FOOTNOTES:

Fig. 171.

Concentration of Light.—Two kinds of concentrating apparatus are supplied in combination with the automatic lamp, both of which are capable of giving a powerful parallel beam, which will reach to an enormous distance, and are well adapted for naval purposes. The one kind consists of a parabolic reflector of stout metal, its concave surface being silvered and burnished. The apparatus is mounted with a ball-and-socket joint upon a wooden stand, as shown inFig. 171.

The other kind is the Fresnel catadioptric lens or holophote,Fig. 172, which may be substituted for the reflector, and gives a more powerful beam than one given by reflection. The lens is surrounded by a metal case or lantern, in which is placed the electric lamp upon a slide for focussing. Behind the carbon points a hemispherical reflector is placed, to catch all the back rays, and reflect them back through the lamp focus. The entire lantern is capable of revolving onhorizontal rollers, and swings upon pivots. Two handles are placed at the back to manipulate it.

Fig. 172.

As the electric arc is much too bright to be looked into with the naked eye, both concentrating apparatus are supplied with a lens, called a focus or flame observer, by means of which an image of the burning carbons is thrown upon small screens at the back, so that the lamp can be easily adjusted without fatigue to the eye. The focus observer is shown on the lamp in holophote,Fig. 172.

Precautions.—Before starting the apparatus, the electric lamp terminals and those of the dynamo machine must beconnected upby means of the leading wires provided with each set of apparatus. The terminals are markedCandZrespectively, and they should be connected,Cof machine toCof the lamp, andZof the machine toZofthe lamp, in order that the electric current may be sent in the proper direction through the carbons of the lamp. Should it, however, be found that the top carbon (which should consume twice as fast as that of the bottom one) does not consume so fast as the bottom one, it may be assumed that the dynamo machine has reversed its poles, and the leading wires will consequently require changing across. This reversal of poles, though possible, is ofvery rareoccurrence.

Fig. 173.

The dynamo-electric machine should not be driven without its proper leading wires to lamp and lamp being connected up, or at least an external resistance equivalent to that of the lamp (which is approximately one Siemens' unit) must be inserted. In other words, the machine must not be driven when a wire of small resistance connects the two terminalsCandZ. This is expressed more briefly by saying the machine must not beshort-circuited. If it is short-circuited when in motion the electric current becomes so powerful that it will leap from segment to segment of the commutator, where very bright and large sparks will be seen, and if continued would destroy the insulation, thus weakening the current generated.

The leading wires should never be disconnected suddenly while the machine is revolving at its full speed, as such a sudden interruption will produce an intense spark, which will burn the ends of the wire where the contact is suddenly broken. When it becomes necessary to disconnect the wires, the belt should be pushed on to the loosepulley by means of the striking gear, or the steam engine should be stopped.

It may be here stated that all connections should be cleaned bright and screwed tightly, to ensure perfect metallic contacts being made.

Coupling two Machines.—AtFig. 174is shown a diagram of how to make the connections when coupling two machines in parallel circuit.MM',m,m', represent the ends of the wires of the electro-magnets;BB'are the branches;CandZare the terminals of each machine respectively.

Fig. 174.

The three ways in which the various wire connections of these machines are joined up, and which are enough for all ordinary purposes, are given below in paragraphs (a), (b), and (c).

(a) When the machine is workingsinglyand revolving in the direction indicated inFig. 166, the following connections are made:—

Mis connected withB,M'"B',m"Z,m'"C,

and the leading wires of the lamp are connected withCand withZas explained.

(b) When workingsinglyand revolving in the direction indicated in Fig. 168:—

Mis connected toB',M'"B,m"Z,m'"C.

Thus the only change necessary when the machine is to be driven in the opposite direction to that for which it is made, is to disconnect atBthe wire fromMtoBand atB'the wire fromM'toB', and to cross them. The machine will then be connected as above (b).

(c) When workingtwomachines in parallel circuit, as inFig. 174, they must be connected as follows (that on the left of the page being called the first machine, and that on the right the second machine):—

Cof first toCof the second.Z"Z"M"B"B"M"M'"B'"B'"M'"

and then connectCandZof the second machine with the leading wires of the lamp.

