E1= E0150 - 30. . . . . . . . (1),300 +n
wherenis the resistance of the batteryE0.
The electromotorE2is now to be inserted in the place ofE1, and the galvanometer needle, when it deflects, again brought back to zero by moving the pointerZ. If for instance the pointer has to be pushed to 40° on theBside to obtain equilibrium we have—
E2= E0150 - 40. . . . . . . . (2).300 +n
By eliminatingnfrom 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 pointerZfrom 150° on theAside 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. 3aand7.
The manipulationsa,b,c, anddsame 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 pointerZ, which must be left stationary; the sine of the angle indicated byZwill 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. 4shows the same connections asFig. 7, but without the shunt box, and with the battery commutator. Fig. 3ashows diagram of the same connections but with the keyK, andFig. 3bthe 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 atFig. 89the shunt diminishes the amount of the current flowing along the circuit betweenAandB.
If only 1/Nth of the current is to pass along the circuit betweenAandB(of resistanceR) 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 atFig. 90, consists of a small base of hard wood on which is an ebonite plate, this by means of the handlemis turned about a central axis between two stopscandc'. On the disc are fixed two copper plates, one of whichois always positive, being connected by the axis and by a plate (+) with the binding screwP, which receives the positive electrode of the battery; the other copper platei,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 electrodeN. On the opposite side of the board are two binding screwsb, andb', to which are attached two elastic metal platesr, andr'.
On the disc being turned as shown in the figure, the current coming by the binding screwPpasses into the pieceo, the plater, and finally the binding screwb, which by means of a copper wire leads the current to the apparatus in connection withb; then returning to the binding screwb', the current reaches the plater', the piecei,e, and so to the battery by the binding screwN.
If the disc is turned so that the handlemis half way betweencandc', the piecesoandi,e, being no longer in contact with the platesrandr', the current will not pass. Ifmis turned as far asc, the plateowill then touchr', and the current pass tob', and return byb, 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.Plate XXV
SHUNT, COMMUTATOR, RHEOSTAT.
Wheatstone's rheostat, which is shown in elevation atFig. 91, consists of two cylindersAandB, 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 handleC. The surface of the non-conducting cylinderBhas 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 screwsD,D'connected with the ends of the copper wire are provided, to which the circuit wires are connected. A scale is attached atE, by means of which the number of convolutions onBcan be read off; and parts of a revolution are indicated on a circle at one end. The handleCcan 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 cylinderA, 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 cylinderB, 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 cylinderB. This instrument is often used in connection with the thermo galvanometer.
Resistance Box.—The general arrangement of a resistance box is shown in the diagramFig. 92.
Between two terminal binding screwsTandT1secured on a vulcanite slab are fixed a series of brass junction piecesa,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 betweenTanda, oraandb.
With all the plugs inserted, the electrical current will flow direct fromTtoT1, 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.
AtFig. 94is shown Wheatstone's balance (Post-office pattern),and atFig. 93the apparatus is reduced into the form of a parallelogram, which is the usual diagram of Wheatstone's bridge. The theory of the bridge is as follows:
Four conductorsA B,B C,A D, andD Care joined atAandCto the poles of a batteryZ; the resistance betweenAandBisR; that betweenAandDisr; that betweenDandCisR1; and that betweenBandCisx, the unknown resistance to be measured. A convenient constant ratio is chosen forR1andr, such as equality 1 to 10, 1 to 100, or 1 to 1000; and thenR1is adjusted until no current flows through the galvanometerG; when this is the case we have R :r=R1:x, orx= (r/R) × R1; so that ifr= R/100,xwill be equal to R1/100.
Two keysaandbare inserted; the current is wholly cut off the four conductors until contact is made ata; and then after the currents in the four conductors have come to their permanent condition, contact is made atbto test whether any current flows through the galvanometer. The three resistancesR,R1andrand the resistance of the galvanometer should be small ifxis small, and great ifxis great.
