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 Fig. 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, m1; then between these bobbins m, m1 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, m1, 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, m1.
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 m1 are connected, the one to the metal strip s1, the other to a piece of brass under the ebonite support l; from this latter piece of brass a wire w1 is led to the brass screw x. The wires w2, w3, 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 w4 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 w4, through the telephone which has a high resistance, then by the wire w2 through the fuzes, and to earth by the wire w3.
On the circuit closer being struck, by which cause the armature n is brought up to the cores of the bobbins m, m1, and the piece of brass r in contact with the metal strips s, s1, 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 s1, round the bobbin m1 (thus forming an electro magnet of m, m1), 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:—
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 Fig. 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 w1, 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 w2, 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 w1, and w2; 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 w1 and w2 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 w1 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:—
Methods of Firing by Observation.—There are several methods of firing by observation, of which the following are the ones principally used:—
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 m1, m2, m3, &c., and n1, n2, n3, &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 s1 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 m1, A m2, A m3, &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, m1, m2, &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 m5, C, and therefore the key of the mine m5, 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 m5 exploded, but when the vessel s reaches the position N, that is over the mine m3, she being on the prolongation of the lines A m3, and B E, the key (m3) at A, and the key at B would both be pressed down, and therefore the mine m3 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 s1,) 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, m1, m2, &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 Fig. 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.
Fig. 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, S1, S2, 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:—
In connection with a system of defence by electrical submarine mines, the following batteries are required:—
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:—
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:—
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:—
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 w1 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 w1, 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:—