Chapter IV: DEFENSIVE TORPEDO WARFARE--continued (3)

_Insulation Test for Electrical Cables._--To test an electrical cable for insulation, it should first be put in a tank of water, or in the sea, and allowed to soak for at least forty-eight hours. The object of this is to allow the water to penetrate the outer protection of hemp and iron wires, &c., and to search out and get into any weak places there may be in the insulation under the armouring. At Fig. 96 is shown the method of performing this test. _A_ is a tank holding the electrical cable, which has been in soak for forty-eight hours; _B_ is an astatic galvanometer; _C_, _Z_ a Leclanché or Daniell battery of great power; and _C_ is an ordinary firing key. One end of the electric cable _D_ is connected to the galvanometer _B_ through the firing key _C_; the other end of the cable is very carefully insulated; one pole of the battery is connected to the galvanometer _B_, the other is put to earth in the tank at _F_; should the insulation be perfect, no deflection of the needle should follow on the key being pressed down. A very slight deflection might be observed on a moderately sensitive galvanometer, due to the current passing through the insulation; its whole length being immersed, the surface through which such a current would pass would be large, and the sum of the infinitesimally small quantities escaping over the whole length, would in the aggregate be sufficient to deflect the needle to a small extent in completing the circuit of the battery. Should any considerable deflection occur, it would indicate a defect or leak in the insulation of the cable, the extent of which would be roughly measured by the amount of such deflection.

By using a reflecting galvanometer a very much more delicate test would be obtained, but for the comparatively short lengths of electric cables used in connection with submarine mines, such accuracy is hardly necessary.

To test an electric cable for conductivity, it would be only necessary to expose the metallic conductor _G_, and put it in the water of the tank. If the conductivity were good, then the whole of the current would pass through the cable and the needle of the galvanometer would be violently deflected. If the continuity were broken, no deflection would be observed.

_Defects observed in the Conductivity of the Cable._--To ascertain the position of a defect in the insulation of a cable, as indicated by the tests above described, it would be only necessary to keep a continuous current flowing through the cable, and gradually take it out of the tank. If the fault existed at a single point, the deflection of the needle would be suddenly reduced at the moment of that point of the cable being lifted out of the water, and therefore its position would be determined with considerable accuracy. Should several defects exist as each was lifted out, a sudden reduction of the deflection would occur.

_Discharge Test._--The conductor of an electrical cable may be broken without destroying the insulation, and on applying the foregoing tests, good insulation would be indicated, but no conductivity, and no information would be given as to the position of the fault. Under such circumstances the following test must be applied:--

Put one pole of a very powerful battery to earth, and charge one end of the defective cable, then immediately discharge it through a reflecting galvanometer, and note the extreme limit of the swing of the needle, then, charge the other end of the cable in a similar manner, and discharge it through the same galvanometer, noting as before the swing of the needle. This should be done three or four times, and the average of the deflections taken. Then the position of the fault would be indicated by the proportion between the average deflections in each case, and the cable might safely be cut at that point. Should the precise position of the fault not be discovered in thus cutting the cable, each section should be tested again for conductivity, and that in which a fault was still found to exist should be again tested by the discharge as before.

_Test of Electrical Resistance of Cable._--This is effected by balancing it against the Wheatstone balance, in a similar manner to that explained for a fuze. The electrical resistance of the conductor of a cable affords a very correct indication of the quality of the metal of which it is composed. For a very delicate test the reflecting galvanometer should be used.

_Electrical Test of Insulated Joints._--Insulated joints and connections, whether of a permanent or temporary nature, should be tested electrically, in a precisely similar manner to that explained for electric cables.

They should be soaked for forty-eight hours, and then tested for insulation, conductivity, and electrical resistance.

In testing permanent joints special tests are carried out, which are described by Mr. Culley in his 'Handbook of Practical Telegraphy.'

Voltaic batteries should be subjected to the following tests:--

1.--For potential.
2.--For internal resistance.
3.--For electromotive force.

For the purpose of testing the potential of a battery, one pole should be put to earth, and with the other one pair of the quadrants of a Thomson's reflecting galvanometer should be charged; when this is done, a certain deflection of the spot of light will occur, and the amount of such deflection, as compared with that produced by a standard cell applied to the instrument in a similar manner, would give the relative value of the potential of the battery.

The following method of determining the internal resistance of a battery is that recommended by Mr. Latimer Clark in his book on electrical measurements.

