Chapter IV: DEFENSIVE TORPEDO WARFARE--continued (2)

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_.

PLATE XX]

_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.

PLATE XXI]

Mechanical tests are applied to ascertain that the mechanical arrangements of the shutter apparatus, circuit closers, and all similar appliances work efficiently and easily; that the several parts of the mine case when put together for service are thoroughly watertight; that the chains, wire cables, and ropes in connection with the mooring apparatus are of sufficient strength to perform the work required of them; that the weights of the anchors, or sinkers, are such as to keep the mines in position after submersion; and that the case of the mine be sufficiently strong to enable it to bear the external pressure due to the depth at which it may be submerged for a considerable time without any leakage.

The foregoing tests of the mine case and moorings would of course be performed during the process of manufacture, but to prevent any chance of failure they should be repeated before being employed on actual service.

_Electrical Tests._--Electrical tests are those which are applied to the several component parts of the system, to ascertain that the electrical conditions necessary to a successful result exist.

The importance of being able to carry out the above in its entirety is understood when it is remembered that a submarine mine becomes practically valueless unless it acts efficiently at the single instant of time that it would be required so to do.

_List of Instruments used in Testing._--The following are some of the instruments that are employed in connection with a system of electrical tests:--

1.--Thomson's electrometer.
2.--Thomson's reflecting galvanometer.
3.--Astatic galvanometer.
4.--Differential galvanometer.
5.--Detector galvanometer.
6.--Three coil galvanometer.
7.--Thermo galvanometer.
8.--Siemens's universal galvanometer.
9.--A shunt.
10.--Commutator.
11.--Rheostat.
12.--Resistance coils.
13.--Wheatstone's balance.

Electrometers indicate the presence of a statical charge of electricity, by showing the force of attraction or repulsion between two conducting bodies placed near together. This force depending in the first place on the quantity of electricity with which the conducting bodies are charged, ultimately depends on the difference of potential between them; an electrometer is therefore strictly an instrument for measuring difference of potential.[J]

Sir William Thomson's quadrant electrometer is the most perfect form of electrometer yet constructed, and the one usually employed in cable testing. It consists of a very thin flat aluminium needle spread out into two wings, and hung by a wire from an insulated stem inside a Leyden jar, which contains a cupful of strong sulphuric acid, the outer surface of which forms the inner coating of the Leyden jar. A wire stretched by a weight connects the aforesaid needle with this inner coating. A mirror, rigidly attached to this needle by a rod, serves to indicate the deflection of the needle by reflecting the image of a flame on to a scale. The needle hangs inside four quadrants, which are insulated by glass stems: each pair of opposite quadrants are in electrical connection. Above and below the quadrants two tubes, at the same potential as the needle, serve to screen it and the wires in connection with it from all induction except that produced by the four quadrants. Suppose the needle charged to a high negative potential (-), then if the quadrants are symmetrically placed, it will deflect neither to the right nor to the left, so long as the near quadrants are at the same potential. If one of these be positive relatively to the other, the end of the needle under them will be repelled from the negative quadrant to the positive one, and at the same time the other end of the needle will be repelled from in the opposite direction. This motion will be indicated by the motion of the spot of light reflected by the mirror, and the number of divisions which the spot of light traverses on the scale measures in an arbitrary unit the difference of potential between the + and - quadrants.

The reflecting electrometer being a very delicate instrument, requires careful handling, and should only be used by a practised electrician. Its use would therefore be restricted to important stations, and special tests of a delicate nature.

_Thomson's Reflecting Galvanometer._--A galvanometer is an instrument intended to detect the presence of a current and measure its magnitude.

The most sensitive galvanometer as yet constructed is the reflecting galvanometer of Sir William Thomson, a diagram of which is shown at Fig. 84.

A small piece of magnetised steel watch spring, 3/8ths of an inch long, is fastened with shellac on the back of a little round concave mirror, and of about the size of a fourpenny piece. This is suspended by a piece of unspun silk thread in the centre of a coil of many hundred turns of fine copper wire insulated with silk, and well protected between the turns with varnish. The two ends of the coils are soldered to terminal screws _a_, _b_, so that any conducting wire can be joined up to it as required. The little mirror hangs in the middle of its coil, with the magnet lying horizontally. By means of a lamp _L_ placed behind the screen, the light of which passes through a slit _M_, and is thrown on the face of the mirror, a spot of light is reflected on the scale _N_.