The connectionsmtoZandm'toCin each machine are the same as in cases (a) and (b). They do not require to be altered, and may therefore be left out of consideration in all three cases (a), (b), and (c). The whole of the connections here indicated can be quickly made by means of a cross-bar commutator or switch, which is supplied with the machines in cases where such changes are likely to be required frequently. This is usually attached to a wall, leading wires being taken to it from the dynamo machines separately, and others from the switch being led to the electric lamps.

The leading wires from machine to lamp should, whenever possible, be keptseparate, to prevent them rubbing together and making contact. A distance of two inches is quite sufficient to prevent accidents of any kind.

When the leading wires are erected in places where they are likely to rub and chafe against hard substances, it is advisable to enclose each wire separately in india-rubber tubing at all the points where they are likely to be rubbed. This becomes very important on board ship, where everything is in motion, and special care is in consequence required.

Some dynamo machines are coupled direct to the crank shaft of the steam engines; they require the same kind of attention as others, that is to say, they should be driven at a uniform speed, should be well oiled as well as the steam-engine, and they should be kept clean and free from sharp grit.

Application.—The electric light used in the case of adirectattack by torpedo boats, without the assistance of guard boats, will not prove of much assistance, on account of the very small space covered by the beam of light, and therefore if the direction of attack is not exactly known, the beam of light must be kept continually sweeping round the horizon on the chance of picking out the attacking boats, and thus, while flashing in one direction, they may be approaching in another, and effect their deadly mission.

Every man-of-war should be fitted with at least three electric lights, whereby the above-mentioned want of space covered would be to a considerable degree obviated.

If a powerful beam of light be thrown in a particular direction, and there kept stationary, all boats or vessels crossing its path at a distance not exceeding 1600 yards from the ship using the electric light, would become distinctly visible to observers placed behind the light; these vessels remaining visible as long as they continue in such a position that the beam of light acts as a background to them. Under very favourable circumstances, the distance at which the above effect may be observed is much increased.

The parabolic reflector extends only about an arc of 33° at 540 yards' distance from the light.

One defect of this form of reflector is, that it is rapidly dimmed by spray, rain, and by the particles given off by the carbons.

The catadioptric lens, or holophote, gives a far more powerful but a more concentrated beam than the parabolic reflector. By means of such a beam of light, a torpedo boat may be discerned at about one mile distance. By adding divergent lens to the holophote, a less powerful and less concentrated beam of light will be thrown out; in this case about 20° of surrounding water would be well illuminated at about 900 yards' distance, while without the divergent lens there would be only about 5° so illuminated but far more brilliantly.

The distance at which objects can be detected by the electric light depends on their size andcolour, more particularly on the latter.

The observer should as a rule be well removed from the light.

In the case of an electric light being thrown on the observer, the vessel, &c., using it would to that observer be invisible, the light only being seen; also when directed on any particular object, surrounding objects would be thrown into shade.

The electric light will be found very useful for signal purposes by fitting a plane mirror in front of the catadioptric lens; so arranged that it be turned to any desired angle to the axis of the beam of light. By altering the angle of the mirror, the reflected beam of light can be swept from the horizon on one side, through the zenith, to the horizon on the other side. The time of passing the zenith being equivalent to the long and short flashes of the usual night signal code.

In addition to using the electric light to detect the approach of torpedo boats, it may be used by the boats themselves to prevent the attacked vessel from discerning them.

In turret ships, electric lights may be so arranged that the instant an object is brought into the field of the beam of light, the turret guns will be bearing on it.

One great disadvantage of electric lights is the impossibility of protecting them from the enemy's fire, and this is a defect that cannot be eradicated, though it may be lessened, by manipulating them from the tops of a ship.

