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.
[Illustration: GALVANOMETERS FOR TESTING.
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.
[Illustration: SIEMEN'S UNIVERSAL GALVANOMETER.
PLATE XXIII]
[Illustration: SIEMEN'S UNIVERSAL GALVANOMETER.
PLATE XXIII^A]
[Illustration: SIEMEN'S UNIVERSAL GALVANOMETER.
PLATE XXIV]
[Illustration: SIEMEN'S UNIVERSAL GALVANOMETER
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.
[Illustration: SHUNT, COMMUTATOR, RHEOSTAT.
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_.
[Illustration: WHEATSTONE'S BRIDGE.
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.
_Insulation Test for Electrical Cables._--To test an electrical cable
for insulation, it should first be put in a tank of water, or in the
sea, and allowed to soak for at least forty-eight hours. The object of
this is to allow the water to penetrate the outer protection of hemp
and iron wires, &c., and to search out and get into any weak places
there may be in the insulation under the armouring. At Fig. 96 is
shown the method of performing this test. _A_ is a tank holding the
electrical cable, which has been in soak for forty-eight hours; _B_ is
an astatic galvanometer; _C_, _Z_ a Leclanché or Daniell battery of
great power; and _C_ is an ordinary firing key. One end of the electric
cable _D_ is connected to the galvanometer _B_ through the firing key
_C_; the other end of the cable is very carefully insulated; one pole
of the battery is connected to the galvanometer _B_, the other is put
to earth in the tank at _F_; should the insulation be perfect, no
deflection of the needle should follow on the key being pressed down.
A very slight deflection might be observed on a moderately sensitive
galvanometer, due to the current passing through the insulation; its
whole length being immersed, the surface through which such a current
would pass would be large, and the sum of the infinitesimally small
quantities escaping over the whole length, would in the aggregate be
sufficient to deflect the needle to a small extent in completing the
circuit of the battery. Should any considerable deflection occur,
it would indicate a defect or leak in the insulation of the cable,
the extent of which would be roughly measured by the amount of such
deflection.
By using a reflecting galvanometer a very much more delicate test
would be obtained, but for the comparatively short lengths of electric
cables used in connection with submarine mines, such accuracy is hardly
necessary.
To test an electric cable for conductivity, it would be only necessary
to expose the metallic conductor _G_, and put it in the water of the
tank. If the conductivity were good, then the whole of the current
would pass through the cable and the needle of the galvanometer would
be violently deflected. If the continuity were broken, no deflection
would be observed.
_Defects observed in the Conductivity of the Cable._--To ascertain the
position of a defect in the insulation of a cable, as indicated by the
tests above described, it would be only necessary to keep a continuous
current flowing through the cable, and gradually take it out of the
tank. If the fault existed at a single point, the deflection of the
needle would be suddenly reduced at the moment of that point of the
cable being lifted out of the water, and therefore its position would
be determined with considerable accuracy. Should several defects exist
as each was lifted out, a sudden reduction of the deflection would
occur.
_Discharge Test._--The conductor of an electrical cable may be broken
without destroying the insulation, and on applying the foregoing
tests, good insulation would be indicated, but no conductivity, and no
information would be given as to the position of the fault. Under such
circumstances the following test must be applied:--
Put one pole of a very powerful battery to earth, and charge one
end of the defective cable, then immediately discharge it through a
reflecting galvanometer, and note the extreme limit of the swing of the
needle, then, charge the other end of the cable in a similar manner,
and discharge it through the same galvanometer, noting as before the
swing of the needle. This should be done three or four times, and the
average of the deflections taken. Then the position of the fault would
be indicated by the proportion between the average deflections in each
case, and the cable might safely be cut at that point. Should the
precise position of the fault not be discovered in thus cutting the
cable, each section should be tested again for conductivity, and that
in which a fault was still found to exist should be again tested by the
discharge as before.
_Test of Electrical Resistance of Cable._--This is effected by
balancing it against the Wheatstone balance, in a similar manner to
that explained for a fuze. The electrical resistance of the conductor
of a cable affords a very correct indication of the quality of the
metal of which it is composed. For a very delicate test the reflecting
galvanometer should be used.
