Chapter IV · DEFENSIVE TORPEDO WARFARE—continued.
Reading edition
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like a practicable form of self-acting apparatus; but within the last few years, the former has almost entirely superseded the latter method, except in very exceptional cases; this revolution being due to the vast improvements that have been, and still are being effected in the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.
MATHIESON'S CIRCUIT CLOSER.
Circuit Breaker.—By altering the mode of connecting the wires, the above apparatus may be used as a circuit breaker, that is to say, the signal may be given, and the mine exploded by the cessation of a passing current, instead of by the closing of the electric circuit. This system was specially designed for use with platinum wire fuzes, but is rarely used.
Circuit Closer of Electro Contact Mines.—When the inertia circuit closer is employed in connection with electro contact mines, the circular ebonite piece m is replaced by a similar shaped piece of brass, and which is in metallic connection through the brass pillars l, l, l with the mass of the metal of the apparatus which forms the earth plate.
The insulated wire of the base plug is connected to one pole of a platinum wire fuze, the other pole of which is connected by another wire to the outer metal rim of the disc of the spindle. As long as the circuit closer remains undisturbed, a break will remain in the circuit, which is due to the ebonite insulation between the spindle and the outer metal rim of the disc; but the moment the apparatus is struck, which causes the spindle to vibrate, the outer metal rim will come in contact with one of the springs completing the circuit, through the circular metal portion and the pillars of the circuit closer to earth.
Adjustment of Circuit Closer.—The sensitiveness of Mathieson's inertia circuit closer is determined by the distance between the disc 4 and the springs 5, 5, 5, which is regulated by means of the adjusting screws 7, 7, 7, which press against the inner faces of the springs. Owing to the great weight of the leaden ball, when by any cause the circuit closer is inclined for a length of time, a permanent set is given to the spindle, thereby destroying the adjustment of the instrument.
Improvements in the Inertia Circuit Closer.—To remedy this very serious defect, a cylinder of india rubber is substituted for the leaden ball; a circuit closer so fitted is also less affected by the action of counter mines, which is a very important advantage.
Mathieson's Spiral Spring Circuit Closer.—A sectional elevation of this form of circuit closer is shown at Fig. 57. It consists of a brass base a, provided with a grooved flange for carrying a gutta percha washer, and it has also an hexagonal projection for the purpose of screwing the circuit closer into the gun-metal mouth of its air-tight cylinder, or buoy; b is a brass dome enclosing the apparatus for the purpose of protecting it from injury, and also by means of india rubber washers to prevent an ingress of water, should the circuit closer case become injured, and leak; c is a brass collar to which the brass contact springs i, i are attached, and which are regulated by the set screws j, j; a brass spiral spring d carries a metal rod e, which supports a brass ball f, surrounded by an india rubber band h. A contact disc g is secured to the base of the spindle e, but insulated from it by an ebonite boss; k is an ebonite base plug with two channels in it, through which the wires m, m1 pass.
An Improvement on the Inertia Circuit Closer.—This instrument is a vast improvement on the inertia apparatus previously described, being more simple and more certain in its action, a desideratum in all circuit closers; but notwithstanding, up to the present time Mathieson's inertia apparatus has been used by our government, to the exclusion of all other instruments of a similar nature, some of which were proved to be far superior when subjected to the crucial test of actual practice.
Austrian Self-acting Circuit Closer.—This form of circuit closing apparatus, which is purely a self-acting one, that is to say, a mine so fitted cannot be fired at will, is shown at Fig. 58.
It consists of several buffers a, a, a, which by means of strong springs are held in position, their heads projecting outside the torpedo case b; on being pressed in by the contact of a passing vessel, the ends of these buffers would be forced against a ratchet wheel c, which is also kept in position by means of a spring. Several strong pieces of wood d, d within the case keep the buffers and their attached arms in the proper direction, and also afford rigidity to the torpedo case. The brass ratchet wheel c being put in motion carries round with it a central arrangement e, the lower part of which is shown at Fig. 58, A.
This portion consists of a cylinder of brass f divided into two parts insulated one from the other by a piece of ebonite g; on one side of this cylinder there are three arms of brass, h, i, and k, and on the other there are two arms, l and m, all of which are insulated from each other.
AUSTRIAN CIRCUIT CLOSER, MERCURY CIRCUIT CLOSER.
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.
Mc.EVOY'S MAGNETO ELECTRO CIRCUIT CLOSER.
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:—
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 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:—
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 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.
RUSSIAN SUBMARINE MINE, FIRING BY OBSERVATION.
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.
APPARATUS FOR FIRING BY OBSERVATION.
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:—
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.
SYSTEMS OF DEFENCE BY SUBMARINE MINES.
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.
FIRING BATTERIES, TESTING BATTERIES.
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:—
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.
FIRING KEYS, SHUTTER APPARATUS.
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.
SHUTTER APPARATUS.
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., Fig. 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.
GALVANOMETERS FOR TESTING.
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. h1, h2, h3, h4, 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, 3a and 3b; 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 l1, 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 l1 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 l1 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 Fig. 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.
Fig. 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.
Fig. 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 · n.
150 - a
but if read off on the B side of the disc—
X = 150 - a · n.
150 + a
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.
SIEMEN'S UNIVERSAL GALVANOMETER.
SIEMEN'S UNIVERSAL GALVANOMETER.
SIEMEN'S UNIVERSAL GALVANOMETER.
SIEMEN'S UNIVERSAL GALVANOMETER
Measuring Electrical Resistances.—For this purpose the instrument is arranged as a Wheatstone's balance. The connections are made as shown at Pl. xxiv., Fig. 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. xxiiiA.):—
X : 100 = 150 + 50 : 150 - 50
X 150 + 50 × 100
150 - 50
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. xxiiiA., Fig. 2 and 6.
For comparing two electromotive forces E1 and E2, a third electromotor of higher electromotive force E0 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 E0 are to be connected to the terminals III. and V.
f.—The poles of the battery whose electromotive force E1 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—
E1 = E 0 150 - 30 . . . . . . . . (1),
300 + n
where n is the resistance of the battery E0.
The electromotor E2 is now to be inserted in the place of E1, 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—
E2 = E0 150 - 40 . . . . . . . . (2).
300 + n
By eliminating n from equations 1 and 2 we have
E1 : E2 = (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 connectionsare made as shown at Pl. xxiv., Figs. 3a 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. 3a shows diagram of the same connections but with the key K, and Fig. 3b 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.
SHUNT, COMMUTATOR, RHEOSTAT.
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 T1 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 T1, 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: