Torpedo:—As in the 5th experiment. It was placed on the ground, immediately under the edge of the outer bottom, 39-3/4' from the target, and opposite No. 18 frame, 70' from the stern; 50' below the surface of the water.
Effect of explosion:—Outer and inner bottom broken entirely asunder at No. 19 frame on the starboard side, and between Nos. 16 and 17 on the portside. A fracture was caused in the outer bottom extending from the shelf plate to upper edge of strake next the keel on the starboard side, and from the shelf plate to upper edge of flat keel plate on the port side. A fracture was also caused in the inner skin extending from the topside to the outer edge of the garboard strake on the starboard side, and from the topside to upper edge of garboard strake on the port side; this including a fracture of the keel at No. 17. The vertical keel, the longitudinals, as well as numerous bracket plates and angle irons, were broken, and about 2000 rivets in the outer bottom were rendered defective.
The outer bottom was indented over a considerable length, the indentation being greatest between the frames, and the maximum being 8 inches. The inner bottom was not indented or damaged, with the exception of the fractures before mentioned.
Experiments at Pola, Austria, 1875.—These experiments were carried out to determine the effect of very heavy charges of dynamite on an iron pontoon fitted with a double bottom, similar to that of H.M.S.Hercules.
Target:—An iron pontoon 60' long and 40' beam, with circular ends and fitted with a double bottom, also a condenser and two Kingston valves.
Torpedo:—617 lbs. of dynamite. It was 62' horizontally from the keel, 53' actual distance from the side, and opposite amidships, 40·5' below the surface of the water, and 20' from the ground.
Pontoon:—Draught of water 19', and moored in 62' of water.
Effect of explosion:—The pontoon moved away bodily a distance of 13 feet; a few rivets in the outer bottom were started, and the outer skin was slightly indented between the frames; the maximum indentation being 1·5". No other damage was sustained by the hull. Several of the screws securing the flanges of the Kingston valves were slightly loosened.
Torpedo:—585 lbs. of dynamite. It was placed 60' horizontally from the keel, 48' actual distance from the side, and opposite amidships; 36' below the surface of the water, and 42' from the ground.
Pontoon:—Draught of water 19·5', and moored in 74' of water.
Effect of explosion:—The pontoon, which had been more rigidly moored than in the previous experiment, was moved bodily away a distance of 4 feet. Many rivets were loosened, and a few connecting the angle irons were sheared; also the outer skin was slightly indented. No damage was done to the condenser or Kingston valves.
Experiment in the Sea of Marmora, 1875.—This experiment was carried out by Turkish officers attached to their naval school at Halki, an island in the Sea of Marmora, about eight miles from Stamboul. It consisted in destroying a Turkish schooner by the explosion of an 100-lb. gun-cotton mine in contact with her, moored in 58 feet of water, and 10 feet beneath the surface.
Experiment at Carlscrona, Sweden, 1876.—This experiment was a continuation of those previously carried out in 1874-75, and which have been detailed atpage 224, &c.
Target:—The same as had been used for the previous experiments (1874-75), and which had been thoroughly repaired.
Torpedo:—660 lbs. of gunpowder, enclosed in a buoyant cylindrical 1/4" steel case with domed ends, and contained in an inner 1/16" steelcase. It was ignited by two Von Ebner fuzes placed in a charge of 1/4 lb. of gunpowder and enclosed in a glass bottle. It was placed 5' horizontally from the water line, 23·75' actual distance from target, and opposite No. 5 (middle) frame of target, 29' below the surface of the water.