Torpedo Guns.—Hitherto by torpedo guns has been meant small guns mounted on carriages so constructed that a shot may be fired into the water only a few feet from the ship's side, or mitrailleuses, Gatlings, &c. Here the term is applied only to machine guns, which are constructed to fire either volleys, or, extremely rapidly, single shot, each shot of which would be capable ofpenetratingandsinkingtorpedo boats, such as Messrs. Yarrow and Thornycroft are daily launching from their yards. Of such weapons there are at present only two, viz., the "Nordenfelt" and "Hotchkiss" gun. The former has, after very exhaustive experiments, been adopted by the English, Austrian, Swedish, and other naval authorities, while the latter has been adopted by the French government.

Nordenfelt Torpedo Gun.—This gun, as it at present is constructed, consists of four barrels of 1 inch calibre.

The barrels are fixed in a horizontal plane, and are not movedduring the firing; and the movement of the lever, the loading, the firing, and the extracting are all performed in the same plane, so that theelevationof the gun is not disturbed by the firing.

The gun is fed by means of hoppers, each of which contains ten rounds per barrel,i. e., forty shots.

The continuous supply of cartridges, as well as the firing and extracting, are all performed by one motion of the lever, thus enabling the gunner to use his left hand to lay the gun.

A volley of four shots can be fired at the same moment, or one shot can be fired separately. Eight shots can be fired in 1-1/4 seconds; twenty, thirty, or forty shots can be fired at a rapidity of two hundred shots per minute without difficulty.

The recoil being taken up by the whole framework of the gun does not in the least disturb the aim.

The entire mechanism of the gun can be opened up without undoing a single screw, in less than 20 seconds.

All the four spiral firing springs can be taken out, without opening the rest of the mechanism, in 1-1/2 seconds.

All the parts of the mechanism are made interchangeable, so that reserved parts can at any time be substituted. The gun can be placed on half cock, so that the strikers do not act; and for further security the lever can be locked. The carrier block, without which the gun cannot be fired, is loose, and can be taken away, in case it becomes necessary to abandon a gun, which is thus made useless to the enemy.

The bullets are solid steel, weighing about 1/2 lb. At 1760 yards at right angles this gun will penetrate a 3/16 inch steel plate, which represents the thickness of the plates of a torpedo boat.

At 200 yards at right angles it will penetrate one 3/16 inch steel plate placed in front of a 1/2 inch steel plate with a space of 3 feet between them, this target representing the plates and boiler of a torpedo boat.

At the same distance, at 30° angle against the line of fire, it will penetrate a 1/2", 1/4", or 3/16" steel plate.

The holes in some instances are from 6 to 11 inches in length, and 2-1/2 inches in height. Angle of depression 20°, of elevation 30°, and of direction 360°.

Weight of the gun 3-3/4 cwt., and weight of carriage 2-1/2 cwt.

Hotchkiss Torpedo Gun.—This gun consists of a group of five barrels, revolving on a central shaft, a breech block, containing the firing mechanism, a feeding hopper, and the necessary hand crank for training and firing. The gun is mounted on trunnions attached to a vertical column, which rests in a suitable socket bolted to the ship's side; by this means a universal motion is obtained.

The essential difference between this and the Nordenfelt gun is, that thebarrelsand mechanism are put into rotatory motion.

Another point of difference is that single shots only can be fired, and not a volley, as in the Nordenfelt gun.

With the Hotchkiss gun, only some thirty shots can be fired in one minute at an advancing torpedo boat. The weight of the Hotchkiss steel shot is about 1 lb., but owing to the low velocity of the gun, its penetrative power is little more than that of the Nordenfelt 1/2 lb. bullet.

The object to be gained in firing at an attacking torpedo boat is to sink her, and not merely to kill or disable her crew, for supposing the attack to be made with a contact spar torpedo, and the boat to have reached within 300 yards' distance from the ship, then, even if all the crew (probably two or three men) were disabled or killed, the boat would, if not sunk, still carry out its work of destruction; therefore the projectiles to be used under such circumstances should be only those capable of penetrating a torpedo boat's plates,i. e., solid steel shot, not shells.

Diving.—In laying down and in picking up submarine mines, divers will be found extremely useful; also in clearing a passage in a river, &c., of an enemy's torpedoes in time of war. During the late Turco-Russian war, the harbour of Soukoum Kaleh taken by the Turks waspopularlysupposed to have been cleared of its mines by native divers (Lazees), but as the torpedoes so captured were never seen at Stamboul, it must have been a stretch of imagination; probably such would have been done, had there been any mines in the harbour to clear away.

