History of the Atlantic Cable & Undersea Communications
- Nerves of the World
J. Munro, Author of Heroes of the Telegraph
A submarine cable is a more highly developed underground wire, adapted for the bottom of the sea and the operations of laying and lifting it. Owing to its great length, the conductor must have a low resistance to the passage of the electricity, and hence it must be of pure copper, since arsenic and other impurities weaken the current in some mysterious way. For the like reason it should be is thick as may be consistent with the cost and other requirements of the line. In the early Atlantic cables the weight of the conductor was 300 to 400 lb and in the latest one not less than 600 lb a nautical mile.
The conductor is usually a flexible strand of seven wires, six wound spirally about a central one, and the whole is covered with a mixture of resin, gutta percha, and Stockholm tar, called Chatterton's compound, to form an adhesive bed for the gutta percha insulator. Thus prepared, the strand is run continuously through a vat of melted gutta percha, and afterwards through a die hole, the diameter of the layer, and a bath of cooling water. Three layers of the gum are generally applied, one above the other, and cemented together by Chatterton's compound. Then india rubber is employed as the insulator, strips of it are wound spirally about the wire until the proper thickness is attained, and the whole may be afterwards cooked, or vulcanised, with sulphur in a special oven for the purpose.
The wire thus insulated is technically termed cable core, and it is now served a spiral covering of untarred hemp, or jute yarn either tanned or soaked in brine to preserve it. It is wound on the core by means of a revolving disc, carrying on one face a series of bobbins holding yarn. The core is passed through a hole in the centre of the disc, and the yarn is twined about it as the disc rotates. This covering serves as a pad or cushion or an outer sheathing or armour to protect the core from injury, which is applied in much the same way as the yarn. The revolving disc in this case is seven or eight feet in diameter, and iron bobbins filled with iron wire are hung on its face in such a way as to keep their perpendicular as the disc turns round, so as not to twist the wires. The served core passes through a hole in the centre of the disc, and is thus overlaid by a spiral of the iron wires, as in the case of the yarn. The sheathing wires are commonly of the best homogeneous iron, which combines some of the toughness of wrought iron with the strength of steel, and they are galvanised with zinc on the surface. For the light, but strong, cable to be laid in deep water, or main cable as it is termed the iron wires are sometimes covered with tarred manilla hemp before they are wound on.
For coast and river cables, to be laid in comparatively shallow water, and sometimes called intermediate cable, the iron wires form a close spiral round the core. For the heaviest type of cable, to be laid on shelving and rocky beaches, or shore end cable, the sheathing wires are sometimes a strand of three iron wires, not merely a single wire, as in the other types. They are intended to guard the core from ships anchors, or the abrasion of boulders shifted by the storm waves. The shore end is seldom more than ten or twelve miles in length, and it is joined to the intermediate by a taper piece. The intermediate cable is often coated with coarse hemp and Bright's compound, a protective cement of mineral pitch, sand, and tar. It is connected to the main cable by another tapering piece.
The lengths of cable are coiled in huge iron tanks and covered with sea water, which is changed from time to time.
During the manufacture, and afterwards, every part of it is submitted to the most searching tests with the electrician and mechanical engineer can devise. The conductor is tested for its resistance or, what comes to the same thing, its conductivity for the electric current. The insulator is tested for its leakage ; that is to say, the resistance which it offers to the escape of the electric current from the conductor to the sea or earth a property which is called its insulation resistance. The finished core is tested for its inductive capacity, or, simply, its power of holding a charge of electricity as a jug holds water. This capacity arises from the fact that a cable core is a sort of Leyden jar, the wire and the sea water being two conductors, and separated by an insulator, like the tinfoil inside and outside the jar, separated by the glass. Accordingly, when a charge of electricity is given to the wire it induces an opposite charge in the water, and these attract and hold one another. The charge, that is to say, the signal, of the message cannot pass along the wire without inducing this opposite charge in the water around it, and, is this takes time, it is delayed. The speed of signalling, in fact, depends on this inductive capacity as well as the resistance of the wire and its total length. Even on aerial lines it comes into play, the ground taking the place of the water, and the air being the insulator; but in this case it is very slight. Moreover, the sheathing wires of the cable are also tested for stretching, twisting, and breaking, and even for galvanisation.
The electrical tests are performed with ease and precision by a variety of ingenious and delicate instruments. The chief of these is Lord Kelvin's mirror galvanometer, which is based on the principle that a current flowing in a wire has power to move a magnetic needle. It consists of a tiny magnet of watch spring, fixed on the back of a mirror the size of a wafer, which is suspended by a single filoselle, or fibre of floss silk, in the heart of a coil of fine wire. A ray of lamplight is reflected from the mirror to a screen, which is divided into a scale of degrees. When a current of electricity is sent through the core, the suspended magnet swings one way or the other, according to the direction of the current, and through in angle proportional to the strength of the current. The mirror of course moves with the magnet, and hence the ray of light is thrown up or down the scale as the case may be, thus magnifying the movement of the needle to the eye of the observer, and enabling him to read the corresponding strength of the current on the scale.
Now, the strength of a current in a wire depends on the electromotive force of the battery which is sending it through the wire, and on the internal resistance of the wire. It follows that if we know the electromotive force of the battery beforehand, and can measure the strength of current by a galvanometer in the way described, we can find the resistance of the wire. We can even measure it direct without reference to the electromotive force of the battery, by comparing the current which a given battery sets up in the wire with the current it sets up in a standard resistance, just as we compare the weight of a quantity of sugar with a standard weight in an ordinary balance. Not only are the resistance of the conductor and insulator of the cable measured in such ways, but the inductive capacity is got by charging the cable full of electricity and discharging it through the galvanometer to see how much it held. The electrical character of every mile of the core and cable is found, and registered for future reference. The record enables us to detect the existence of a fault in the line when it breaks out, and by special tests, based on the same principles, to calculate the distance of the fault from either end of the line, and proceed to extract it. A daily test is made of the cable while it lies in the tanks of the factory, as also during the voyage, and it continuous test during shipment.
Laying a Cable.
Before the days of steam navigation it would have been almost impossible for a ship to lay a cable, as she requires to be independent of wind, weather and currents. Her movements must be governed by her superior destiny, which is to connect two distant parts by a sympathetic bond. She is the slave of her cargo that mystic cord which, like a spider, she seems to create and spin out of her own bowels. As a matter of fact, she is privileged at sea, and flies a special signal, warning other ships to give her a wide berth, as her duty renders her unmanageable, and, like a genius amongst men, a law unto herself.
In the early days of submarine telegraphy, the cables were laid by ordinary steamers adapted for the purpose with tanks in the hold, and paying gear on deck, and a testing room wherever it was convenient; but now they are submerged by vessels built expressly for the purpose. Some of these are very fine illustrations of naval architecture; for example, the Silvertown, belonging to the India Rubber and Gutta Percha Telegraph Works, and the Faraday, of Messrs. Siemens Brothers. The latter is a vessel of 5,000 tons, and capable of laying 1500 to 2000 miles of deep sea cable. She is fitted with twin screws fore and aft, so that her motion can be reversed without turning round in the water. She is thus able to haul in a cable which she had previously payed out, by a simple change of gear. Three cylindrical cable tanks of iron, upwards of 50 feet in diameter and from 30 to 45 feet deep, are situated in the body of the vessel, one fore, one aft, and the other amidships. Her deck is furnished with massive gear and winches for coiling, paying out, and picking up the line; as well as an electrical testing room, electric light apparatus, and a stock of iron buoys lashed to the gunwales for employment as sea marks.
The cable ship having been moored alongside the factory, the cable is run over pulleys and coiled into her tanks round a large core in the centre, and each layer is separated from the next by a coat of whitewash to keep them from sticking together, and producing a foul flake during the laying: that is, an entanglement of the line as it runs into the sea. When it is all aboard, the tanks are filled with salt water till the cable is soaked, and the vessel starts on her voyage.
The expedition may be divided into the cable engineers, who are charged with the mechanical work of submerging the line; the electricians, who make the electrical tests; and the navigators, including the captain, the and the sailors, who work the ship. The captain is, of course, at the head of the ship, and allows no interference with his own command. The electrician in chief is also the head of the electrical staff; but the engineer in chief is, in a manner, head, not merely of his own staff, but of the entire expedition, since it is he. who instructs the captain where he is to put the ship, and requires the chief electrician to let him know the electrical condition of the cable.
Surveying the Sea.
It is highly important to survey the sea before the cable is deposited, and thus select a proper route, with a favourable bottom for it to lie on, and a suitable depth of water for repairing it. Neglect of this precaution the ruin of more than one expensive. It is not only that the heavy shore end may be laid on mud soft as butter, and main cable on the jagged surface of coral reefs; but lines may be rotted through the chemical action of minerals or organic matter, and even bitten by voracious fishes. Preliminary soundings should always be taken, to ascertain the depth and nature of the bottom, the strength and direction of the currents, and also the temperature of the water at the bottom, as this has an influence on the electrical properties of the line, and therefore on the tests. The best route for the cable can then be chosen, and marked on the charts as a guide to the navigator and the engineer.