The instrument employed is a double shunt differential galvanometer, a diagram of which is shown at Fig. 97. Connect the battery and a set of resistance coils in circuit between the terminals _A_ and _D_, and insert plugs in the resistance coils so that they give no resistance; insert plugs at _A_ and _C_, and also both the shunt plugs at _A_ and _D_. The current will now flow through one half of the galvanometer circuit only, being, however, reduced to 1/100 of its amount by the shunt _D_; the deflection of the needle must be carefully read. The plug _A_ must now be removed to _B_, which causes the battery current to flow through both halves of the galvanometer (each being shunted). The circuit will now be as shown in the figure, and the needle will of course be deflected somewhat more than before. Now unplug the resistance coils which are in circuit with the battery until the deflection of the needle is reduced to its original amount, and the resistances unplugged will be equal to the internal resistance of the battery.

The following is another method of ascertaining the internal resistance of a battery cell.

A circuit is formed, consisting of the battery cell, a rheostat, and a galvanometer, and the strength _C_ is noted on the galvanometer. A second cell is then joined with the first, so as to form one of double the size, and therefore half the resistance, and then by adding a length _l_ of the rheostat, the strength is brought to what it originally was, _C_.

Then if _E_ is the electromotive force, and R the resistance of cell, _r_ the resistance of the galvanometer, and other parts of the circuit, the strength _C_ in the one case is C = E / (R + _r_), and in the other = E / ((1/2)R + _r_ + _l_), and since the strength in both cases is the same, R = 2_l_, i.e., the internal resistance of the cell is equal to twice the resistance corresponding to the length _l_ of the rheostat wire.

The comparative electromotive force of a battery may be determined by means of a double shunt differential galvanometer in the following method, as recommended by Mr. Latimer Clark.

"This can only be done relatively in terms of some other standard battery. First determine the resistance of the standard and of the other cells to be measured; then insert the shunt plugs at _A_ and _D_, Fig. 97, and also at _C_ and _B_, and join up the standard cell in circuit with a resistance coil to the terminals _A_ and _D_, and unplug the resistance coils until a convenient deflection is obtained, say 15°; note the sum of the resistances in circuit, including that of the battery galvanometer, resistance coil and connecting wires; now change the battery for another, and by unplugging the resistance coils bring the needle again to the same deflection, 15°; having again found the total resistance in the circuit, the relative electromotive force will be directly proportional to these resistances."

The electromotive force of a battery may also be measured statically by means of Thomson's quadrant electrometer, the poles of the battery being connected with the two chief electrodes of the instrument, in which arrangement no current will pass, and the electromotive force will be directly indicated by the difference of potential observed.

In the case of a quantity battery, that is, a battery capable of fusing a fine platinum wire, its electromotive force and internal resistance may be determined by means of the resistance coils _K_, and thermo galvanometer _M_, shown at Fig. 95.

_Tests after Submersion._--After an electrical submarine mine has been placed in position, it should be immediately tested to ascertain that all is right, and similar tests should be applied at intervals to ascertain that the charge remains dry; that the insulation and conductivity of the electric cable remains the same; and that its electrical resistance indicates a state of efficiency.

The nature of the tests applied to determine these points will depend upon the nature of the combination in which the mine is arranged.

The manner of applying the "sea cell" test, by which is ascertained the condition of a system of electrical submarine mines, will be readily understood from the following examples.

The arrangements for testing to ascertain whether a charge is dry, or wet, is shown at Fig. 98.

_z_ is a plate of zinc introduced in the circuit within the charge, and between the fuze and the shore; another earth plate of carbon _x_ is connected with the electric cable beyond the fuze, forming the ordinary earth connection of the system at that point; and at home a copper earth plate _c_ is used.

First, in the case of a dry charge with the insulation and conductivity of the cable, good; under these circumstances there would be formed a sea cell between the earth plates _x_, and _c_, which would produce a certain deflection of the needle of a galvanometer _g_, which is placed in the circuit, and in a certain direction.

Secondly, in the case of a charge becoming wet, through leakage, with the insulation and conductivity of the cable, good; under these circumstances, a sea cell would be formed between the plates _c_ and _z_, causing a different deflection of the needle in amount and in direction, by which it would be at once indicated that the charge had become wet.

PLATE XXVII]

_"Sea cell" Test for Insulation._--Again, in the case of the insulation of the electric cable being damaged to such an extent as to expose the copper conductor. Under these circumstances there would be formed a sea cell between the copper earth plate _c_, and the exposed copper conductor of the cable, by which a certain definite deflection of the galvanometer would be observed, which deflection would differ in character from that produced by the copper carbon sea cell, when the insulation of the cable was good, and the system in working order, and therefore it would indicate that some change in the electrical conditions of the system had occurred. The fact that a leak existed in the insulation would be proved by changing the earth plate at home from copper to zinc, carbon, tin, &c.