When a current passes through the coil, the little magnet is deflected, and since the magnet is attached to the mirror, which is very light, both are deflected as forming one body, and the spot of light moves accordingly along the scale _N_.

A powerful steel magnet _S_ is placed above the coil, and can be moved up or down, whereby the directive force of the earth may be increased or weakened. This magnet _S_ is used to steady the spot of light, which otherwise would shake about, and there would be no certainty about the measurement. A second magnet _T_ is placed perpendicular to the magnetic meridian, to adjust the zero of the instrument, i.e., to bring back the spot of light to a fiducial mark at the centre of the scale when no current is passing.

This instrument should only be used at important stations, and when special tests of a delicate nature are required to be applied.

_Astatic Galvanometer._--An astatic galvanometer is that in connection with which an astatic needle is employed, by the use of which the sensitiveness of a galvanometer is greatly increased.

An astatic needle is a combination of magnetised needles _with their poles turned opposite ways_.

At Fig. 85 a diagram of such an instrument is shown. Two magnets _D_ and _C_ are joined, with the north pole of one over the south pole of the other, forming one suspended system. In the ordinary form of astatic galvanometer the needles _D_ and _C_ are about two inches long, and are each covered by a coil, these latter being so joined that the current must circulate in opposite directions round the two so as to deflect both magnets similarly. The deflection of the needles _D_ and _C_ is observed by means of a pointer or glass needle _A_, _B_, rigidly connected with the astatic system by a prolongation of the brass rod connecting the needles _D_ and _C_. The coils are flat and of the shape indicated in Fig. 85, and are also made in two halves, placed side by side with just sufficient space between them to allow the rod to hang freely.

This form of galvanometer, though less delicate than the preceding one, is still a very sensitive one, and should only be applied in the case of fine and delicate tests.

_Differential Galvanometer._--A differential galvanometer consists of a magnetic needle surrounded by two separate coils of equal length and material carefully insulated from each other and wound in opposite directions. In using it one circuit acts against the other. If a current of equal strength were passing through each there would be no deflection of the needle, because the influence in both directions is equal. If one current were stronger than the other, the needle would be deflected by the stronger.

This form of galvanometer will be found extremely useful in connection with a system of electrical tests.

Latimer Clark's double shunt differential galvanometer is the instrument best adapted for submarine mine tests.

_Detector Galvanometer._--A detector galvanometer is usually made with a vertical needle, and is employed to detect and roughly estimate the strength of a current where no particular accuracy is required.

It consists of a magnetic needle pivoted in the centre of a coil of insulated wire, and having an index needle attached to move with it, the latter appearing on a dial, divided into 360 equal arcs or portions: a diagram of such an instrument is shown at Fig. 86.

This instrument should be of small size and portable form, and as sensitive as it is possible to make it, under such conditions.

_Three Coil Galvanometer._--The three coil galvanometer is provided with a vertical needle, and is in other respects very similar in appearance to the detector galvanometer before described. It is formed with three coils of 2, 10, and 1000 ohms resistance; each coil is connected with a brass plate on the top of the box which encloses the whole, and may be switched into circuit by means of a plug at will. The object of the three resistances is to suit the different resistances that may occur, with a perfect, or imperfect state of the electrical combination in connection with each mine. A diagram of this instrument is shown at Fig. 87, the dotted portions are inside the case.

_Thermo Galvanometer._--A thermo galvanometer is an instrument used to ascertain the power of a firing battery which is employed to ignite platinum wire or low tension fuzes.

The form of thermo galvanometer generally used in connection with a test table, is arranged as follows:--

Two ebonite studs, fitted with brass connecting screws, are fixed to the lid of a box containing some resistance coils, and placed in circuit with them; these studs, placed about ·3 of an inch apart, are arranged to receive a piece of platinum wire which is stretched from one stud to the other; the firing battery being placed in circuit with the platinum wire, and the resistance coils, its working power would then be tested by the fusion of the wire through a given electrical resistance, as indicated by the resistance coils put in circuit.