The conductorsA BandA Dof the bridge are each formed of three resistance coils having a resistance of 10, 100, and 1000 ohms respectively, inserted between the terminalsBandDof the balance,Fig. 94.
The conductorD Cis formed of a set of resistance coils from 1 up to 4000 ohms, amounting altogether to 11,110 ohms, inserted between the terminalsDandCof the balance; in the balance, a brass plug being inserted between the terminalsDandD1, they may be considered as one terminalD. The conductorB Cis the wire to be tested, and is connected to the terminalsBandCof the balance.
Measurement of Resistances.—When a resistance is to be measured that is within the range of the coils inR1,Randrare made equal. The needle of the galvanometer will move in a different direction, either to the right or to the left, according as the resistance inR1is greater or less than the line wirex. The needle remains at zero only when the resistance inR1is equal to that inx. Forr:R::R1:x.
WHEATSTONE'S BRIDGE.Plate XXVI
WHEATSTONE'S BRIDGE.
When the resistance ofxis greater than that ofR1, as in an insulation test, the resistance inris madelessthan that inR, in order thatrandRmay have such a proportion one to the other as will enable the coils inR1to balance a resistance inx, greater than their own, that is to say, greater than 11,100 ohms; thusr:R::R1:x, or 10 : 1000 :: 10,000 : 1,000,000, the resistance in the line to be tested would be 1,000,000 ohms, supposing the values ofr,RandR1to be respectively 10, 1000, and 10,000 ohms.
When the resistance to be tested is less than that of the least coil inR1(1 ohm), then the resistance inris made greater than inR. Thusr:R::R1:x, or 100 : 10 :: 2 : 0·2; the resistance of the line to be tested would in this case be 1/20 of an ohm.
Manipulation.—In all cases the key in connection with the battery should first be depressed, then the galvanometer key, making very short contacts by the latter, just sufficient to show the direction of the deflection, until the coils inR1are nearly adjusted, otherwise considerable time will be lost in making a series of tests, owing to the swing given to the needle, which will take some little time before it again remains steady at zero. When once the coils inR1are adjusted, and a balance obtained, it should be ascertained whether the needle will remain steady when contact is made and broken.
Test Tables.—In connection with a system of testing electrical submarine mines, for the sake of convenience and simplicity it is necessary to use a table (termed a "Test Table"), on which all the apparatus used for the purpose of testing are fixed. Several forms of tables have been designed for such a purpose. AtFig. 95is shown the method of arranging such a table.[L]
Ais an astatic galvanometer placed between two switch plates,BandC; ten other similar switch plates, 1, 2, 3, 4,D, 5, 6, 7,E, and 8, are arranged in front of the galvanometerA;F,G, andHare three terminal plates;Kis a box of resistance coils used in connection with the thermo galvanometerM;Lis a firing key, andNa battery commutator;Ois a three-coil galvanometer;Ris a Wheatstone balance (Post-office pattern).
The ten switch plates, 1, 2, 3, 4,D, &c., are used for the connection of any particular line to be tested, as well as for the earth connections and instruments employed in that operation.
"Sea Cell" Tests.—The arrangement shown in the figure is that required in connection with the sea cell test, and Mr. Brown's method of keeping certain earth plates in a bucket instead of in the sea.
If two plates of suitable metal to form a Voltaic battery are placed in salt water and connected by a metallic conductor, a battery is at once formed capable of producing considerable deflection on a moderately delicate galvanometer. Testing by this arrangement has been termed the "sea cell" test.
Arranging Earth Plates.—Mr. Brown's, Assistant-Chemist to the War Department, method of arranging the earth plates is as follows:—
A series of earth plates, such as copper, carbon, tin, zinc, &c., are placed in a bucket filled with sea water, and which is placed in the testing room. The water in the bucket is put in connection with the water of the sea by means of a conducting wire, terminating at one end with a zinc plate in the bucket, and at the other with a zinc plate in the sea. By this means the tests made with the different earth plates in the bucket are identical with those made with corresponding earths placed absolutely in the sea, and therefore these latter may be done away with, the sea cell tests being entirely carried out by means of the bucket earth plates.