Effect of explosion:—The ship was moored in 54' of water. She was lifted by the explosion, rolled over to port, and then settled to starboard, sundry large pieces of timber being thrown up in the air. The outer bottom of the target was broken through above the second longitudinal frame, from the fourth to the seventh frames laterally, and from the top of the target to the second longitudinal frame vertically, the hole made measuring about 9' high by 12' wide, or about 100 square feet in area. The inner bottom was also broken through between the top of the target and second longitudinal frame, and between the fourth and seventh vertical frames, the hole made being about 75 square feet in area. The bracket frames within the damaged area were but little damaged. The wing passage bulkhead was broken through opposite to Nos. 5 and 7 frames, the holes made being respectively 18 and 17 square feet in area. Through these holes the force of the explosion had made its way to the horizontal iron deck, forming the top of the target, which was completely broken through a little abaft No. 5 frame, the hole made measuring about 100 square feet in area. A piece of this iron deck, weighing, with the iron fastenings attached to it, about 1650 lbs., was thrown 16' against the upper deck beams. The target below the second longitudinal frame was comparatively but little injured. The outer bottom was indented and cracked in one or two places, but the inner bottom was uninjured. In addition to the damage to the target, the ship herself sustained serious injury, eleven of the lower deck beams, with their knees being broken (six being broken completely across). The main keel immediately under the target was also opened at the scarf, and the back of the ship was apparently broken. The hull had given out laterally to such an extent as to prevent the ship being taken into dock.
Experiments at Portsmouth, England, 1876.—The object of the following experiments was to determine the effect of comparatively small charges of gunpowder and gun-cotton exploded in actual contact with an ironclad, as would be the case in a torpedo attack either with locomotive towing or spar torpedoes.
Target:—the same as used in the experiments of 1874-5, which have been detailed atpage 229, &c., viz., theOberonfitted to represent H.M.S.Herculeswithout the armour. Her mean draught was 11', and she was moored in 26-1/2' of water. TheOberonhad been placed in a thorough state of repair.
Torpedo:—60 lbs. of gun-cotton in slabs, saturated with water. Total weight of charge 75 lbs. It was enclosed in a 1/4" iron case with cast iron ends. It was placed at 15' actual distance from the nearest side of the case to the target, and opposite No. 4 frame on the port side, 10' below the surface of the water.
Effect of the explosion:—The effect upon the vessel was unappreciable. This charge represented the large Whitehead fish torpedo, and its position corresponded to that of this torpedo when striking a net at a small angle with the keel.
Torpedo:—The Harvey towing torpedo, charged with 66 lbs. of gunpowder, primed with gunpowder, and fired by means of an electric fuze. It was placed at 3' actual distance from the target, measuring from the centre of the torpedo, and opposite No. 4 solid frame on the starboard side, the vertical axis of the torpedo being at right angles to the vessel's side, 9-1/4' below the surface of the water.
Effect of explosion:—This and the two following torpedoes were fired simultaneously. The outer bottom was blown in from the upper edge of the flat keel plate to the underside of the water-tight longitudinal, and fore and aft from No. 2 to No. 6 frames; an area 16' × 8-5/6'. Flat keel plates were broken between No. 2 and No. 4 frames, and the 4th strake of the bottom plating was broken, and the frames for that space blown in. Two holes were blown through the inner bottom, measuring respectively 2' × 2' and 7' × 1', making the total area of the inner bottom destroyed, 11 square feet.
Torpedo:—33 lbs. of granulated gun-cotton, saturated with water; total weight of charge being about 41 lbs. It was enclosed in a1/4" iron case, 12-1/2" × 12" × 12-1/2", the primer being 2-1/2 lbs. of slab gun-cotton, included in the 33 lbs. It was placed at 4' actual distance from the target, measuring from the centre of the case, and opposite No. 30-1/2 solid frame on the starboard side; 9-1/4' below the surface of the water.
Effect of explosion:—Outer bottom blown in from upper edge of the lower longitudinal to the lower edge of the upper longitudinal between Nos. 28 and 32 frames; an area of 18 × 11 feet. The butts of the flat keel were started and the plating broken across No. 30-1/2 frame from the flat keel plate to the upper deck. Shelf plate at Nos. 30-1/2 and 32-1/2 frames was broken. Nos. 29, 30, and 31 frames were blown in from first to third longitudinal; lower longitudinal from No. 28 to 31 also blown in. Two holes were blown through the inner bottom, measuring respectively 6 × 1·5' and 5' × ·25', making the total area of inner bottom destroyed 10 square feet. A steam launch with steam up and outrigger torpedo gear in place, one pole being rigged out, was placed with the stem of the boat 22' horizontally from the torpedo. She was uninjured and shipped very little water.