Another form of thermo galvanometer, which is very compact and portable, is shown at Fig. 88. It consists of a wooden box _a_, with a cover of ebonite _b_, within the box is placed a resistance coil _c_; _d_ and _e_ are two ebonite standards ·3" apart, the former of which is connected by a copper wire with the terminal _f_, the latter to the terminal _g_; the terminal _h_ is similarly connected to the contact piece _k_, and the terminal _l_ to the firing key _m_, at _n_; the resistance coil _c_ is connected to the terminal _g_ and to the copper wire _n_; the platinum wire (of which several lengths are used, according to the resistance of the coil _c_) is placed between the standards _d_ and _e_. To test a battery, it is only necessary to connect it to the terminals _f_ and _h_, when by pressing down the key _m_ the power of the battery, according as to its fusing or not the platinum wires, will be ascertained; the use of the terminals _g_ and _l_ is to cut out the resistance, which is effected by connecting them by means of a copper wire.

_Siemens's Universal Galvanometer._--Siemens's universal galvanometer is an instrument combining in itself all the arrangements necessary for the following operations:--

1.--For measuring electrical resistances.
2.--For comparing electromotive forces.
3.--For measuring the intensity of a current.

The instrument which is shown in elevation and plan at Pl. xxiii., Figs. 1 and 2 respectively, consists of a sensitive galvanometer which can be turned in a horizontal plane, combined with a resistance bridge (the wire of which bridge instead of being straight is stretched round part of a circle). The galvanometer has an astatic needle, suspended by a cocoon fibre, and a flat bobbin frame wound with fine wire. The needle swings above a cardboard dial divided in degrees; as however, when using the instrument the deflection of the needle is never read off, but the needle instead always brought to zero, two ivory limiting pins are placed at about 20 degrees on each side of zero.

The galvanometer is fixed on a graduated slate disc, round which the platinum wire is stretched. Underneath the slate disc three resistance coils of the value of 10, 100, and 1000 Siemens' units are wound on a hollow wooden block, which protrudes at one side, and on the projection carries the terminals for the reception of the leading wires from the battery and unknown resistance. The adoption of three different resistance coils enables the measuring of large as well as small resistances with sufficient accuracy.

PLATE XXII]

The whole instrument is mounted on a wooden disc, which is supported by three levelling screws, so that it may be turned round its axle. On the same axle a lever is placed which bears at its end an upright arm, carrying a contact roller. This roller is pressed against the platinum wire round the edge of the slate disc by means of a spring acting on the upright arm, and forms the junction between the _A_ and _B_ resistances of a Wheatstone's bridge, which resistances are formed by the platinum wire on either side of the contact roller, one of the three resistance coils forming the third resistance of the bridge. _G_ is the galvanometer, _k_ a milled head from which the needles are suspended, and by turning _k_ they can be raised or lowered, _m_ is the head of a screw which arrests or frees the needle when in motion. _h__{1}, _h__{2}, _h__{3}, _h__{4}, are the terminals of the respective ends of the three resistance coils, viz., 10, 100, and 1000 units, which are wound on the wooden block _C_; these terminals may be connected to each other by means of stoppers, and therefore one or more of the resistances may be brought into circuit as desired, and to the ends of these terminals the wires of the artificial resistances are connected as shown on diagrams Pl. xxiv., Figs. 1, 2, 3_a_ and 3_b_; _f_ is the graduated slate disc, round which the platinum wire is stretched in a slight groove at the edge of the disc, and is inserted in such manner that about half its diameter protrudes beyond the slate. The ends of the platinum wire are soldered to two brass terminals _l_ and _l_^{1}, which are placed at the angles formed by the sides of the gap in the slate disc, and which form the junctures, as in the ordinary resistance bridge, between _A_, _n_, and the galvanometer on one side, and _B_, _X_, and the galvanometer on the other side, of the parallelogram. The terminal _l_ is permanently connected by a thick copper wire or metal strip to terminal _h__{1}, and the other terminal _l_^{1} is connected in a similar manner to terminal III.

Slate is adopted for the material of which to make the disc _f_, because it is found by experience to be the material which is the least sensitive to variations in the weather or temperature.

The slate disc is graduated on its upper edge through an arc of 300 degrees, zero being in the centre, and the graduations figured up to 150 on each side at the terminals _l_ and _l_^{1} of the bridge wire.

In the centre of the circular plate _E_ of polished wood, supported upon three levelling screws _b_, _b_, _b_, a metal boss is inserted, in which turns the vertical pin _a_ which carries the instrument. This pin, being well fitted to the boss, supports the instrument firmly, but at the same time allows it to be turned freely round its vertical axis without losing its horizontal position when once obtained.