In addition to the bucket earth plates there will be several other earth plates in connection with the testing room, these being placed in the sea, such as the zinc earth for the firing battery, the zinc earth for the signalling battery, &c.
Connections of Switch Plates.—The switch plateDis used for the connection of any particular mine cable which it may be required to test. The switch plateEis connected with a zinc earth plate used for testing the firing battery. This must always be in the sea. The switch plate 1 is in connection with a zinc earth in the bucket; 2 is attached to a copper earth plate in the bucket; 3 is attached to a carbon earth plate in the bucket; 4 to a tin earth plate in the bucket; 5 is used for connection with the zinc signalling earth connection in the sea; 6 is attached to a copper earth plate used for the sea cell test, or any other purpose required, in the sea; 7 is attached to a zinc earth plate in the sea; and 8 is a common zinc earth in the sea.
The terminal platesGandHare used for the connection, for testing purposes of the negative and positive poles, of the firing battery, andFis connected with a zinc earth in the sea, for a similarpurpose. These plates are in connection with the resistance coilsKand the thermo galvanometerM, employed for testing the firing battery, the circuit being closed by the firing keyL. Other ways of using these plates may of course be adopted if desired. The resistance coilsKrange from 0·5 to 100 ohms, and are composed of wire adapted for the passage of a quantity current. A reversing key is generally used in connection with a testing battery and the three-coil galvanometerO. This reversing key would consist of two bridges completely insulated from each other, the upper one attached to the negative, the lower one to the positive pole of the test battery. In their normal position both keys press against the upper bridge, and until one or other of the keys is pressed down no current will pass, the direction of the current being altered by pressing down a different key. The point of each key is provided with a terminal and connected, the one to a zinc earth through the switch plate 8, the other to one terminal of the three-coil galvanometer when the tests are to be applied.
The Wheatstone balanceRis used in finding the resistances of electrical cables, balancing fuzes, &c. By means of a commutator,N, the necessary number of cells for any particular test may be thrown in circuit when required.
Test of Platinum Wire Fuze for Conductivity.—The platinum wire fuze may be tested electrically as follows:—
If placed in circuit with a few cells of a Daniell or Leclanché battery and a detector galvanometer, before the platinum wire bridge of the fuze is fixed, there should be no deflection of the needle, for no metallic circuit exists; if it did, such would be fatal to the efficiency of the fuze. If similarly placed in circuit after the bridge has been fixed, a considerable deflection of the needle should result, such deflection being due to the current passing through the metallic bridge, which to be efficient ought to be the sole medium through which the circuit is completed.
Test of Resistance of Platinum Wire Fuze.—The electrical resistance of a platinum wire fuze is ascertained by means of the Wheatstone's balanceRand galvanometerA,Fig. 95. The terminals of the fuze are connected to the binding screws of the balance, the commutatorNand galvanometerAbeing connected up in circuit. The resistance of the coils is then adjusted by taking out plugs until theneedle of the galvanometerAis brought to zero, when the sum of the resistances indicated by the unplugged coils will be equal to that of the fuze. The resistance of a platinum wire fuze might also be ascertained by means of a differential galvanometer instead of a Wheatstone balance.
The electrical resistance of 3/10" of fine platinum wire, weighing 1·9 grains to the yard, is 3/10 of an ohm nearly (Schaw).
Testing High Tension Fuzes.—High tension fuzes require very delicate and careful management in testing them, due to the high electrical resistance of such fuzes, which ranges from 1500 to 2000 ohms, combined with the danger of premature explosion when testing even with a small number of battery cells. Very sensitive galvanometers, such as the reflecting galvanometer, should if possible be used, otherwise the mode of making the tests for conductivity and resistance of a high-tension fuze is similar to that already given for a platinum wire fuze.
Detonating fuzes should always be placed in an iron case during the process of testing.