Torpedo:—31 lbs. 14 oz. of gun-cotton in slabs, saturated with water, total weight about 40 lbs. It was enclosed in a 1/4" iron case 12-1/2" × 12-1/2" × 6"; primer being 20 oz. of gun-cotton, included in the 31 lbs. 14 oz. It was placed at 4' actual distance from the target measuring from the centre of the case, and opposite No. 30-1/2 solid frame on the port side; 9-1/4' below the surface of the water.
Effect of explosion:—Outer bottom and frames injured in a similar manner to that described in the third experiment. Outer angle irons of the 1st, 2nd, and 3rd longitudinals were started in the wake of the broken place. A hole was blown through the inner bottom, measuring 9·5' × 1', or about 10 square feet in area. The bolts of the outer bottom plate of stern post much open, and at Nos. 16 and 17 on the port side the upper two strakes were buckled and the shelf plate started.
A steam launch, arranged in the same manner as in the fourth experiment, was uninjured, and shipped but little water.
Experiments with Countermine.—The following experiments have been carried out in England and other countries to ascertain some reliable data for countermining operations.
Experiments in the Medway, England, 1870.—Countermine:—432 lbs. of compressed gun-cotton, enclosed in a 3/16" iron case. It was moored at a depth of 37' below the surface of the water.
Submarine mines:—A series of similar cases containing coal dust, &c., were moored at distances of 50' to 100' from the countermine, and 37 feet below the surface.
Effect of explosion:—The submarine mine at 80' distance was completely destroyed; the dome of its circuit closer was dented in.
Countermine:—As before, but moored 27' below the surface.
Submarine mines:—As before, but moored at distances of 70' to 120' from the countermine, and 27' below the surface.
Effect of explosion:—The submarine mine case at 120' distance was dented, but remained water-tight; the copper guard of fuze piece collapsed, and the earth connection of the fuzes was ruptured; the dome of its circuit closer was dented.
Countermine:—As before, but moored 47' below the surface.
Submarine mines:—As before, but moored at distances of 70' to 200' from the countermine.
Effect of explosion:—The submarine mine case at 200' distance was dented, but it did not leak.
Experiments at Stokes Bay, England, 1873.—Countermine:—500 lbs. of gun-cotton, enclosed in a 3/16" iron case. It was placed on the ground, in 47' of water.
Submarine mines:—Six ground mines, 1/4" thick cases, fitted with circuit, 10' below the surface, at distances of 100' to 200' from the countermine.
Effect of explosion:—Submarine mines at 100' and 120' distance were destroyed, and their circuit closers thrown out of adjustment;submarine mines at 140' and 170' distance were much bulged, and leaked, and their circuit closer spindles were bent; submarine mine at 200' distance was uninjured, but its circuit closer was thrown out of adjustment.
Countermine:—100 lbs. of gun-cotton enclosed in case, thickness No. 12 B. W. G. It was moored 10' below the surface, in 35' of water.
Submarine mines:—Five similar mines placed at same depth, at distances of 50' to 150' from the countermine.
Effect of explosion:—The submarine mine at 50' distance showed continued or dead earth, two screws broken, and its case dented; the other mines were uninjured.
Experiments at Carlscrona, Sweden, 1874.—Countermines:—226 lbs. of dynamite, enclosed in a case 17-1/2" × 20" × 1/8". It was moored 9-3/4' below the surface, the depth of water being 41 feet.
Submarine mines:—(a) cast iron ground 600 lb. mines, dome shaped, 48-3/4" × 21-1/2" × 2"; (b) cylindrical cases, wrought iron, empty, 11-1/2" × 11-1/2" × 1/8"; (c) cylindrical cases, wrought iron, charged, 11-1/2" × 11-1/2" × 1/8"; (d) cylindrical cases, wrought iron, 30-1/4" × 30-1/4" × 1/8"; (e) spherical cases, wrought iron, 32-1/2" × 1/8"; (f) spherical cases, tinned steel, 12" × 1/8".