On the arm _D D_, which turns on the pin _a_, and somewhat behind the handle _g_, there is a small upright brass arm _d_ turning between two screw points _r_, and carrying in a gap at its upper end a small platinum jockey pulley _e_ turning on a vertical axis. This pulley forms the movable contact point along the bridge wire, against which it is kept firmly pressed by means of a spring acting on the arm _d_. The arm _D D_, which is insulated from the other parts of the apparatus, is permanently connected with the terminal I. On the top of _d_ a pointer _Z_ or a vernier is fixed, which laps over the upper edge of the slate disc and points to the graduations.

To the pin _a_ is attached a circular disc of polished wood _C_, about one inch thick, and having a groove turned in its edge for the reception of the insulated wires composing the resistances. The disc _C_ has a projection _c_, which carries the five insulated terminals marked I., II., III., IV., V., as shown on Figs. 1 and 2, Pl. xxiii. Terminals III. and IV. can be connected by a plug, II. and V. by the contact key _K_. Terminal I. is in connection with the lever _D D_.

Figs. 3 and 4, Pl. xxiii. show the shunt box supplied with the galvanometer if specially desired; the copper connecting arms _a_, _a_ are screwed to the terminals II. and IV. By inserting a plug at _c_ (Fig. 4, Pl. xxiii.), the galvanometer is put out of circuit altogether, whilst by plugging either of the other holes shunts of the value of 1/9, 1/99, or 1/999, are introduced into the circuit, and the effect upon the galvanometer is reduced to 1/10, 1/100, 1/1000, respectively of what it would have been without the insertion of the shunt.

Figs. 5 and 6, Pl. xxiii., show a battery commutator allowing to bring into the circuit four different amounts of battery power. It is placed in the battery circuit whenever consecutive tests with different batteries are desired to be made, it being only necessary to change the place of the stopper in the battery commutator, the terminal screw _a_ of the battery commutator being connected to terminal V. of the galvanometer, and the screws _b_, _b_, _b_, _b_ to various sections of the battery: see diagram of connections, Fig. 4, Pl. xxiv.

The application of the universal galvanometer will be clear from the diagrams on Pl ii.; instructions, however, for its practical use are added further on, and also tables for use when measuring conducting resistances.

As will be seen from diagram, Fig. 1, Pl. xxiv., the proportion between the unknown resistance X, and the artificial resistance _n_ is, when the deflection is read off on the side of the slate disc marked _A_:

X : _n_ = 150 + _a_ : 150 - _a_

or, X = ((150 + _a_) / (150 - _a_)) × _n_.

but if read off on the _B_ side of the disc--

X = ((150 - _a_) / (150 + _a_)) × _n_.

The values of these two fractions, for every half degree, will be found in the columns headed _A_ and _B_ of the table in the Appendix.

PLATE XXIII]

PLATE XXIII^A]

PLATE XXIV]

PLATE XXIV^A]

_Measuring Electrical Resistances._--For this purpose the instrument is arranged as a Wheatstone's balance. The connections are made as shown at Pl. xxiv., Figs. 1 and 5, where _X_ is the unknown resistance.

_a._--The needle _i_ is to be brought to the zero
point of the small cardboard scale by turning the
galvanometer _G_ round its vertical axis, taking care
that the needle moves with perfect freedom.

_b._--The pointer or vernier _Z_ is to be brought, by
means of the handle _g_, to the zero point of the large
scale on the slate disc.

_c._--A plug is to be inserted between the terminals
marked III. and IV.

_d._--The holes 10, 100, and 1000 are, two of them, to
be plugged, and one left open, according to the extent
of the unknown resistance to be measured; either 10 or
100 must be left open if the resistance is small, and
1000 if it is large.

_e._--The two ends of the unknown resistance are to be
connected to terminals II. and IV.

_f._--The two poles of some galvanic battery are to be
connected to terminals I. and V.

When the above-mentioned connections have been made, and on depressing the key _K_, the battery current is sent into the combination and deflects the needle, say, to the right-hand or _B_ side of the instrument, the pointer or vernier _Z_ must then be pushed, by means of the handle _g_, to the _B_ side of the instrument. If this is found to increase the deflection of the needle _i_, the pointer _Z_ should be pushed to the other or _A_ side of the instrument beyond the zero point of the large scale until the needle remains stationary when the key _K_ is depressed.

The number indicated by the vernier _Z_ should be read off carefully, and notice taken whether it is on the _A_ or _B_ side of the large scale. This number must then be referred to the galvanometer table,[K] when the figure opposite to the number, multiplied by the resistance unplugged, is the resistance of _X_. The value of the resistance to be determined will be thus found by a single operation.