The following is a general description of Messrs. Siebe and Gorman's improved diving apparatus.

The apparatus consists of

Air-pump.—This improved air-pump consists of two double action cylinders, each cylinder capable of supplying about 135 cubic inches per revolution. The advantage of this air-pump is, that it can supply air to two divers, working independently and at different levels, each diver being in direct connection with one of the cylinders. The air-pipes are in lengths of 45 feet and 30 feet, made of vulcanised india-rubber with a galvanised iron wire imbedded; this protects from corrosion, and allows the air to pass through the pipes with less friction.

Diving Dress.—The diving dress is made of solid sheet india-rubber, covered on both sides with tanned twill; it has a double collar, the inner one to pull up round the neck, and the outer one of vulcanised india-rubber to go over the breast-plate and form a water-tight joint. The cuffs are also of vulcanised india-rubber, and fit tightly round the wrist, making, when secured by the vulcanised india-rubber rings, a water-tight joint, at the same time leaving the diver's hand free.

Breast-plate.—The breast-plate is made of tinned copper, and has a valve in front, by which the diver can regulate the pressure of air inside his dress and helmet. The outer edge of the breast-plate is of brass, and is secured by screws to the outer collar of the dress.

Helmet.—The helmet is made of tinned copper, and has a segment bayonet screw at the neck, corresponding to that of the breast-plate, which enables the helmet to be removed from the breast-plate by one-eighth of a turn. It has three strong plate glasses in brass frames, protected by guards; two oval at sides, and a round one on the front; the front one can be unscrewed, to enable the diver to give and take orders. At the side is an outlet valve, which, by inserting a finger, the diver can close, and so rise to the surface. The valve allows the foul air to escape, and prevents the entrance of the water. An elbow tube is securely fitted on the helmet, to which is fixed an inlet valve, to which the air-pipe is attached. The inlet valve is made that the air can enter, but in case of a break in the air-pipe it cannot escape.

The front and back weights are of lead, heart-shaped, and weigh about 40 lbs. each.

Boots.—The boots are made of stout leather, with leaden soles, and are secured over the instep by a couple of buckles and straps. Each boot should weigh at least 20 lbs.

Crinoline.—The crinoline or shackle is used for deep water; it is placed round the body and tied in the front of the stomach: being supported by braces, it affords protection to the stomach, and enables the diver to breathe more freely.

Ladder.—An iron ladder should be provided with stays to bear against the side of the boat from which the diving is carried on, to which may be attached (if working in deep water) an ordinary rope ladder, with ash rounds, and weighted at the end. Some divers have the ladder only 20 feet long, to the last round a rope with a weight attached, which rests on the ground; by that means they descend.

Directions for using the Apparatus.—The ladder having been fixed, the position of the pump should be decided on, and it should be securely lashed by means of the ropes attached to the handles down to a stage, into which thescrew-eyesshould be fastened if necessary; the pump should be placed out of the way of the divers, the men attending on them, and all the men employed. The best position for the pump is facing the head of the ladder, and about six feet from it.

While the diver is dressing, the pump should be prepared for use, the winch handles should be taken out of the pump case, the nipples protecting the crank axles removed, the nuts being replaced on their screws. The nuts for the ends of the crank axles are taken off, the fly-wheel placed on the shaft, and the winch handles put on, and secured by the nuts, which are screwed home with the spanner. The pump is always worked in its case.

The flaps covering the pressure gauges and that at the back of the pump case should be opened, the screw on the overflowing nozzle of the cistern removed, and the cistern filled with water; the caps of the air delivery pipes should be removed, the necessary lengths of air-pipe should be put together carefully with washers in place, and all the screws must be worked home by means of thetwodouble-ended spanners. The air-pipes should be tested by holding the palm of the hand to the end of the pipe, till the pressure shown on the pressuregauge is considerably above that corresponding to the depth the diver is to descend.