Effect of explosion:—(b) mine, at 34' distance, was destroyed, and one at 92' distance was slightly bulged; (c) mine, 58' distance, mouthpiece injured and case leaky; (d) mine, 244' distance, a rivet started.
Countermine:—As before, but moored at 29-1/4' below the surface; depth of water, 41 feet.
Submarine mines:—As before.
Effect of explosion:—(a) mine, at 146' distance, split in two; (b) mine, 34' distance, destroyed; at 49' distance, fractured; at 68' distance, indented but not fractured; (c) mine, 58' distance, case much bulged, and leaky; (d) mine, at 244' distance, rivets started, case half full of water; at 195' distance, sunk, several rivets started; (e) mine, at 195' distance, bolt loosened; (f) mine, at 68' distance, not injured.
Countermine:—453 lbs. of dynamite, enclosed in a case, 24-1/2" × 28-1/4" × 1/8". It was moored 9-3/4' below the surface; depth of water as before.
Submarine mines:—As before.
Effect of explosion:—(b) mine, at 49' distance, sunk and not recovered; at 58' distance, very much indented; (c) mine, at 58' distance, case much indented and leaky; (f) mine, at 48-1/2' distance, uninjured.
Countermine:—As before, but moored 29-1/4' below the surface.
Effect of explosion:—(a) mine, at 195' distance, completely stove in; (c) mine, at 58' distance, case indented but charge dry; (e) mine, at 175' distance, slightly leaky; (f) mine, at 48-1/2' distance, upper half indented in three places. It was also discovered during the above experiments that submarine mines charged with dynamite can be caused to explode by the detonation of a charge of the same explosive, at distances from it considerably beyond those at which the cases themselves are damaged by a similar charge. To prevent the foregoing, it is necessary to pack the dynamite very carefully, using at the same time special precautions.
ELECTRIC lights combined with fast steam launches as guard boats and specially constructed torpedo guns, such as the Nordenfelt and Hotchkiss machine guns, are at the present time the onlytruly practicablemeans afforded to a man-of-war of defending herself against the attack of torpedo boats, whether these latter are armed with the spar, fish, or towing torpedo; the torpedo gun sinking the boats after the electric light and guard boats have detected their approach and position.
As has been before stated, nets, shields, booms, &c., placed around a vessel of war, must, however slightly constructed, affect to a considerable degree her efficiency, by decreasing her power of moving quickly in any desired direction, which is essential to the utility of such a vessel in time of war; and thus on electric lights, guard boats, and torpedo guns must the safety of ships in future wars really depend, when attacked by torpedo boats.
The Electric Light.—The phenomenon of theVoltaic arcwas first discovered by Sir Humphry, then Mr., Davy at the beginning of the present century. The following is an account of the matter as given by him in his "Elements of Chemical Philosophy":—
"The most powerful combination that exists, in which number of alternations is combined with extent of surface, is that constructed by the subscription of a few zealous cultivators and patrons of science in the laboratory of the Royal Institution. It consists of 200 instruments, connected together in regular order, each composed of ten double plates arranged in cells of porcelain, and containing in each plate thirty-two square inches; so that the whole number of double plates is 2,000, and the whole surface 128,000 square inches. This battery, when the cells were filled with sixty parts of water, mixed with one part of nitric acid, and one part of sulphuric acid, afforded a series ofbrilliant and impressive effects. When pieces of charcoal about an inch long and one-sixth of an inch in diameter were brought near each other (within the thirtieth or fortieth part of an inch), a bright spark was produced, and more than half the volume of the charcoal became ignited to whiteness, and by withdrawing the points from each other, a constant discharge took place through the heated air, in a space equal at least to four inches; producing a most brilliant ascending arch of light, broad, and conical in form in the middle. When any substance was introduced into this arch, it instantly became ignited. Platina melted as readily in it as wax in the flame of a common candle; quartz, the sapphire, magnesia, lime, all entered into fusion; fragments of diamond, and points of charcoal and plumbago, rapidly disappeared, and seemed to evaporate in it, even when the connection was made in a receiver exhausted by the air pump; but there was no evidence of their having previously undergone fusion."