Supposing the reading to be 50 on the _A_ side of the large scale, the resistance _n_ unplugged having been 100 units, we get according to the before-mentioned law of resistance bridge the following proportion (see Fig. 5, Pl. xxiv.):--

X : 100 = 150 + 50 : 150 - 50

X = ((150 + 50) / (150 - 50)) × 100

X = 200 units.

For measuring very small resistances a single cell will be found sufficient; but for large resistances more should be used, say, 15 to 20. If very accurate measurements of small resistances are to be taken, the screw at the end of the moving arm _D D_ should receive one battery wire, terminal V. receiving the other.

_Comparing Electromotive Forces._--For this purpose Professor E. du Bois-Reymond's modification of Poggendorff's compensation method is used.

The connections are made as shown at Pl. xxiv., Figs. 2 and 6.

For comparing two electromotive forces _E__{1} and _E__{2}, a third electromotor of higher electromotive force _E__{0} is used, and two separate tests taken.

The manipulations _a_ and _b_ are to be the same as before.

_c._--The hole between III. and IV. to be left
unplugged.

_d._--Plugs to be inserted in 10, 100 and 1000.

_e._--The two poles of the electromotor of an
electromotive force _E__{0} are to be connected to the
terminals III. and V.

_f._--The poles of the battery whose electromotive
force _E__{1} is to be compared are connected to
terminals I. and IV. in such a manner that the similar
poles of the two electromotors are joined to terminals
I. and III., and to IV. and V. respectively.

When depressing the key _K_ the galvanometer needle will be deflected and can be brought back to zero by turning the pointer _Z_ either to the right or to the left. Should for instance the pointer have to be brought to 30° on the _A_ side we have the following equation--

E_{1} = E_{0} × ((150 - 30) / ( 300 + _n_)) (1),

where _n_ is the resistance of the battery _E__{0}.

The electromotor _E__{2} is now to be inserted in the place of _E__{1}, and the galvanometer needle, when it deflects, again brought back to zero by moving the pointer _Z_. If for instance the pointer has to be pushed to 40° on the _B_ side to obtain equilibrium we have--

E_{2} = E_{0} × ((150 + 40) / ( 300 + _n_)) (2).

By eliminating _n_ from equations 1 and 2 we have

E_{1} : E_{2} = (150 - 30) / (150 + 40) = 12 : 19 (3).

The two electromotive forces are in the same proportion as the two observed distances of the pointer _Z_ from 150° on the _A_ side of the instrument.

_For measuring the Intensity of a Current._--For this purpose the instrument is simply used as a sine galvanometer. The connections are made as shown at Pl. xxiv., Figs. 3_a_ and 7.

The manipulations _a_, _b_, _c_, and _d_ same as in the second case.

_e._--Connect one pole of a battery to terminal II. and
put the other pole to earth.

_f._--Connect the line to terminal IV.

The galvanometer is then to be turned in the same direction as the needle is deflected until the needle coincides with the zero point. Whilst this is being done the large scale on the slate disc will move under the pointer _Z_, which must be left stationary; the sine of the angle indicated by _Z_ will thus give the value proportionate to the strength of the current. Should the shunt box be required, it has to be connected with terminals II. and IV.

Fig. 4 shows the same connections as Fig. 7, but without the shunt box, and with the battery commutator. Fig. 3_{a} shows diagram of the same connections but with the key _K_, and Fig. 3_{b} the same without the key.

_A Shunt._--A "Shunt" is a second path offered to a current traversing a given circuit, or portion of a circuit, so as to diminish the amount of the current flowing through that portion of the circuit. In the diagram shown at Fig. 89 the shunt diminishes the amount of the current flowing along the circuit between _A_ and _B_.

If only 1/Nth of the current is to pass along the circuit between _A_ and _B_ (of resistance _R_) then the resistance of the shunt must equal R/(N - 1).

By the aid of shunts it is quite possible to make use of very sensitive instruments to measure powerful currents.

_Commutators or Switch Plates._--A commutator or switch plate is an apparatus by which the direction of currents may be changed at will, or by which they may be opened or closed. Bertin's commutator, which is represented at Fig. 90, consists of a small base of hard wood on which is an ebonite plate, this by means of the handle _m_ is turned about a central axis between two stops _c_ and _c'_. On the disc are fixed two copper plates, one of which _o_ is always positive, being connected by the axis and by a plate (+) with the binding screw _P_, which receives the positive electrode of the battery; the other copper plate _i_, _e_, bent in the form of a horse-shoe, is connected by friction below the disc with a plate (-), which plate is connected with the negative electrode _N_. On the opposite side of the board are two binding screws _b_, and _b'_, to which are attached two elastic metal plates _r_, and _r'_.