Dressing the Diver—Crinoline only for Deep Water.—The diver having taken off his own clothes, puts on a guernsey, a pair of drawers, very carefully adjusted outside the guernsey, and securely fastened by the tape round the waist, to prevent them from slipping down, and then a pair of inside stockings. If the water be cold, the diver may put on two or more of each of the above articles. He then puts on the crinoline and woollen cap, drawing the latter well over his ears; some divers find relief from putting cotton saturated with oil in their ears.

Theshoulder padis then put on, and tied under the diver's arms. He then gets into the diving-dress, which in cold weather should be slightly warmed, drawing it well up to his waist; he next puts his arms into the sleeves, an assistant opening the cuffs by means of the cuff expanders, or by inserting the first and second fingers of both hands, taking care to keep his fingers straight. The diver, by pushing, forces his hand through the cuff. He puts on a pair of outside stockings and a canvas overall to preserve the dress from injury.

The diver then sits down, and the inner collar of the dress is drawn well up and tied round the neck with a piece of spun yarn, and the breast-plate put on, great care being taken that the india-rubber of the outer collar is not torn in putting it over the projecting screws of the breast-plate. The four pieces of the breast-plate band, which with the thumbscrews had been previously placed for safety in one of the boots, are then put over the outer collar, and secured to the projecting screws by means of the thumbscrews; the centre screw of each plate should be tightened first. It will generally be sufficient if the thumbscrews be screwed up hand-tight, the spanner being only used when necessary. The canvas overall is now adjusted and the boots are put on.

The rings are passed over the cuffs, and the sleeves of the overall are drawn down to cover them. If gloves are to be used, the rings will be put on over them, as well as the cuffs. The helmet (without the front bull's-eye) is then put on; before doing so, the attendant should blow through the outlet valve of the helmet; he can do so by placing his head in the interior, and placing his mouth to the holewhere the air escapes. Blow strongly; if in proper working order, the valve will vibrate. A loop of the life line is placed round the diver's waist, the line brought up in front of the man's body, and secured with a piece of small rope passing round his neck, or to the stud on the helmet. The waist-belt is buckled on with the knife on the left side, the end of the air-pipe being passed from the front, through the ring on the belt on the man's left, and up to the inlet valve on the helmet, to which it is secured; the upper part of the pipe is then made fast by a lashing to the stud on the left of the helmet. The diver then steps on the ladder, and two men are told off toman the pump.

The weights are then put on, the front weight first, the clips being placed over the studs on the breast-plate. The back weights are then put on, and the clip lashings over the hooks on the helmet, and the two are secured to the diver's body by means of the lashing from the back weight, which is passed round the waist, through the thimble beneath the front weight, and tied to the other end of the lashing at the back weight.

When the signalman is sure that all is right, and that the diver understands all the signals, he gives the wordPump, and screws the centre bull's-eye into the helmet securely; this done, he takes hold of the life line and "pats" the top of the helmet, which is the signal for the diver to descend.

Signals employed.—The signalman is the responsible person, and must be very vigilant all the time the diver is down; occasionally he will give one pull on the life line, and the diver should return the signal by one pull signifying "all right;" if the signal be not returned, the diver must be hauled up, but if the diver wishes to work without being interrupted by signal, he gives one pull on the line, independently, for "All right; let me alone." If the signalman feels any irregular jerks, such as might be occasioned by the diver falling into a hole, he should signal to know if he is all right, and if he does not receive any reply, he should haul him up immediately. If the diver from any cause is unable to ascend the ladder, and wishes to be pulled up, he gives four sharp pulls on the life line. If while being hauled up the diver gives one pull, it signifies "All right; don't haul me any more." The diver should be hauled up slowly and steadily. If the signalman wishes the diver to come to the surface, he gives four sharppulls on the line, on which the diver should answer "All right," return to the foot of the ladder, and signal to be hauled up.

Onepull on the air-pipe signifies that the diver wants more air.Twopulls on the life line andtwopulls on the air-pipe in rapid succession, signify that the diver is foul and cannot release himself, and requires the help of another diver; on receiving such a signal, no attempt should be made to haul the diver to the surface.