The philosopher also showed that, when the Voltaic or electric arc is produced in the exhausted receiver of an air pump, the phenomena are as brilliant in character, and the charcoal points can be more widely separated, thus proving that the electric light is quite independent of the oxygen of the air for its support.
Owing to the crude nature of the Voltaic batteries of that day, and also to the great expense of maintaining a large battery of that nature, nothing practical resulted from Davy's discovery of the electric or Voltaic arc. Professor Faraday, the great physicist, by his discovery of the principle of magneto-electricity, has enabled the electric light to be brought into practical use. As early as 1833 Pixii applied the principle practically in the construction of a magneto-electric machine with revolving magnets; he was followed by Laxton, Clark, Nollet, Holmes, and others, who made machines with fixed magnets. In 1854 Dr. Werner Siemens, of Berlin, introduced the "Siemens' Armature," which, from its compact form, permitted a very high velocity of rotation in an intense magnetic field, giving powerful alternating currents, which, when required, were commutated into one direction.
The latest improvement has been that from the magneto-electric to the dynamo-electric machine. It is due to both Dr. Siemens and Sir C. Wheatstone. Induced currents are directed through the coils of the electro-magnets which produce them, increasing theirmagnetic intensity, which in its turn strengthens the induced currents, and so on, accumulating by mutual action until a limit is reached.
Siemens' Electric Light.—The following is a description of Messrs. Siemens Brothers' dynamo-electric light apparatus, which, for use on board ship against boat torpedo attacks, &c., is equal, if not superior, to any similar apparatus yet produced, and which is extensively used in the German and other European navies. This apparatus was one of many others experimented on by Dr. Tyndal and Mr. Douglas, M.I.C.E., for the Trinity House.
Dr. Tyndal says: "I entirely concur in the recommendation of Mr. Douglas, that the Siemens machine recently tried at the South Foreland be adopted for the Lizard. From the first I regarded the performance of this handy little instrument as wonderful. It is simple in principle, and so moderate in cost that a reserve of power can always be maintained without much outlay. By coupling two such machines together, a great augmentation of the light is moreover obtainable."
Principle.—When a closed electrical circuit is moved in the neighbourhood of a magnetic pole, so as to cut the lines of magnetic force, a current is generated in the circuit, the direction of which depends upon whether the magnetic pole is N or S; it also depends on the direction of motion of the circuit, and according to the law of Lenz, the current generated is always such as to oppose the motion of the closed circuit.
All magneto-electric and dynamo-electric machines are based on the principle stated above, and are subject to many modifications.
The namedynamo-electric machine is given to it, because the electric current is not induced by apermanent magnet, but is accumulated by the mutual action of electro-magnets and a revolving wire cylinder or armature. It is found that, as the dynamic force required to drive the machine increases, so also does the electric current; it is therefore called a dynamo-electric machine.
Description.—In the machine here described, of whichFig. 164is an elevation,Fig. 173a part elevation, andFig. 165a longitudinal section, the electric current is produced by the rotation of an insulated conductor of copper wire or armature coiled in several lengths, 8, 12, 16, &c., up to 28, and in several layers, longitudinally, upon a cylinder with a stationary iron corenn' ss', so that the whole surface ofthe armature is covered with longitudinal wires and closed at both ends, as inFig. 165. This revolving armature is enclosed to the extent of two-thirds of its cylindrical surface by curved soft iron barsNN1,SS1.
Fig. 164.
Fig. 164.
The curved bars are the prolongations of the cores of the electro-magnetsE E E E. They are held firmly together by screws to the sides or bottom of the cast iron frame of the machine, making it compact and strong.