On the disc being turned as shown in the figure, the current coming by the binding screw _P_ passes into the piece _o_, the plate _r_, and finally the binding screw _b_, which by means of a copper wire leads the current to the apparatus in connection with _b_; then returning to the binding screw _b'_, the current reaches the plate _r'_, the piece _i_, _e_, and so to the battery by the binding screw _N_.

If the disc is turned so that the handle _m_ is half way between _c_ and _c'_, the pieces _o_ and _i_, _e_, being no longer in contact with the plates _r_ and _r'_, the current will not pass. If _m_ is turned as far as _c_, the plate _o_ will then touch _r'_, and the current pass to _b'_, and return by _b_, thus reversing its direction.

"Peg" switches are also often used; they are arranged so that the removal or insertion of a brass peg or plug cuts out, or completes a circuit.

_Rheostat._--A rheostat is an instrument used for the comparison of resistances.

PLATE XXV]

Wheatstone's rheostat, which is shown in elevation at Fig. 91, consists of two cylinders _A_ and _B_, one of brass and the other of non-conducting material, so arranged that a copper wire can be wound off the one on to the other by turning a handle _C_. The surface of the non-conducting cylinder _B_ has a screw thread cut in it for its whole length, in which the turns of the copper wire lie, so that its successive convolutions are well insulated from each other. Two binding screws _D_, _D'_ connected with the ends of the copper wire are provided, to which the circuit wires are connected. A scale is attached at _E_, by means of which the number of convolutions on _B_ can be read off; and parts of a revolution are indicated on a circle at one end. The handle _C_ can be shifted from one cylinder to the other.

Supposing the rheostat introduced into a circuit, and the whole of the copper wire wrapped on the metal cylinder _A_, then, on account of the large section of this metal cylinder, its resistance may be entirely neglected, but for every convolution of the wire on the non-conducting cylinder =B=, a specific resistance is introduced into the circuit. The amount of resistance can thus be varied as gradually as desired by winding on and off the cylinder _B_. This instrument is often used in connection with the thermo galvanometer.

_Resistance Box._--The general arrangement of a resistance box is shown in the diagram Fig. 92.

Between two terminal binding screws _T_ and _T__{1} secured on a vulcanite slab are fixed a series of brass junction pieces _a_, _b_, _c_, _d_; each of these is connected by a resistance coil to its neighbour, as shown at 1, 2, 3, and 4. A number of brass conical plugs with insulating handles of vulcanite are provided, which can be inserted between any two successive junction pieces, as between _T_ and _a_, or _a_ and _b_.

With all the plugs inserted, the electrical current will flow direct from _T_ to _T__{1}, the large metallic junction pieces directly connected by the plugs would offer no sensible resistance; but if all the plugs were removed, then the current would flow through each of the coils 1, 2, 3, and 4, and the resistance in the circuit would be the sum of the resistances of those four coils. With the plugs arranged as in the figure, the current would flow through coil 4 only, and the resistance in the circuit would be equal to the resistance of that coil.

_Wheatstone's Balance._--The electrical conductivity of a body is determined by ascertaining the ratio between the resistance of a certain length of the conductor in question, having a given section, to that of a known length of a known section of some substance taken as a standard.

For this purpose Wheatstone's bridge in connection with a box of resistance coils is the most convenient method.

At Fig. 94 is shown Wheatstone's balance (Post-office pattern), and at Fig. 93 the apparatus is reduced into the form of a parallelogram, which is the usual diagram of Wheatstone's bridge. The theory of the bridge is as follows:

Four conductors _A B_, _B C_, _A D_, and _D C_ are joined at _A_ and _C_ to the poles of a battery _Z_; the resistance between _A_ and _B_ is _R_; that between _A_ and _D_ is _r_; that between _D_ and _C_ is _R__{1}; and that between _B_ and _C_ is _x_, the unknown resistance to be measured. A convenient constant ratio is chosen for _R__{1} and _r_, such as equality 1 to 10, 1 to 100, or 1 to 1000; and then _R__{1} is adjusted until no current flows through the galvanometer _G_; when this is the case we have R : _r_=R_{1} : _x_, or _x_ = (_r_/R) × R_{1}; so that if _r_ = R/100, _x_ will be equal to R_{1}/100.