The above signals are to be invariably used; but other signals may be arranged as is most convenient for any particular work, as a great variety can be made with the life line and air-pipe. The diver can communicate with the surface by means of a slate.

Further information on this subject, especially with regard to the foregoing diving apparatus, will be found in Messrs. Siebe and Gorman's "Manual for Divers."

FOOTNOTES:[V]In wrought iron there is always some residual magnetism; there is therefore no necessity to start the magnetism with a permanent magnet.[W]Siemens' unit.

[V]In wrought iron there is always some residual magnetism; there is therefore no necessity to start the magnetism with a permanent magnet.

[W]Siemens' unit.

THEORY of Electricity.—The theory most readily understood, and which most satisfactorily explains the various electrical phenomena, is as follows:—

"That every substance and every atom of the world is pervaded by a peculiar, subtle, imponderable fluid which is termedElectricity, but which is not known to exist, or remains in a state ofelectrical equilibrium, until evoked by certain causes."

The effect of causing a disturbance of this equilibrium is to increase the normal, or natural, electricity in some particles, and to equally decrease it in other particles, i.e. what one loses the other gains. An excess of natural electricity is denoted by the termpositive, or mathematical symbol (+) while a deficiency is denoted by the termnegative, or symbol (-).

Like electricities repel each other.

That is to say, two bodies charged with an excess of, or positive, electricity, being brought together repel each other, neither wishing to increase the excess that has been evoked in them.

Similarly in the case of two bodies charged with a deficiency of, or negative, electricity, neither wish to add to the deficiency already there.

In both these cases there can be no tendency to electrical equilibrium, which is the principle at work. In the former case, there being already too much, more will but increase the disturbance.

In the latter case, further deficiency will but add to the irregularity.

Unlike electricities attract each other.

That is to say, if two bodies, one charged with positive, or having an excess of electricity, the other charged with negative, or having a deficiency of electricity, be brought together, they will attract each other; both being desirous of altering their existing state, theone by decreasing its excess, and the other by decreasing its deficiency of electricity.

In this case, there will be a tendency to equilibrium, caused by attraction. The earth is supposed to be a vast reservoir of electricity, from which a quantity can be drawn to fill up a deficiency, and which is always ready to receive an excess from other bodies. Every body in nature has its own natural quantity of electricity, and when an object is negatively electrified, or has a deficiency in its normal quantity, there is a tendency to receive a supply from any convenient source. Such an object would receive electricity from the earth if means were afforded; and a bodypositivelyelectrified, would tend to part with its excess in the same manner. Where such facilities for establishing electrical equilibrium are afforded, the result is the passage of acurrentof electricity.

Conductors.—Sensible effects can be produced by electricity at great distances from the source, provided there be a medium of communication, that is, goodconductorsto transfer it. When a glass rod is rubbed with a piece of silk, it becomes charged with an excess of, or positive, electricity, and at the same time the silk becomes charged with negative electricity.

The glass rod will retain the positive electricity upon it for some time, unless touched with the wet hand, a wet cloth, a metal, &c., when it will instantly cease to be electrified. The electricity is then said to have been conducted away, and the bodies which allow it to run off the glass are calledconductorsof electricity. Metals, water, the human body, charcoal, damp wood, and many other bodies are conductors.

Those bodies which conduct electricity hardly at all, such as the air, silk, glass, sealing wax, gutta percha, india rubber, &c., are termednonconductorsorinsulators.

Strictly speaking, all substancesconductelectricity in some degree, and anonconductoris merely abadconductor.

In the following table the bodies are arranged in their order of conductivity, i.e. each substance conducts better than that which precedes it; the first-named body is the best insulator, and the last-named one is the best conductor.

Though two substances are near one another in the above list, they do not necessarily approach one another in their power of conducting. For instance, taking the conducting power of pure silver as represented by the number 100, then

PureCopper will be equal to 99·9,Gold will be equal to 78·0,whileZincwill be only equal to 29·0,

and pure water, which is half-way down the list, will offer 6,754 millions more resistance than silver to the passage of the electric current.