The coils of the electro-magnet form with the wires of the revolving armature one continuous electric circuit, and, when the armature is caused to rotate, an electric current (which at first is very feeble) is induced by the remanent magnetism in the soft iron bars and directed through the collecting brushes into the electro-magnet coils, thus strengthening the magnetism of the iron bars,[V]which again induce a still more powerful current in the revolving armature.
The electric current thus becomes stronger and stronger, and the armature therefore revolves in a magnetic field of the highest intensity, the limit of which is governed by the limit of saturation of the soft iron.
At each revolution the maximum magnetic effect upon each convolution of the armature is produced just after it passes through the middle of both magnetic fields, which are in a vertical plane passing through the axis of the machine (i. e.N1S1in Fig. 173). The minimum effect is produced when in a plane at right angles to it, i. e. horizontal.
Fig. 165.
Fig. 165.
According to the law of Lenz already referred to, when a circuit starts from a neutral position on one side of an axis towards the pole of a magnet, it has a direct current induced in it, and the other part of the circuit which approaches the opposite pole of the magnet has an inverse current induced in it; these two induced currents are, however, in the same direction as regards circuit. A similar current will also be induced in all the convolutions of wire in succession as they approach the poles of the magnets.
These currents, almost as soon as they are induced, are collected by terminal rollers or brushesB, usually the latter, placed in contact with the commutator in the position which gives the strongest current. The position giving the strongest current gives also the least spark, so that when there are no sparks at the commutator the best lighting effect is produced.Fig. 166shows position of brushes when the armature revolves in the direction indicated by the arrow.
The circumference of the revolving armature is divided into an even number of equal parts, each opposite pair being filled with convolutions of insulated wire wound parallel to the axis of the armature.
The ends of these wires are brought to a commutator and connected to the segments either by screws or by soldering.
The brushes collect the electric currents as they are induced, which is nearly constant and continuous.
The collecting brushes are combs of copper wire placed tangentially to the cylindrical commutator, and press lightly upon it with an elastic pressure.
Fig. 166.
Fig. 166.
Power and Light produced.—An increase of the armature speed produces a corresponding increase in the current produced, but not in the same proportion. The current increases more rapidly than the speed, and could be made to reach any intensity but for considerations explained below. With increase of current there is also increase of heat.
The speed for continuous work must not be taken too high, because the heat developed at high velocities might destroy the insulation of the coils of the electro-magnet. The speed given for this machine produces no such injurious heating effect.
The strength of the current is also influenced by the resistance of the electric lamp and its leading wires. With an electric lamp in a circuit of proper resistance the armature should revolve at the rate given in the following Table. The heating will then reach its maximum, which is very moderate, in about three hours after which there will be no further change.
Table.
Size.Number of revolutions of armature.Intensity of light in standard candles.HP (actual) to drive.Medium800 to 8504,000 to 6,0003½ to 4
The intensity of the unassisted light is given in standard candles. The standard here used is a stearine candle consuming 10 grammes per hour.
Regulation.—From the fact that a closed circuit rotating in a magnetic field experiences resistance to its motion which a broken circuit does not, motive power to any extent is only required when the circuit is closed. An interruption of the current is therefore equivalent to removing the load from the motor, which for mechanical reasons may be injurious to it and for electrical reasons to the dynamo machine.
The sudden interruption of the circuit of the large machine produces an electric tension so dangerously high as to strain or destroy the insulation of the machine. When contact is again made after such interruption, the increase of speed resulting from the interruption causes a momentary current of great intensity, accompanied by sparks at the commutator.
In order that the light may be quite steady the speed should be as uniform as possible. As too high an increase of speed may result in temporary extinction of the light, it ought never to be permitted. The motor should therefore be provided with a good and sensitive governor, that will keep the speed perfectly uniform however the steam and load may vary. A large and heavy fly-wheel is also very useful in keeping the speed nearly uniform during change of load.