Two keys _a_ and _b_ are inserted; the current is wholly cut off the four conductors until contact is made at _a_; and then after the currents in the four conductors have come to their permanent condition, contact is made at _b_ to test whether any current flows through the galvanometer. The three resistances _R_, _R__{1} and _r_ and the resistance of the galvanometer should be small if _x_ is small, and great if _x_ is great.

The conductors _A B_ and _A D_ of the bridge are each formed of three resistance coils having a resistance of 10, 100, and 1000 ohms respectively, inserted between the terminals _B_ and _D_ of the balance, Fig. 94.

The conductor _D C_ is formed of a set of resistance coils from 1 up to 4000 ohms, amounting altogether to 11,110 ohms, inserted between the terminals _D_ and _C_ of the balance; in the balance, a brass plug being inserted between the terminals _D_ and _D__{1}, they may be considered as one terminal _D_. The conductor _B C_ is the wire to be tested, and is connected to the terminals _B_ and _C_ of the balance.

_Measurement of Resistances._--When a resistance is to be measured that is within the range of the coils in _R__{1}, _R_ and _r_ are made equal. The needle of the galvanometer will move in a different direction, either to the right or to the left, according as the resistance in _R__{1} is greater or less than the line wire _x_. The needle remains at zero only when the resistance in _R__{1} is equal to that in _x_. For _r_ : _R_ :: _R__{1} : _x_.

PLATE XXVI]

When the resistance of _x_ is greater than that of _R__{1}, as in an insulation test, the resistance in _r_ is made _less_ than that in _R_, in order that _r_ and _R_ may have such a proportion one to the other as will enable the coils in _R__{1} to balance a resistance in _x_, greater than their own, that is to say, greater than 11,100 ohms; thus _r_ : _R_ :: _R__{1} : _x_, or 10 : 1000 :: 10,000 : 1,000,000, the resistance in the line to be tested would be 1,000,000 ohms, supposing the values of _r_, _R_ and _R__{1} to be respectively 10, 1000, and 10,000 ohms.

When the resistance to be tested is less than that of the least coil in _R__{1} (1 ohm), then the resistance in _r_ is made greater than in _R_. Thus _r_ : _R_ :: _R__{1} : _x_, or 100 : 10 :: 2 : 0·2; the resistance of the line to be tested would in this case be 1/20 of an ohm.

_Manipulation._--In all cases the key in connection with the battery should first be depressed, then the galvanometer key, making very short contacts by the latter, just sufficient to show the direction of the deflection, until the coils in _R__{1} are nearly adjusted, otherwise considerable time will be lost in making a series of tests, owing to the swing given to the needle, which will take some little time before it again remains steady at zero. When once the coils in _R__{1} are adjusted, and a balance obtained, it should be ascertained whether the needle will remain steady when contact is made and broken.

_Test Tables._--In connection with a system of testing electrical submarine mines, for the sake of convenience and simplicity it is necessary to use a table (termed a "Test Table"), on which all the apparatus used for the purpose of testing are fixed. Several forms of tables have been designed for such a purpose. At Fig. 95 is shown the method of arranging such a table.[L]

_A_ is an astatic galvanometer placed between two switch plates, _B_ and _C_; ten other similar switch plates, 1, 2, 3, 4, _D_, 5, 6, 7, _E_, and 8, are arranged in front of the galvanometer _A_; _F_, _G_, and _H_ are three terminal plates; _K_ is a box of resistance coils used in connection with the thermo galvanometer _M_; _L_ is a firing key, and _N_ a battery commutator; _O_ is a three-coil galvanometer; _R_ is a Wheatstone balance (Post-office pattern).

The ten switch plates, 1, 2, 3, 4, _D_, &c., are used for the connection of any particular line to be tested, as well as for the earth connections and instruments employed in that operation.

_"Sea Cell" Tests._--The arrangement shown in the figure is that required in connection with the sea cell test, and Mr. Brown's method of keeping certain earth plates in a bucket instead of in the sea.

If two plates of suitable metal to form a Voltaic battery are placed in salt water and connected by a metallic conductor, a battery is at once formed capable of producing considerable deflection on a moderately delicate galvanometer. Testing by this arrangement has been termed the "sea cell" test.