The old method of taking a sounding is to carry the leadline (a line rope of tarred manilla yarn) from stern to stem along the side of the vessel, and there drop the lead into the water. As it sinks, the line runs off a reel, and when it strikes the bottom the line slackens. The depth is estimated from the length of line payed out, and the nature of the bottom from the grains adhering to a piece of tallow in the base of the lead. Lord Kelvin's sounding machine is always used in cable work, as it is quicker and more accurate than the leadline. In it the rope is replaced by a steel wire, and the depth is recorded by a pressure gauge. The late Sir William Siemens invented a bathometer, to show the depth of water under the Faraday without plumbing it at all, and merely by the variation of gravity on a mercury column produced by the varying depths of water but it failed in practice, owing, as I have been told by Mr. Jacob, one of his engineers, to the disturbing action of the waves.
The place from which the cable is to be laid is usually a quiet cove, near a seaport, but retired from the harbour and anchorage. A cable hut of galvanised iron, or stone and lime, has been erected on the shore, and a trench dug in the beach to the water's edge to contain the shore end. A land line has also been made, to connect the cable with the telegraph office in the town.
The Shore End.
The shore end is sometimes coiled on a raft or barge towed by a steam launch and payed out by hand from the cable ship towards the shore, until the barge grounds. The cable hands, or natives hired for the purpose, then jump into the water and pull the end up the beach, into the trench, and through a hole in the foot of the wall into the cable hut. Ladies and gentlemen, visitors from the neighbourhood, generally join in this work, to the detriment of their kid gloves, and the bystanders vent their enthusiasm in cheers. In a few moments a test from the ship to the cable hut or vice versa announces that the shore end has been successfully laid. On some occasions the end is hauled on shore from the ship by a line passing through a spider pulley fixed on shore, and so back to a steam winch on board.
On board the ship everything has been prepared to begin the paying out, The steam is up, the anchor is weighed, the cable is passed through the paying-out gear, and held fast at the stern, the electrical test is applied to it, and the men are all at their appointed places. As soon as the work party have returned to the ship, leaving a staff of electricians at the cable hut to watch there during the laying, the word is given to start. The engines throb, the screw churns the water, the stoppers holding the cable at the stern are cut away, and it runs freely over the great stern pulley into the blue water like a black serpent. Night and day the work of laying goes on without intermission, until the ship comes to anchor at her destination in order to lay the other shore end and thus complete the line.
Breaking the Record.
The speed varies with circumstances, but on an average it is five or six miles an hour. The Faraday broke the record last July, by laying the newest Atlantic cable in twelve days.
The cable is pulled out of the ship by its own weight hanging in the water; and to give it the proper amount of slack for repairing purposes, its rate of egress is controlled by a friction brake and altering the speed of the ship. It runs from the tank over pulleys to the brake, where it makes three turns round a heavy iron drum, which revolves against the friction of wooden blocks which are adjusted to press upon it with greater or less force. From the drum it passes through a dynamometer that shows the strain upon it at any moment, and thence to a large iron pulley, or grooved wheel, called a sheave, projecting from the stern, or, as the case may be, the bows of the vessel, and so into the sea. Soundings are taken it intervals to get the depth of the water, on which the speed depends. The strain or tension of the cable, the rate of the engines that is to say, the number of revolutions of the screw in a minute are constantly under observation, and a careful watch is kept on the tank for foul flakes. In the electrical testing room the same watchful activity prevails, A continuous test is applied to the cable to detect any flaw in the insulator the moment it occurs. This is done by charging the line throughout, and observing its leakage with a galvanometer. A pulse of electricity, or signal current, is also sent through it from ship to shore or shore to ship every few minutes, generally five, to show that the conductor is intact. A more searching test of the line is also taken from time to time. Meanwhile the navigating officers are busy keeping the ship as near as possible to her prescribed course, by observations of the heavenly bodies and other means.
If a foul flake happens, or a fault is reported from the testing room, the engines are immediately reversed to stop the ship, and the cable held back. In the case of a fault, the line cut, and the sections tested. If the fault is on board, a fresh piece of cable is spliced to that in the water, and the laying proceeds; but if it is in the sea, the cable must either be hauled in, or under run, till the fault is reached and cut out; or, should it be too distant for this plan, the end must be sealed up to keep the water out, and buoyed on the spot. The ship then returns on her course, and, grappling the cable on the bottom, lifts it up and extracts the fault. This done, she returns to the buoy, picks up the end, splices on more cable, and resumes the paying out until she arrives at her goal, where she lands the shore end as before. A careful test of the whole line is then made, and messages are exchanged from end to end. The cable is afterwards submitted to daily tests for a period of thirty days, as a rule and if it behaves properly during the trial, is finally handed over to the telegraph company which owns it, for the public service.
Last revised: 15 March, 2022