Although the circuit, when the machine is in full action, should never be suddenly interrupted, interruption arising from the extinction of the light isnotdangerous, because it is always preceded by a decrease in the strength of the current. When it is desired to divert the current into another circuit it is advisable to stop the machine. Although in practice with small machines this is rarely done, with large machines it is necessary.
Self-acting Shunt.—For great security, especially with the two machines coupled together, where the electric current is strong andthe light equivalent to about 14,000 candles, it is advisable to insert in the circuit a self-acting shunt.
Fig. 167.
Fig. 167.
This is placed between the lamp and machine and connected to both leading wires. Its principle is as follows:—
The terminalM,Fig. 167, is joined by a short connecting wire to one terminal of the machine. The terminalL Mis connected to the remaining terminal of the machine and also to one of the lamp terminals.
The terminalLis connected to the other terminal of the lamp.
The shunt contains a small electro-magnetEmounted upon a square wooden slab or baseboard with its armature a, a contact c, and, below the slab, a resistance coilW, which is equal to the resistance of the electric arc of the light, about 1 S.u.[W]
As long as the lamp is burning well, the current circulates in the coils of the electro-magnet, and the armatureabeing strongly attracted, there is no contact atc. The resistance coilWis therefore not in electrical circuit. When the light is extinguished the current in the coils of the electro-magnet ceases, and the armature is withdrawn by the springfmaking contact atc. This offers to the electric current a path throughWof equal resistance to that of the lamp, and the current is subjected to scarcely any change, so that the motor has practically no cause to alter its rate.
When the carbon points of the lamp again touch, the electric current returns to them, breaking contact atc, re-establishing the former conditions.
Direction of Rotation.—The armature may revolve in either direction. If it becomes necessary to drive it in the opposite direction to that for which the machine has been made, it is only necessary to reversethe brushes, placing their points in the direction of motion, and to change two of the wire connections, which operations can be effected in a few minutes.Fig. 166shows the position of brushes for one direction of rotation andFig. 168that for the other.
Fig. 168.
Fig. 168.
Conducting or Leading Wires.—The leading wires are usually of copper of high electrical conductivity. They must be insulated from one another the whole of their length and not placed too close together. As their resistance affects the intensity of the light very much, the section must be carefully proportioned to the distance of the lamp from the machine.
The best practical result is obtained when their resistance together with that of the lamp is equal to the total internal resistance of the dynamo machine. Wires of various sizes are therefore required.
Decrease in strength of the current caused by a leading wire of too high resistance can be overcome by a higher velocity, which is obtained only by increased motive power, but if the wire is much too small, it will become heated. The proper remedy is to increase the sectional area of the leading wire.
Bright sparks should never be allowed to appear at the commutator and brushes, as sparks result from a rapid burning of the metallic parts. They can easily be avoided by properly inclining the two arms which carry the brushes.
The position of the brushes yielding the least spark at the commutator is that giving the highest intensity of light in the electric arc.
The commutator should, while in motion, be freely oiled, to prevent the brushes wearing away too rapidly. The sticky oil should from time to time be removed by washing with paraffine oil or benzoline.
Wear and Tear.—The chances of stoppage so common to the old forms of electric light apparatus have in this form been reduced to a minimum, and now do not exceed those that arise with machines generally. The Trinity House Report states that the Siemens' machine worked well for a month without any necessity for stopping. The brushes are the only parts which wear away, and they are very easily replaced.
In thick weather they should be connected in what is called parallel circuit (or parallel arc, or for "quantity"), because it has been found that when they are so arranged the intensity of the electric light produced exceeds by some twenty per cent. the intensity of the sum of the two when worked separately. Thus the two machines, giving respectively a candle power of 4,446 and 6,563 when worked separately (total 11,009), have given when coupled up in parallel circuit a light equivalent to 13,179 candles; just as in telegraphy it has been found that the rate of sending can be increased from 20 to 25 per cent. when the apparatus is coupled up in parallel arc. For this reason it is usual to employ two machines of medium size instead of one machine of large size. The intense light so produced is also much more uniform than from one large machine.
