History of the Atlantic Cable & Undersea Communications
|Telegraph Cables (1899)
Transactions and Proceedings of the Royal Society of New Zealand 1868-1961
By C. J. Cooke.
[Read before the Hawke’s Bay Philosophical Institute.]
The subject of telegraph cables is one of considerable interest to myself, as I spent some years in testing cables electrically, both during manufacture and the laying in South American waters and among some of the islands of the West Indies. Our subject divides itself into four branches—the history of submarine telegraphy, the manufacture of the cables, their laying, and their working.
The electric telegraph dates from the year 1837, and for some years after only land-lines were in use. Some ten or twelve years later Faraday suggested the use of guttapercha as an insulator for submarine lines. This substance was unknown in Europe before 1843. Faraday’s suggestion was acted on, and a telegraph cable was laid in 1850. This prompt recognition of the value of guttapercha adds another to the many instances on record of the benefits of scientific investigation to human progress. Guttapercha is a very suitable substance as an insulator for telegraph cables; it is tough, flexible, will stand a knock, a shake, or a strain, is thoroughly impervious to water, and is unacted upon by saltwater. Of the different materials used as insulators in various descriptions of electrical apparatus, some, as glass and porcelain, are too brittle for cable-work, others, as wood, are not impervious to water. Hence guttapercha and indiarubber are the only substances which have been used as insulators in telegraph cables.
The following extract from Cassells’s “Technical Educator” may be in place here: “The first cable from Dover to Calais consisted of a wire coated with guttapercha, but this was so imperfect that it failed the following day.
In the cable which was laid the following year the conductor consisted of four copper wires, each of which was separately insulated by being covered with guttapercha. The wires were laid side by side, a little hemp being placed between them to prevent their chafing. Tarred hemp was then laid on so as to form a solid rope, and outside all, as a protection against external injury, there were galvanised-iron wires spirally wound. The cable when complete weighed about 7 tons per mile, and possessed very great strength. It was found to answer admirably, and has remained in working-order ever since.”
Thus early in the history of cables—in the second year of their existence—we have all the essentials of the cable of to-day. Other cables soon followed this one from Dover to Calais, a total of two thousand five hundred miles having been laid by the end of 1857, and fifteen thousand miles before the end of 1863. In 1858 the first Atlantic cable from Ireland to Newfoundland, two thousand miles long, was laid, but in a very short time it ceased to work. There were several causes to which its non-success was attributed, but the principal one was the iron sheathing-wires not being strong enough. We have seen that the Dover and Calais cable weighed 7 tons per mile. The deep-sea part of this cable weighed only 1 ton per mile.
The year 1866 saw the successful laying of two Atlantic cables, since which time there has been permanent telegraphic communication between Europe and North America. Ten years later there were five cables crossing the North Atlantic, besides one from Lisbon to Brazil. South Africa was the last place of any importance to be connected by telegraph with Europe. The troubles in Zululand and the Transvaal previous to 1880 having shown the extreme necessity for it, it was at once arranged to lay a cable, that the South African colonies might have the advantage of the prompt communication which the telegraph affords, and which has become a necessity to all civilised communities.
II. The Manufacture of Cables.
There are four parts of a cable to be considered—(1) The wire that transmits the current, hence known as the conductor; (2) the covering to prevent the dissipation of the electricity, known as the insulator; (3) a padding of some soft substance, as jute; (4) the iron sheathing.
1. The Conductor.—This, of course, is the essential part, the other parts being subsidiary to it. In cables it is always made of copper, on account of the high conducting-power of that metal. Copper has been considered by many physicists to have the highest conducting-power for electricity of all known substances, though other physicists have thought silver to be slightly superior. The question is not one of any importance to the telegraph engineer, as the high price of the latter metal precludes its use. It is interesting, however, to observe that very carefully devised experiments in this matter have not been attended with uniform results. These discrepancies are easily accounted for by the difficulty of getting the metals absolutely pure. Alloys are usually of much greater resistance, not only than the mean of their constituents, but greater than of either constituent. A very small admixture of a second metal with one of high conductive power such as copper produces a very marked effect in the resistance, while a trace of a non-metallic body increases the resistance enormously. It is stated that the copper of commerce has six or seven times the resistance of the pure metal. Thus telegraphy has created a demand for pure copper. It has also done much to supply that demand, as the residue from batteries in which copper is reduced to the state of metal furnishes a copper very suitable for telegraphic purposes. It may be mentioned here that, while silver and copper have the highest conductivity for a given volume, aluminium has the highest for a given weight. The latter metal, therefore, may prove of service for some sorts of electrical communication, but it is not likely to be used for submarine cables. In the first place, the weight of the copper, varying from 100 lb. to 400 lb. per nautical mile, is only a fraction of the weight of the cable. In the next place, an aluminium conductor of less weight but of greater sectional area would require a greater amount of insulating material, and this, again, would require a greater amount of sheathing-wire.