_Electrical Test of Insulated Joints._--Insulated joints and
connections, whether of a permanent or temporary nature, should be
tested electrically, in a precisely similar manner to that explained
for electric cables.
They should be soaked for forty-eight hours, and then tested for
insulation, conductivity, and electrical resistance.
In testing permanent joints special tests are carried out, which are
described by Mr. Culley in his 'Handbook of Practical Telegraphy.'
Voltaic batteries should be subjected to the following tests:--
1.--For potential.
2.--For internal resistance.
3.--For electromotive force.
For the purpose of testing the potential of a battery, one pole should
be put to earth, and with the other one pair of the quadrants of a
Thomson's reflecting galvanometer should be charged; when this is done,
a certain deflection of the spot of light will occur, and the amount
of such deflection, as compared with that produced by a standard cell
applied to the instrument in a similar manner, would give the relative
value of the potential of the battery.
The following method of determining the internal resistance of a
battery is that recommended by Mr. Latimer Clark in his book on
electrical measurements.
The instrument employed is a double shunt differential galvanometer,
a diagram of which is shown at Fig. 97. Connect the battery and a set
of resistance coils in circuit between the terminals _A_ and _D_, and
insert plugs in the resistance coils so that they give no resistance;
insert plugs at _A_ and _C_, and also both the shunt plugs at _A_ and
_D_. The current will now flow through one half of the galvanometer
circuit only, being, however, reduced to 1/100 of its amount by the
shunt _D_; the deflection of the needle must be carefully read. The
plug _A_ must now be removed to _B_, which causes the battery current
to flow through both halves of the galvanometer (each being shunted).
The circuit will now be as shown in the figure, and the needle will
of course be deflected somewhat more than before. Now unplug the
resistance coils which are in circuit with the battery until the
deflection of the needle is reduced to its original amount, and the
resistances unplugged will be equal to the internal resistance of the
battery.
The following is another method of ascertaining the internal resistance
of a battery cell.
A circuit is formed, consisting of the battery cell, a rheostat, and
a galvanometer, and the strength _C_ is noted on the galvanometer.
A second cell is then joined with the first, so as to form one of
double the size, and therefore half the resistance, and then by adding
a length _l_ of the rheostat, the strength is brought to what it
originally was, _C_.
Then if _E_ is the electromotive force, and R the resistance of cell,
_r_ the resistance of the galvanometer, and other parts of the circuit,
the strength _C_ in the one case is C = E / (R + _r_), and in the other
= E / ((1/2)R + _r_ + _l_), and since the strength in both cases is the
same, R = 2_l_, i.e., the internal resistance of the cell is equal to
twice the resistance corresponding to the length _l_ of the rheostat
wire.
The comparative electromotive force of a battery may be determined by
means of a double shunt differential galvanometer in the following
method, as recommended by Mr. Latimer Clark.
"This can only be done relatively in terms of some other standard
battery. First determine the resistance of the standard and of the
other cells to be measured; then insert the shunt plugs at _A_ and _D_,
Fig. 97, and also at _C_ and _B_, and join up the standard cell in
circuit with a resistance coil to the terminals _A_ and _D_, and unplug
the resistance coils until a convenient deflection is obtained, say
15°; note the sum of the resistances in circuit, including that of the
battery galvanometer, resistance coil and connecting wires; now change
the battery for another, and by unplugging the resistance coils bring
the needle again to the same deflection, 15°; having again found the
total resistance in the circuit, the relative electromotive force will
be directly proportional to these resistances."
The electromotive force of a battery may also be measured statically
by means of Thomson's quadrant electrometer, the poles of the battery
being connected with the two chief electrodes of the instrument, in
which arrangement no current will pass, and the electromotive force
will be directly indicated by the difference of potential observed.
In the case of a quantity battery, that is, a battery capable of fusing
a fine platinum wire, its electromotive force and internal resistance
may be determined by means of the resistance coils _K_, and thermo
galvanometer _M_, shown at Fig. 95.