Automatic Electric Lamp.—Automatic electric lamps have been constructed with spring clockwork to cause the carbons to approach one another to a certain point, when, by means of an electro-magnet, the clockwork is checked, and the carbon points are allowed to burn away to such a distance that, by the decrease of current, the clockwork is released and the carbons caused to approach again. With such lamps the clockwork has been a source of trouble, and it is liable to get out of order.
Siemens' Patent Electric Lamp.—The lamp here described is actuated without clockwork; it also automatically separates the carbons after they have approached too closely or touch, and, by this combined action of approaching and separating, the carbon points are kept at a proper distance apart, and a steady light is obtained.
The working parts are represented in the diagramFig. 169, and atFig. 170is shown the size employed on board ship.
Eis the horse-shoe magnet with the armatureAplaced in front of its poles a short distance from them. A regulating screwbwith the spiral springfis attached to the leverA', forcing it against the stopd, and withdrawing the armature from the poles of the electro-magnet.When a current traverses the coils of the latter of sufficient strength to attract the armature and overcome the tension of the springf, contact is made atc, which diverts the current from those coils. The consequent release of the armature breaks contact atc, the armature is again attracted, and this action is repeated, producing a vibrating motion of the lever and armature, which continues as long as there is sufficient current to overcome the tension of the spring.
Fig. 169.
Fig. 169.
The spring pawlsat the upper end of the leverA', and oscillating with it, actuates a ratchet-wheelu, which is in gear with a train of wheels and the carbon holders; it thus opposes their tendency to approach by pushing them apart, tooth by tooth, until the current is so much weakened by the increased length of electric arc that the armature and lever cease to oscillate enough to move the teeth of the ratchet-wheel, and it rests near the stopd.
While in this position the spring pawl is released from the ratchet-wheel and the preponderating weight of the upper carbon holder causes the carbon points to approach again. Increase of current follows decrease of resistance, the armature again oscillates, and this cycle of action is continuously repeated.
When in action the movements of the carbons are scarcely perceptible, but when, by any external cause, the carbons are separated so as to extinguish the light, they immediately run together until they touch, when they ignite and separate to a proper working distance by means of the electro-magnet above described.
The only operation requiring attention in the use of this lamp is the adjustment of the tension of the springf. When this tension is once regulated to the current at disposal, the lamp will continue to give a steady light as long as the current remains uniform.
The relative rate of consumption of the two carbon points differs. The positive carbon burns away rather more than twice as quickly as the negative carbon.
Fig. 170.
Fig. 170.
The duration of the light depends mainly on the lengths and sizes of the carbons.
Provision is made in this lamp that the rack which supports the negative carbon may be made to gear either into the teeth of the same pinion as that of the positive carbon, or into one of about half the size. By these means the light, when once focussed in a reflector, will remain in focus as long as the carbons last, whether permanent or reversed currents are employed.
Besides its twofold application, the lamp is very compact, is simple in construction, and therefore not likely to get out of order, and it is capable of being regulated with great precision.
There is no spring to be wound up. The contact need not be cleaned, as the sparks are scarcely perceptible.
By removing two screws in the outside casing, all the chief working parts can be easily removed and inspected.
Carbons are made from the hard carbon deposited in the interior of gas retorts, also from graphite. Various sizes, both square and round in section, of from 5 to 20 mm. in diameter, are used in the electric lamp according to the intensity of the electric current. Those commonly employed are from 10 to 12 mm. in diameter.
The carbons supplied with the Siemens patent lamp are coated with a thin film of copper. This enhances the cost somewhat, but it greatly improves the result, as the carbons burn longer, and do not split, when so coated.
By coating them the resistance is diminished, except at the points, so that all the heat is concentrated in the electric arc, and a brighter light is the result.
When two dynamo machines are coupled together (seepage 248), to give a very powerful current, the sizes up to 20 mm. are required.
The consumption varies a little, but the average is from 3 to 4 inches per hour.