Insulation Test for Electrical Cables.—To test an electrical cable for insulation, it should first be put in a tank of water, or in the sea, and allowed to soak for at least forty-eight hours. The object of this is to allow the water to penetrate the outer protection of hemp and iron wires, &c., and to search out and get into any weak places there may be in the insulation under the armouring. AtFig. 96is shown the method of performing this test.Ais a tank holding the electrical cable, which has been in soak for forty-eight hours;Bis an astatic galvanometer;C,Za Leclanché or Daniell battery of great power; andCis an ordinary firing key. One end of the electric cableDis connected to the galvanometerBthrough the firing keyC; the other end of the cable is very carefully insulated; one pole of the battery is connected to the galvanometerB, the other is put to earth in the tank atF; 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 todeflect 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 conductorG, 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, eachsection 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:—
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 atFig. 97. Connect the battery and a set of resistance coils in circuit between the terminalsAandD, and insert plugs in the resistance coils so that they give no resistance; insert plugs atAandC, and also both the shunt plugs atAandD. The current will now flow through one half of the galvanometer circuit only, being, however, reduced to 1/100 of its amount by the shuntD; the deflection of the needle must be carefully read. The plugAmust nowbe removed toB, 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 strengthCis 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 lengthlof the rheostat, the strength is brought to what it originally was,C.
Then ifEis the electromotive force, and R the resistance of cell,rthe resistance of the galvanometer, and other parts of the circuit, the strengthCin 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 = 2l, i.e., the internal resistance of the cell is equal to twice the resistance corresponding to the lengthlof 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 atAandD,Fig. 97, and also atCandB, and join up the standard cell in circuit with a resistance coil to the terminalsAandD, 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 batterybeing 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 coilsK, and thermo galvanometerM, shown atFig. 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 atFig. 98.
zis a plate of zinc introduced in the circuit within the charge, and between the fuze and the shore; another earth plate of carbonxis 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 platecis 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 platesx, andc, which would produce a certain deflection of the needle of a galvanometerg, 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 platescandz, 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.
TEST TABLE, DIFFERENTIAL GALVANOMETER.Plate XXVII
TEST TABLE, DIFFERENTIAL GALVANOMETER.
"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 thecopper conductor. Under these circumstances there would be formed a sea cell between the copper earth platec, 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 atFig. 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 atFig. 99.ais the electric cable leading from the shore;bthe cable attached to a polarised relayc, and connecting the charge through the fuzefto the earth;b'the cable, attached to another polarised relayc', and connecting the mine with the circuit closer; the polarised relayc, 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 armatured, 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 relayc'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 ofc.
Then, a positive current passing along the line wirea, the armaturedin the charge will be attracted, whiled'will remain unaffected;again, if a negative current be circulated, the armatured'within the circuit closer will be attracted, while the armaturedwill remain unaffected. Two insulated wires forked together are wound round each electro magnet, one a thin wire (gandg') having a considerable resistance, about 1000 ohms, being connected direct to the earth plateseande', and the other a thick wire (handh') 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.
METHODS OF TESTING.—ARMSTRONG,—AUSTRIAN.Plate XXVIII
METHODS OF TESTING.—ARMSTRONG,—AUSTRIAN.
Its design is shown at Fig. 100;c zrepresents the battery with one pole to earth ate, and the other in connection with an intensity coila, through which the current passes to the contact plateb. 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 platesbandf, and the current passes through the galvanometerg, and electrically charges the conducting wires connecting the mines with the battery, through the severalbinding 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 galvanometerg.
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 celldis employed. This cell is in connection through a galvanometerg'(a more sensitive instrument than the galvanometerg) with the pivot of the keyh, and rheotomeR, 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 galvanometerg'. During the testing process the firing batteryc zmust be disconnected; this is done by raising one of the bridgesi iwith which each group of ten mines is provided.
Insulation Test.—The rheotome and testing galvanometerg'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 galvanometerg'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 permanentlyarranged, 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 platesbandf; 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:—
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 andcertaininformation 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:—
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 frameworkdefence 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.:—