_Tests after Submersion._--After an electrical submarine mine has
been placed in position, it should be immediately tested to ascertain
that all is right, and similar tests should be applied at intervals
to ascertain that the charge remains dry; that the insulation and
conductivity of the electric cable remains the same; and that its
electrical resistance indicates a state of efficiency.
The nature of the tests applied to determine these points will depend
upon the nature of the combination in which the mine is arranged.
The manner of applying the "sea cell" test, by which is ascertained the
condition of a system of electrical submarine mines, will be readily
understood from the following examples.
The arrangements for testing to ascertain whether a charge is dry, or
wet, is shown at Fig. 98.
_z_ is a plate of zinc introduced in the circuit within the charge, and
between the fuze and the shore; another earth plate of carbon _x_ is
connected with the electric cable beyond the fuze, forming the ordinary
earth connection of the system at that point; and at home a copper
earth plate _c_ is used.
First, in the case of a dry charge with the insulation and conductivity
of the cable, good; under these circumstances there would be formed a
sea cell between the earth plates _x_, and _c_, which would produce a
certain deflection of the needle of a galvanometer _g_, which is placed
in the circuit, and in a certain direction.
Secondly, in the case of a charge becoming wet, through leakage,
with the insulation and conductivity of the cable, good; under these
circumstances, a sea cell would be formed between the plates _c_ and
_z_, causing a different deflection of the needle in amount and in
direction, by which it would be at once indicated that the charge had
become wet.
[Illustration: TEST TABLE, DIFFERENTIAL GALVANOMETER.
PLATE XXVII]
_"Sea cell" Test for Insulation._--Again, in the case of the insulation
of the electric cable being damaged to such an extent as to expose
the copper conductor. Under these circumstances there would be
formed a sea cell between the copper earth plate _c_, and the exposed
copper conductor of the cable, by which a certain definite deflection
of the galvanometer would be observed, which deflection would differ
in character from that produced by the copper carbon sea cell, when
the insulation of the cable was good, and the system in working order,
and therefore it would indicate that some change in the electrical
conditions of the system had occurred. The fact that a leak existed in
the insulation would be proved by changing the earth plate at home from
copper to zinc, carbon, tin, &c.
In the case of no deflection being produced on the galvanometer, on
applying the sea cell test, a want of continuity, or inefficient
connections would be indicated.
The foregoing afford examples of the vast utility of the "sea cell"
in connection with a system of electrical tests for submarine mines,
numerous variations of which may be effected by employing a series of
earth plates, of different metals, at the home end of the circuit, in
connection with a carbon and zinc earth plate at the other end. And
the mode of manipulating these tests may, by means of numerous switch
plates, as shown at Fig. 95, be made extremely simple and efficient.
_Armstrong's System of Electrical Testing._--A very simple method of
testing electrical submarine mines, with which low tension fuzes are
used, has been devised by Captain Armstrong, R.E., and is shown at
Fig. 99. _a_ is the electric cable leading from the shore; _b_ the
cable attached to a polarised relay _c_, and connecting the charge
through the fuze _f_ to the earth; _b'_ the cable, attached to another
polarised relay _c'_, and connecting the mine with the circuit closer;
the polarised relay _c_, in the mine, is arranged to be worked by a
positive current, that is to say, the wire surrounding the core is
so wound as to increase the polarity of the electro magnet, near the
armature _d_, when a positive current is passed through it, and to
diminish the polarity when a negative current is passed through the
wire surrounding the core; the polarised relay _c'_ within the circuit
closer is arranged to be worked by a negative current, the coil being
so wound as to produce an influence exactly the reverse of _c_.
Then, a positive current passing along the line wire _a_, the
armature _d_ in the charge will be attracted, while _d'_ will remain
unaffected; again, if a negative current be circulated, the armature
_d'_ within the circuit closer will be attracted, while the armature
_d_ will remain unaffected. Two insulated wires forked together are
wound round each electro magnet, one a thin wire (_g_ and _g'_) having
a considerable resistance, about 1000 ohms, being connected direct to
the earth plates _e_ and _e'_, and the other a thick wire (_h_ and
_h'_) offering a very small resistance, and so arranged that when the
armature is attracted, they may be in contact with and complete the
circuit through the armature to earth.