_Arranging Earth Plates._--Mr. Brown's, Assistant-Chemist to the War Department, method of arranging the earth plates is as follows:--

A series of earth plates, such as copper, carbon, tin, zinc, &c., are placed in a bucket filled with sea water, and which is placed in the testing room. The water in the bucket is put in connection with the water of the sea by means of a conducting wire, terminating at one end with a zinc plate in the bucket, and at the other with a zinc plate in the sea. By this means the tests made with the different earth plates in the bucket are identical with those made with corresponding earths placed absolutely in the sea, and therefore these latter may be done away with, the sea cell tests being entirely carried out by means of the bucket earth plates.

In addition to the bucket earth plates there will be several other earth plates in connection with the testing room, these being placed in the sea, such as the zinc earth for the firing battery, the zinc earth for the signalling battery, &c.

_Connections of Switch Plates._--The switch plate _D_ is used for the connection of any particular mine cable which it may be required to test. The switch plate _E_ is connected with a zinc earth plate used for testing the firing battery. This must always be in the sea. The switch plate 1 is in connection with a zinc earth in the bucket; 2 is attached to a copper earth plate in the bucket; 3 is attached to a carbon earth plate in the bucket; 4 to a tin earth plate in the bucket; 5 is used for connection with the zinc signalling earth connection in the sea; 6 is attached to a copper earth plate used for the sea cell test, or any other purpose required, in the sea; 7 is attached to a zinc earth plate in the sea; and 8 is a common zinc earth in the sea.

The terminal plates _G_ and _H_ are used for the connection, for testing purposes of the negative and positive poles, of the firing battery, and _F_ is connected with a zinc earth in the sea, for a similar purpose. These plates are in connection with the resistance coils _K_ and the thermo galvanometer _M_, employed for testing the firing battery, the circuit being closed by the firing key _L_. Other ways of using these plates may of course be adopted if desired. The resistance coils _K_ range from 0·5 to 100 ohms, and are composed of wire adapted for the passage of a quantity current. A reversing key is generally used in connection with a testing battery and the three-coil galvanometer _O_. This reversing key would consist of two bridges completely insulated from each other, the upper one attached to the negative, the lower one to the positive pole of the test battery. In their normal position both keys press against the upper bridge, and until one or other of the keys is pressed down no current will pass, the direction of the current being altered by pressing down a different key. The point of each key is provided with a terminal and connected, the one to a zinc earth through the switch plate 8, the other to one terminal of the three-coil galvanometer when the tests are to be applied.

The Wheatstone balance _R_ is used in finding the resistances of electrical cables, balancing fuzes, &c. By means of a commutator, _N_, the necessary number of cells for any particular test may be thrown in circuit when required.

_Test of Platinum Wire Fuze for Conductivity._--The platinum wire fuze may be tested electrically as follows:--

If placed in circuit with a few cells of a Daniell or Leclanché battery and a detector galvanometer, before the platinum wire bridge of the fuze is fixed, there should be no deflection of the needle, for no metallic circuit exists; if it did, such would be fatal to the efficiency of the fuze. If similarly placed in circuit after the bridge has been fixed, a considerable deflection of the needle should result, such deflection being due to the current passing through the metallic bridge, which to be efficient ought to be the sole medium through which the circuit is completed.

_Test of Resistance of Platinum Wire Fuze._--The electrical resistance of a platinum wire fuze is ascertained by means of the Wheatstone's balance _R_ and galvanometer _A_, Fig. 95. The terminals of the fuze are connected to the binding screws of the balance, the commutator _N_ and galvanometer _A_ being connected up in circuit. The resistance of the coils is then adjusted by taking out plugs until the needle of the galvanometer _A_ is brought to zero, when the sum of the resistances indicated by the unplugged coils will be equal to that of the fuze. The resistance of a platinum wire fuze might also be ascertained by means of a differential galvanometer instead of a Wheatstone balance.

The electrical resistance of 3/10" of fine platinum wire, weighing 1·9 grains to the yard, is 3/10 of an ohm nearly (Schaw).

_Testing High Tension Fuzes._--High tension fuzes require very delicate and careful management in testing them, due to the high electrical resistance of such fuzes, which ranges from 1500 to 2000 ohms, combined with the danger of premature explosion when testing even with a small number of battery cells. Very sensitive galvanometers, such as the reflecting galvanometer, should if possible be used, otherwise the mode of making the tests for conductivity and resistance of a high-tension fuze is similar to that already given for a platinum wire fuze.

Detonating fuzes should always be placed in an iron case during the process of testing.