In the case of no deflection being produced on the galvanometer, on applying the sea cell test, a want of continuity, or inefficient connections would be indicated.

The foregoing afford examples of the vast utility of the "sea cell" in connection with a system of electrical tests for submarine mines, numerous variations of which may be effected by employing a series of earth plates, of different metals, at the home end of the circuit, in connection with a carbon and zinc earth plate at the other end. And the mode of manipulating these tests may, by means of numerous switch plates, as shown at Fig. 95, be made extremely simple and efficient.

_Armstrong's System of Electrical Testing._--A very simple method of testing electrical submarine mines, with which low tension fuzes are used, has been devised by Captain Armstrong, R.E., and is shown at Fig. 99. _a_ is the electric cable leading from the shore; _b_ the cable attached to a polarised relay _c_, and connecting the charge through the fuze _f_ to the earth; _b'_ the cable, attached to another polarised relay _c'_, and connecting the mine with the circuit closer; the polarised relay _c_, in the mine, is arranged to be worked by a positive current, that is to say, the wire surrounding the core is so wound as to increase the polarity of the electro magnet, near the armature _d_, when a positive current is passed through it, and to diminish the polarity when a negative current is passed through the wire surrounding the core; the polarised relay _c'_ within the circuit closer is arranged to be worked by a negative current, the coil being so wound as to produce an influence exactly the reverse of _c_.

Then, a positive current passing along the line wire _a_, the armature _d_ in the charge will be attracted, while _d'_ will remain unaffected; again, if a negative current be circulated, the armature _d'_ within the circuit closer will be attracted, while the armature _d_ will remain unaffected. Two insulated wires forked together are wound round each electro magnet, one a thin wire (_g_ and _g'_) having a considerable resistance, about 1000 ohms, being connected direct to the earth plates _e_ and _e'_, and the other a thick wire (_h_ and _h'_) offering a very small resistance, and so arranged that when the armature is attracted, they may be in contact with and complete the circuit through the armature to earth.

The thin wire coils are so arranged that a certain number of Leclanché cells (ten or twelve, as may be desired) will make the electro magnets act, while with fewer cells the current would be too weak, and would therefore pass through them to earth without affecting the armature.

By means of the three-coil galvanometer, a table of the deflections, obtained by the foregoing system of testing, should be carefully recorded, when the circuit is known to be in good working order, so that any defect in the circuit would be at once indicated on the application of the various tests, by the results so obtained differing from those originally recorded. When a system of submarine mines is placed in position for the purposes of practice and experiment, every trouble should be taken to endeavour to fix the exact position of any defect that may exist, also to ascertain its magnitude, &c., but in time of war, should a defect exist in the system, no time must be lost in such operations, but the mine at once lifted, and the fault repaired, or a fresh one laid in its place, unless the presence of an enemy or other imperative cause should prevent such work being done.

_Austrian Testing Table._--The following is a description of the Austrian testing table, and their mode of making electrical tests with it, in connection with their system of self-acting electrical submarine mines.

PLATE XXVIII]

Its design is shown at Fig. 100; _c z_ represents the battery with one pole to earth at _e_, and the other in connection with an intensity coil _a_, through which the current passes to the contact plate _b_. When it is desired to put the system of mines in connection with the table, in a state of preparation to be fired by the contact of a vessel, a plug is inserted between the contact plates _b_ and _f_, and the current passes through the galvanometer _g_, and electrically charges the conducting wires connecting the mines with the battery, through the several binding screws on the contact plates, numbering 1, 2, 3, &c. The fact that the charge has been fired is also at once indicated on the galvanometer _g_.

_Test to discover an Exploded Charge._--It then becomes necessary to ascertain which particular mine of the system has been exploded; for this purpose a separate circuit in connection with a single cell _d_ is employed. This cell is in connection through a galvanometer _g'_ (a more sensitive instrument than the galvanometer _g_) with the pivot of the key _h_, and rheotome _R_, which latter is connected, as shown by the dotted lines, with each individual mine of the system attached to the contact plates numbered 1, 2, 3, &c. The handle of the rheotome is moved round, to each number in succession and directly it is placed in contact with that corresponding to the exploding mine, the electrical circuit is completed through the exposed end of the fractured wire, and this is indicated by the galvanometer _g'_. During the testing process the firing battery _c z_ must be disconnected; this is done by raising one of the bridges _i i_ with which each group of ten mines is provided.