The thin wire coils are so arranged that a certain number of Leclanché
cells (ten or twelve, as may be desired) will make the electro magnets
act, while with fewer cells the current would be too weak, and would
therefore pass through them to earth without affecting the armature.
By means of the three-coil galvanometer, a table of the deflections,
obtained by the foregoing system of testing, should be carefully
recorded, when the circuit is known to be in good working order, so
that any defect in the circuit would be at once indicated on the
application of the various tests, by the results so obtained differing
from those originally recorded. When a system of submarine mines is
placed in position for the purposes of practice and experiment, every
trouble should be taken to endeavour to fix the exact position of
any defect that may exist, also to ascertain its magnitude, &c., but
in time of war, should a defect exist in the system, no time must be
lost in such operations, but the mine at once lifted, and the fault
repaired, or a fresh one laid in its place, unless the presence of an
enemy or other imperative cause should prevent such work being done.
_Austrian Testing Table._--The following is a description of the
Austrian testing table, and their mode of making electrical tests with
it, in connection with their system of self-acting electrical submarine
mines.
[Illustration: METHODS OF TESTING.--ARMSTRONG,--AUSTRIAN.
PLATE XXVIII]
Its design is shown at Fig. 100; _c z_ represents the battery with one
pole to earth at _e_, and the other in connection with an intensity
coil _a_, through which the current passes to the contact plate _b_.
When it is desired to put the system of mines in connection with the
table, in a state of preparation to be fired by the contact of a
vessel, a plug is inserted between the contact plates _b_ and _f_,
and the current passes through the galvanometer _g_, and electrically
charges the conducting wires connecting the mines with the battery,
through the several binding screws on the contact plates, numbering
1, 2, 3, &c. The fact that the charge has been fired is also at once
indicated on the galvanometer _g_.
_Test to discover an Exploded Charge._--It then becomes necessary to
ascertain which particular mine of the system has been exploded; for
this purpose a separate circuit in connection with a single cell _d_
is employed. This cell is in connection through a galvanometer _g'_ (a
more sensitive instrument than the galvanometer _g_) with the pivot of
the key _h_, and rheotome _R_, which latter is connected, as shown by
the dotted lines, with each individual mine of the system attached to
the contact plates numbered 1, 2, 3, &c. The handle of the rheotome is
moved round, to each number in succession and directly it is placed in
contact with that corresponding to the exploding mine, the electrical
circuit is completed through the exposed end of the fractured wire, and
this is indicated by the galvanometer _g'_. During the testing process
the firing battery _c z_ must be disconnected; this is done by raising
one of the bridges _i i_ with which each group of ten mines is provided.
_Insulation Test._--The rheotome and testing galvanometer _g'_ are
also used to test the insulation of the electric cables connecting
the mines to the testing table. This is done in precisely the same
manner as testing for an exploded mine: the handle of the rheotome is
turned round, and each cable connected in succession with the testing
circuit as before; should the galvanometer _g'_ remain stationary,
the insulation is good; but should a defect of insulation exist, the
current passing through it would act on and deflect the galvanometer,
indicating the particular line in which it exists, and, roughly, its
extent in proportion to the deflection shown; should the fault be
considerable, the defective cable should be at once detached, as the
current lost through it might so diminish the working power of the
firing battery, as to prevent it exploding any of the fuzes attached
to the group in connection with it. By the above arrangement, the
insulation of each line can be tested at any moment required.
In making the delicate test for insulation, which should invariably
be done at leisure, and, if possible, when an enemy's vessels are
not in the vicinity of the mines, a large number of Daniell's or
other cells of suitable form should always be used. To do this,
it would only be necessary to connect such a battery in place of
a single cell permanently arranged, as described, in the testing
circuit, and to proceed with the details of the operation as before.
