing harbours of mines. This method could not be operated in waters properly guarded and swept by artillery fire.
There are two distinct methods of laying out countermines, viz.:--
1.--In a boat, which may be either towed, or hauled out to its destination, or may be steered, and controlled by electricity.
2.--By attaching them to buoys so that they are suspended at the proper depths, and then hauled out by means of a warp to an anchor which has been previously placed in position.
Both of the foregoing methods have been successfully manipulated in practice, the first method, where the boat carrying the countermines is towed either by a pulling or steam boat being the most practicable one. A large amount of material would be required for clearing a channel by means of countermines: for example, if the mines to be attacked require 500-lb. gun-cotton charges to be used, 7-1/2 tons of the explosive, besides cables, buoys, &c., would be required to clear a passage about one mile in length and 200 feet in width.
A ship's launch will carry about twelve of these 500-lb. countermines, with all the gear attached thereto.
Experiments to ascertain the effect of countermining have been carried out in England and Europe for the last five years, some of which are given at length in the chapter on "Torpedo Experiments." During the Turco-Russian war, a portion of the Danube was swept in the ordinary and most simple manner by the Turks, and five Russian electro contact buoyant mines were picked up; one other exploded during the process of dragging it to the surface, but no injury occurred to those at work.
_Destruction of Passive Obstructions._--To clear away booms, or other passive obstructions, if not possible to cut them away, they may be destroyed by outrigger boats exploding their torpedoes underneath, and in contact, or by attaching charges of gun-cotton at intervals, and then exploding them simultaneously. When a chain is horizontal, and therefore somewhat taut, a charge of 3-1/2 lbs. of gun-cotton (this explosive, being the most effective and convenient for such purposes, should always be used) will be found sufficient to destroy it, no matter what size, and whether the chain is in or out of the water, the charge being of course placed in contact with it. Great uncertainty must always attend the supposed clearance of a channel, or passage of submarine mines, as was exemplified during the American civil war, when most of the Northerners' vessels were destroyed while moving over ground which had been previously carefully dragged, and buoyed, and this fact, coupled with the tediousness and danger of performing such a service, proves the enormous value of a system of defence by submarine mines.
FOOTNOTES:
[Footnote J: 'Electricity and Magnetism,' by Professor F. Jenkins.]
[Footnote K: See Appendix.]
[Footnote L: As constructed by Mr. J. Mathieson, late R.E., at the Silvertown Telegraph Works, Essex.]