_Insulation Test._--The rheotome and testing galvanometer _g'_ are also used to test the insulation of the electric cables connecting the mines to the testing table. This is done in precisely the same manner as testing for an exploded mine: the handle of the rheotome is turned round, and each cable connected in succession with the testing circuit as before; should the galvanometer _g'_ remain stationary, the insulation is good; but should a defect of insulation exist, the current passing through it would act on and deflect the galvanometer, indicating the particular line in which it exists, and, roughly, its extent in proportion to the deflection shown; should the fault be considerable, the defective cable should be at once detached, as the current lost through it might so diminish the working power of the firing battery, as to prevent it exploding any of the fuzes attached to the group in connection with it. By the above arrangement, the insulation of each line can be tested at any moment required.

In making the delicate test for insulation, which should invariably be done at leisure, and, if possible, when an enemy's vessels are not in the vicinity of the mines, a large number of Daniell's or other cells of suitable form should always be used. To do this, it would only be necessary to connect such a battery in place of a single cell permanently arranged, as described, in the testing circuit, and to proceed with the details of the operation as before. As the cable would, in actual work, always be charged with the full power of a firing battery, the value of its insulation to resist an electrical charge at such a high potential would be an important point to determine. The fuzes being entirely out of the circuit till the moment of the action arrives, no danger of a premature explosion need be apprehended; if a fuze were in such a position as to be fired prematurely, it would be exploded, in connection with the firing circuit, independently of the operation of testing the insulation of the cables.

_To render a Channel Safe._--In order to render the channel safe for a friendly vessel, it is only necessary to remove the plug from between the contact plates _b_ and _f_; this disconnects the firing battery from the circuit.

_Defence of Harbours by Booms, &c._--Booms or cables supported by rafts may also be employed in the defence of harbours, or rivers, either by themselves, or in combination with submarine mines; in the latter case, the booms, &c., may be moored either in advance of the mines, or in rear of the front row, this last method of mooring them being the most effective one.

There are a great variety of forms in which a boom may be constructed. The qualities essential for a good and practicable boom are:--

1.--Great strength.
2.--Great power of resistance.
3.--Convenience in handling.
4.--Easy to manipulate.
5.--Its materials easily procurable.

_Construction of a Boom._--The general construction of a boom consists of a main cable, buoyed up at intervals by floats. The main cable may be either wire, chain, or rope, the former being very much superior for this purpose to chain or rope. The floats consist of balks of timber built round the main cable and bound together by means of iron hoops &c. A space is left between each float, by which a certain amount of flexibility in the boom is obtained, without which it would be of comparatively little use, as it might be easily overrun.

It must be borne in mind, in constructing all such booms, that the smaller the proportion of timber used in forming the floats to the cable, consistent with buoyancy, the stronger will be the structure.

A very important feature in connection with such a mode of defence is the manner of mooring it; for if it be moored so as to be unyielding, then its sole power of resisting a vessel charging it is the actual strength of the materials composing the structure, but if it be moored so that it is capable of yielding to a sudden blow, this force will be to some extent absorbed, and resistance of the defence greatly increased.

The raft employed to support the main cable should be moored by means of very heavy chains (without anchors) in the direction of the attack, and with ordinary anchors and cables on the other side.

As a rule, the booms should be moored obliquely to the direction of the current, where there is any, as the tendency of the current to overrun the boom when so placed will be less, and also a ship ramming it must place herself athwart the current to attack the boom at right angles.

_Clearing a Passage through the Torpedo Defences of an Enemy._--The subject of clearing a passage through the torpedo defences of an enemy is one fraught with innumerable difficulties, on account of the varied nature and impracticability of obtaining accurate and _certain_ information of such defences, and thus it is impossible to lay down any fixed rule or plan for carrying out such an operation.

In fact, it will be only under the most favourable circumstances that such a service will be successfully accomplished, that is to say, in the case of a harbour or river defended by submarine mines but unsupported by guns, or guard boats, or where the electric light is used.

Numerous methods have been devised from time to time to effect the destruction of an enemy's submarine defences, among which are the following:--

1.--Projecting frames, &c., from the bows of a vessel.
2.--Creeping and sweeping by boats.
3.--Countermining.