The metals being the best known conductors, are usually employed as the means of transferring the electric current from one place to another.

Electric Circuit.—The conditions attending this operation are different from those of any other known method of transmission.

A completecircuitmust always be formed by the electric current, i.e. it cannot start from one placeA, travel to another placeB, and cease there, but the current must be completed before it can be said to have reachedB. There cannot be a current of electricity without a means of recombination, which recombination must be at thesource, or place of original disturbance.

This "place of disturbance" orsourcemust be considered as having two sides, i.e. at some spot the normal or natural electrical equilibrium is disturbed, and electricity is separated into too much (positive) on one side, and too little (negative) on the other side. If then no meansof recombination be afforded, the electricities remain separated, and no current exists; but if aconductorbe made to connect the two sides, electricity is set in motion, and a current established. Originally to form a circuit between two stationsAandB, a conducting wire and a return wire were necessary, but in 1837 Steinway discovered that the earth itself answered all the purposes of a return wire, in fact under favourable conditions much better. Thus, to form a circuit betweenAandB, a conducting wire is required, and a buried metal plate atAandB, the earth by these means taking the place of the return wire.

The aforesaid metal plates are technically termedearth plates. The greater the size of the earth plates (up to certain limits), the deeper they are buried, and the better the conducting power of the soil surrounding them, the better conductors the plates become, or the less resistance the earth portion of the circuit offers. If either plate be not in communication with the earth, or else be separated from the wire, the circuit is not complete, or, as it is termed, "it is broken," and no current will flow, the signal not made, torpedo not fired, &c.

"Short" Circuit.—Due to the fact that recombination, or a tendency to equilibrium, is always at work when electricity has been evoked, the conducting path along which the electric current flows must be covered with a nonconducting substance, or, as it termed, "insulated," or else the current would not perform its duty, but escape to earth, and so form what is termed a "short circuit."

A current of electricity always chooses theeasiest pathto effect recombination, or electrical equilibrium.

Insulators, &c.—On land, telegraph wires are as a rule laid above the ground, and therefore require supporting at every few yards; this is done by means of posts, and as these are formed of substances which are conductors of electricity, the wires require to be insulated from them. The insulators generally employed for such purposes are cup-shaped pieces of porcelain, or pottery, fixed to the head of the telegraph posts. By means of these insulators, the current of electricity is prevented from escaping to the earth by the post conductors.

A certain amount of leakage, or loss of electricity, must occur at each of these posts, as there is no such thing as a perfect insulator. When the wires are laid on the ground or under ground, or under water, they are insulated by covering them with gutta percha, india rubber, &c., and any loss of current is thus prevented.

Methods of generating Electricity.—For the purposes of torpedo warfare there are two methods of evoking electricity, viz.—

1.—Bychemical action.

2.—Byfriction.

By Chemical Action.—Chemical actionis the chief source of free electricity, the representative of which is the galvanic, or Voltaic, battery.

The electricity so generated is also termed dynamical electricity, due to there being a constant electric current, so long as the poles of the battery producing it are kept closed; the electricity being thus in adynamicor moving state.

By chemical action is signified that which occurs when two or more substances so act upon one another as to produce a third substance differing altogether from the original ones in its properties, or when one substance is brought under such conditions that it forms two or more bodies differing from the original ones in their properties.

Definition and Properties of a Voltaic Cell.—TheVoltaiccell consists of an insulating jar, containing a liquid, in which are placed two plates or pieces of dissimilar metals; the liquid must be composed of two or more chemical elements, one of which at least tends to combine with one or other of the metals, orwith both in different degrees.

By a Voltaicbatteryis meant a number of cells above one; this term, however, is often applied to a single cell when working by itself.

A "simpleVoltaic cell," "element," or "couple," consists of two metals placed in a conducting liquid. If two metals—for instance, zinc and copper—are placed in water slightly acidulated, without touching each other, no effect is apparent; but if they be made to touch, bubbles of hydrogen gas are formed over the copper plate, and continue forming these until the plates are separated. After being in contact for some time, the copper plate will be found unaltered in weight, but the zinc plate will have lost weight, and the portion so lost will be found in the liquid in the form of sulphate of zinc. The same effects are also produced by connecting the two plates by means of some conducting substance, instead of placing them in contact.