As the cable would, in actual work, always be charged with the full
power of a firing battery, the value of its insulation to resist an
electrical charge at such a high potential would be an important
point to determine. The fuzes being entirely out of the circuit till
the moment of the action arrives, no danger of a premature explosion
need be apprehended; if a fuze were in such a position as to be fired
prematurely, it would be exploded, in connection with the firing
circuit, independently of the operation of testing the insulation of
the cables.
_To render a Channel Safe._--In order to render the channel safe for a
friendly vessel, it is only necessary to remove the plug from between
the contact plates _b_ and _f_; this disconnects the firing battery
from the circuit.
_Defence of Harbours by Booms, &c._--Booms or cables supported by rafts
may also be employed in the defence of harbours, or rivers, either by
themselves, or in combination with submarine mines; in the latter case,
the booms, &c., may be moored either in advance of the mines, or in
rear of the front row, this last method of mooring them being the most
effective one.
There are a great variety of forms in which a boom may be constructed.
The qualities essential for a good and practicable boom are:--
1.--Great strength.
2.--Great power of resistance.
3.--Convenience in handling.
4.--Easy to manipulate.
5.--Its materials easily procurable.
_Construction of a Boom._--The general construction of a boom consists
of a main cable, buoyed up at intervals by floats. The main cable may
be either wire, chain, or rope, the former being very much superior for
this purpose to chain or rope. The floats consist of balks of timber
built round the main cable and bound together by means of iron hoops
&c. A space is left between each float, by which a certain amount of
flexibility in the boom is obtained, without which it would be of
comparatively little use, as it might be easily overrun.
It must be borne in mind, in constructing all such booms, that the
smaller the proportion of timber used in forming the floats to the
cable, consistent with buoyancy, the stronger will be the structure.
A very important feature in connection with such a mode of defence is
the manner of mooring it; for if it be moored so as to be unyielding,
then its sole power of resisting a vessel charging it is the actual
strength of the materials composing the structure, but if it be moored
so that it is capable of yielding to a sudden blow, this force will
be to some extent absorbed, and resistance of the defence greatly
increased.
The raft employed to support the main cable should be moored by means
of very heavy chains (without anchors) in the direction of the attack,
and with ordinary anchors and cables on the other side.
As a rule, the booms should be moored obliquely to the direction of the
current, where there is any, as the tendency of the current to overrun
the boom when so placed will be less, and also a ship ramming it must
place herself athwart the current to attack the boom at right angles.
_Clearing a Passage through the Torpedo Defences of an Enemy._--The
subject of clearing a passage through the torpedo defences of an
enemy is one fraught with innumerable difficulties, on account of the
varied nature and impracticability of obtaining accurate and _certain_
information of such defences, and thus it is impossible to lay down any
fixed rule or plan for carrying out such an operation.
In fact, it will be only under the most favourable circumstances that
such a service will be successfully accomplished, that is to say,
in the case of a harbour or river defended by submarine mines but
unsupported by guns, or guard boats, or where the electric light is
used.
Numerous methods have been devised from time to time to effect the
destruction of an enemy's submarine defences, among which are the
following:--
1.--Projecting frames, &c., from the bows of a vessel.
2.--Creeping and sweeping by boats.
3.--Countermining.
_Projecting Frames, &c., from the Bows of a Vessel._--This method
was adopted by the Federals during the American civil war of 1861-5,
and in many instances it was the means of saving their ships when
proceeding up rivers which had been torpedoed by the Confederates,
though notwithstanding this precaution several vessels were sunk. The
submarine mines against which this mode of defence was used, were in
nine cases out of ten mechanical ones, and therefore the framework
defence afforded a better means of protection then, than would be the
case now that electrical ground mines and circuit closers are used, as
the framework would catch the circuit closer only, and the vessel would
probably be over the mine when the explosion took place. The Americans
moor their circuit closers in rear of their mines, so that a vessel
fitted with a bow frame or not, coming in contact with the former must
be right over the charge at the instant of explosion.
Against ground electrical mines fired at will, the bow net, &c., is
no protection whatever, still under certain circumstances it would be
found extremely useful.