_Projecting Frames, &c., from the Bows of a Vessel._--This method was adopted by the Federals during the American civil war of 1861-5, and in many instances it was the means of saving their ships when proceeding up rivers which had been torpedoed by the Confederates, though notwithstanding this precaution several vessels were sunk. The submarine mines against which this mode of defence was used, were in nine cases out of ten mechanical ones, and therefore the framework defence afforded a better means of protection then, than would be the case now that electrical ground mines and circuit closers are used, as the framework would catch the circuit closer only, and the vessel would probably be over the mine when the explosion took place. The Americans moor their circuit closers in rear of their mines, so that a vessel fitted with a bow frame or not, coming in contact with the former must be right over the charge at the instant of explosion.

Against ground electrical mines fired at will, the bow net, &c., is no protection whatever, still under certain circumstances it would be found extremely useful.

_Sweeping for Submarine Mines._--This method of clearing a channel of submarine mines could not possibly be carried out under artillery fire, but in waters not so defended it would prove of some value.

Where only buoyant mines, or ground mines with circuit closers are to be cleared away, two or more boats dragging a hawser between them would be sufficient to discover them, and so lead to their destruction; but where dummy mines and inverted creepers are moored in addition, another method of sweeping must be resorted to, viz., that of bringing an explosive charge of gun-cotton to act on the obstruction grappled, and thus destroy it. This is effected by lashing a charge to each end of the sweep, so that whatever is grappled may slide along it, until caught by hooks, which are attached for this purpose to the centre of the charge. On grappling an obstruction, the two boats drop their anchors, one hauling in, the other veering out the sweep, until the charge is hooked by the obstruction; this being effected, the boats move out of range, and the charge is fired.

_Creeping for Electrical Cables, &c._--Creeping is the method employed for picking up the electric cables of the enemy's submarine mines, and is effected by boats towing an ordinary grappling iron, or specially prepared creeper on the ground.

In both sweeping and creeping it would be found necessary to employ a diver, who would ascertain the nature of the grappled obstructions which could not be easily raised by the boats.

The Lay torpedo boat, which is fully described in the chapter on offensive torpedoes, is capable of being used for the foregoing purposes.

_Countermining._--Countermining, that is, the destruction of submarine mines by the explosion of other mines dropped close to them, will under certain conditions prove of great use in clearing harbours of mines. This method could not be operated in waters properly guarded and swept by artillery fire.

There are two distinct methods of laying out countermines, viz.:--

1.--In a boat, which may be either towed, or hauled out
to its destination, or may be steered, and controlled
by electricity.

2.--By attaching them to buoys so that they are
suspended at the proper depths, and then hauled out by
means of a warp to an anchor which has been previously
placed in position.

Both of the foregoing methods have been successfully manipulated in practice, the first method, where the boat carrying the countermines is towed either by a pulling or steam boat being the most practicable one. A large amount of material would be required for clearing a channel by means of countermines: for example, if the mines to be attacked require 500-lb. gun-cotton charges to be used, 7-1/2 tons of the explosive, besides cables, buoys, &c., would be required to clear a passage about one mile in length and 200 feet in width.

A ship's launch will carry about twelve of these 500-lb. countermines, with all the gear attached thereto.

Experiments to ascertain the effect of countermining have been carried out in England and Europe for the last five years, some of which are given at length in the chapter on "Torpedo Experiments." During the Turco-Russian war, a portion of the Danube was swept in the ordinary and most simple manner by the Turks, and five Russian electro contact buoyant mines were picked up; one other exploded during the process of dragging it to the surface, but no injury occurred to those at work.

_Destruction of Passive Obstructions._--To clear away booms, or other passive obstructions, if not possible to cut them away, they may be destroyed by outrigger boats exploding their torpedoes underneath, and in contact, or by attaching charges of gun-cotton at intervals, and then exploding them simultaneously. When a chain is horizontal, and therefore somewhat taut, a charge of 3-1/2 lbs. of gun-cotton (this explosive, being the most effective and convenient for such purposes, should always be used) will be found sufficient to destroy it, no matter what size, and whether the chain is in or out of the water, the charge being of course placed in contact with it. Great uncertainty must always attend the supposed clearance of a channel, or passage of submarine mines, as was exemplified during the American civil war, when most of the Northerners' vessels were destroyed while moving over ground which had been previously carefully dragged, and buoyed, and this fact, coupled with the tediousness and danger of performing such a service, proves the enormous value of a system of defence by submarine mines.

FOOTNOTES:

[Footnote J: 'Electricity and Magnetism,' by Professor F. Jenkins.]

[Footnote K: See Appendix.]

[Footnote L: As constructed by Mr. J. Mathieson, late R.E., at the Silvertown Telegraph Works, Essex.]