Zinc is invariably employed as one of the metal plates, on account of the ease with which it dissolves in dilute acids; and the greatest results are obtained when the second metal plate is not acted upon at all by the liquid, for then the whole effect due to the oxidation of the zinc plate is obtained; but when the second plate is also chemicallyacted upon, then only the effect due to the difference between the two chemical actions is obtained, for, as will be explained further on, they each act in directly opposite directions.

Voltaic Current.—The Voltaic current makes its appearance under the general laws of electrical action.

When a body charged with anexcessof, orpositive, electricity, is connected with the earth, electricity is transferredfromthe charged body to the earth; and similarly when a body is charged with adeficiencyof, ornegative, electricity, is connected with the earth, electricity is transferredfromthe earth to the body.

Generally whenever two conductors in different electrical conditions are put in contact, electricity will flow from one to the other. That which determines the direction of the transfer is the relativepotentialof the two conductors. Electricity always flows from a body athigher potentialto one atlower potential, when the two are in contact, or connected by a conductor. When no transfer of electricity takes place under these conditions, the bodies are said to be at thesame potential, which may be eitherhighorlow. Thepotentialof the earth is assumed to bezero.

Definition of Potential.—"Thepotential of a body or point, is the difference between the potential of the body or point, and the potential of the earth."

Difference of potential for electricity is analogous to difference of level for water. Now, since, when a metal is placed in a vessel containing a liquid, electricity is produced, the liquid becomes of a different potential to the metal, each being electrified in an opposite way; and therefore, as above stated, there being adifferenceof potentials, electricity will tend to flow from one to the other.

This is evidence of aforcebeing in action, for there can be no motion without some force to produce it.

Electro-motive Force.—Electro-motive forceis the name given to a peculiar force to which is due the property of producing a difference of potentials. When it is said that zinc and water produce a definite electro-motive force, what is meant is, that by their contact a certain definite difference of potentials is produced.

Theelectro-motive forceof a Voltaic element may be termed itsworkingpower, in the same way as the pressure of steam is the working power of a steam engine, though this is not to be considered as thereal source of power, which, as will be seen, is uncertain. Due to the difference of potential of the metal and the liquid, a current of electricity will flow from one to the other, causing the chemical decomposition of the liquid, and the reaction may be taken as the origin of the power employed.

But while the expenditure of energy (which is necessary to produce aforce) is accounted for by taking the chemical action as the source of power, the preceding cause of this chemical action, viz. the flowing of the current of electricity due to the difference of potential of the metal and the liquid, must also have first involved the expenditure of energy; thus the real source of power is very uncertain.

Electrolytes.—As before stated, a Voltaic cell consists of two plates of dissimilar metals, which must be immersed in a liquid composed of two or more chemical elements, one of which at least will combine with one or other of the metals, or both in a different degree. Those liquids which are thus decomposed by the passage of a current of electricity are termedelectrolytes.

The elements, then, forming the electrolyte may have chemical affinity for both metals, though in a greater degree for one than the other.

"Oxygen" is the most important element of an electrolyte, and to theaffinity for oxygen of the metalsis the magnitude of the result and effect.

Terms Electro-positive and Electro-negative.—All metals have a definite relation to each other as to the potential which any one may have when brought into contact with another. Thus, when zinc is brought into contact with copper, the former has a potential positive to the latter, i.e. a current of electricity will tend to flow from the zinc to the copper. The metals may be so placed in a list that each one would be positive to any of those that follow it; it is then said to be electro-positive to them, and they are electro-negative to it. As those metals which are electro-positive to others have a greater affinity for oxygen, and those that are electro-negative to others a less affinity for this element, the terms electro-positive and electro-negative signify, in effect, greater or less affinity for this element. Conversely, oxygen will combine more readily with the former than with the latter.

The following list shows the commoner metals arranged in electro-chemical order.

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