_Sweeping for Submarine Mines._--This method of clearing a channel of
submarine mines could not possibly be carried out under artillery fire,
but in waters not so defended it would prove of some value.
Where only buoyant mines, or ground mines with circuit closers are
to be cleared away, two or more boats dragging a hawser between them
would be sufficient to discover them, and so lead to their destruction;
but where dummy mines and inverted creepers are moored in addition,
another method of sweeping must be resorted to, viz., that of bringing
an explosive charge of gun-cotton to act on the obstruction grappled,
and thus destroy it. This is effected by lashing a charge to each end
of the sweep, so that whatever is grappled may slide along it, until
caught by hooks, which are attached for this purpose to the centre
of the charge. On grappling an obstruction, the two boats drop their
anchors, one hauling in, the other veering out the sweep, until the
charge is hooked by the obstruction; this being effected, the boats
move out of range, and the charge is fired.
_Creeping for Electrical Cables, &c._--Creeping is the method employed
for picking up the electric cables of the enemy's submarine mines, and
is effected by boats towing an ordinary grappling iron, or specially
prepared creeper on the ground.
In both sweeping and creeping it would be found necessary to employ
a diver, who would ascertain the nature of the grappled obstructions
which could not be easily raised by the boats.
The Lay torpedo boat, which is fully described in the chapter on
offensive torpedoes, is capable of being used for the foregoing
purposes.
_Countermining._--Countermining, that is, the destruction of submarine
mines by the explosion of other mines dropped close to them, will
under certain conditions prove of great use in clearing harbours of
mines. This method could not be operated in waters properly guarded and
swept by artillery fire.
There are two distinct methods of laying out countermines, viz.:--
1.--In a boat, which may be either towed, or hauled out
to its destination, or may be steered, and controlled
by electricity.
2.--By attaching them to buoys so that they are
suspended at the proper depths, and then hauled out by
means of a warp to an anchor which has been previously
placed in position.
Both of the foregoing methods have been successfully manipulated in
practice, the first method, where the boat carrying the countermines is
towed either by a pulling or steam boat being the most practicable one.
A large amount of material would be required for clearing a channel by
means of countermines: for example, if the mines to be attacked require
500-lb. gun-cotton charges to be used, 7-1/2 tons of the explosive,
besides cables, buoys, &c., would be required to clear a passage about
one mile in length and 200 feet in width.
A ship's launch will carry about twelve of these 500-lb. countermines,
with all the gear attached thereto.
Experiments to ascertain the effect of countermining have been carried
out in England and Europe for the last five years, some of which are
given at length in the chapter on "Torpedo Experiments." During the
Turco-Russian war, a portion of the Danube was swept in the ordinary
and most simple manner by the Turks, and five Russian electro contact
buoyant mines were picked up; one other exploded during the process of
dragging it to the surface, but no injury occurred to those at work.
_Destruction of Passive Obstructions._--To clear away booms, or other
passive obstructions, if not possible to cut them away, they may be
destroyed by outrigger boats exploding their torpedoes underneath, and
in contact, or by attaching charges of gun-cotton at intervals, and
then exploding them simultaneously. When a chain is horizontal, and
therefore somewhat taut, a charge of 3-1/2 lbs. of gun-cotton (this
explosive, being the most effective and convenient for such purposes,
should always be used) will be found sufficient to destroy it, no
matter what size, and whether the chain is in or out of the water, the
charge being of course placed in contact with it. Great uncertainty
must always attend the supposed clearance of a channel, or passage
of submarine mines, as was exemplified during the American civil war,
when most of the Northerners' vessels were destroyed while moving over
ground which had been previously carefully dragged, and buoyed, and
this fact, coupled with the tediousness and danger of performing such a
service, proves the enormous value of a system of defence by submarine
mines.
FOOTNOTES:
[Footnote J: 'Electricity and Magnetism,' by Professor F. Jenkins.]
[Footnote K: See Appendix.]
[Footnote L: As constructed by Mr. J. Mathieson, late R.E., at the
Silvertown Telegraph Works, Essex.]