Copper, then, as having practically the highest conductivity, or, in other words, the smallest resistance for its volume, and not being too dear in price, always forms the conductor. In ocean cables there is usually a strand of seven wires twisted together. This arrangement is considered better than one thick wire, in which a flaw involving fracture would render the cable useless till repaired. A fracture of one or more wires of the strand has no effect on the signalling, unless all the wires are broken in the same place. Joints are the weakest places, but there the precaution is always taken to bind the copper strand round with a coil of thin wire fastened only at the ends. If a joint should break the thin wire will still convey the current. In spite of all precautions a rupture of the conductor inside the insulator does sometimes occur. The distance of such a break from either end is easily found by observing the amount of the static charge that part of the broken cable will furnish. Of course, the amount of static charge per mile is always known.
An unusual kind of fault came under my own notice during the laying of a cable in the West Indies from Santa Cruz to Porto Rico. The conductor was broken inside the insulator, which remained perfect, but the ends of the broken pieces were in loose contact. On testing the resistance of the conductor after laying, it was found to be about double of what it had been previously. This had no effect on the signals, so it was not discovered till the laying was completed. Such a fault is difficult of detection. There would always be the danger in such a case of the loose contact not being maintained, when the signals would suddenly cease. It was necessary to cut the cable in one or two places to localise the fault, and replace the faulty length by a good one. The fault was found some distance out at sea.
2. The Insulator.—Most telegraph cables are insulated with guttapercha. I have only had personal experience of those insulated with rubber; I propose therefore to confine my remarks to the latter substance as an insulator. Indiarubber, like guttapercha, is a gum that exudes from several tropical plants. The best rubber comes from the forests of the Amazon. The indiarubber-tree resembles the ash in appearance. On making holes in the bark a juice like milk comes out. This, on being dried and smoked, becomes the black solid we are familiar with. Though generally black, some of the pure rubber, especially that sold at artists’ warehouses, is much lighter in parts, and occasionally approximates to its natural colour—white. The black colour is simply caused by smoke. One of our company in Brazil tried to secure some pure-white rubber by filling a bottle with juice and corking it. In a day or two, however, the cork was forced out of the bottle; some of the rubber, now solid, was also forced out, and stood some inches above the neck of the bottle. It had acquired a very disagreeable odour. We gather, then, that the smoking is a necessary process. Rubber is collected from a large part of the Amazon Valley from trees most of which are growing wild. Pieces as large as saucers and from ½in. to 1 in. thick are strung on a switch and conveyed in canoes to the main stream or the larger tributaries of the Amazon. River steamers then convey them to the seaport of Para, where they are transhipped on to ocean-going vessels.
The insulator of a telegraph cable, whether rubber or guttapercha, must be carefully purified before use, as an admixture of impurities would seriously interfere with its insulating properties. In the case of guttapercha, indeed, that substance after purification is sometimes purposely mixed with other substances whereby its insulating-power is lessened, but which enables signalling to be carried on more quickly. After rubber has been purified it is stretched into long tapes. Great care is needed here, as the manipulation of it renders it highly electrical, and light bodies, such as feathers, are readily attracted by it. If some substance adheres which would, become charred on subsequent heating, the insulation would be destroyed, as charcoal is a good conductor of electricity.
If rubber could be used as an insulator it would have a great advantage over guttapercha, in that for a cable of the same dimensions it would allow of quicker signalling, or, in other words, a cable as effective could be made with a smaller amount of material.
Rubber is sometimes vulcanized—that is, mixed with sulphur and the two substances melted together. This is the common grey-coloured rubber. The mixture of these two insulating bodies forms another valuable insulator. Pure indiarubber is not suitable for cables, as when immersed for any long period the sea-water softens it and destroys the insulation. Vulcanized rubber resists the action of sea-water, but the sulphur attacks the copper, forming copper-sulphide. A consideration of these facts produced what was known as Hooper’s core, a copper-insulated rubber, which at one time threatened to rival the guttapercha-covered wire. In this the conductor was covered with pure rubber, while the outside of the core, which was exposed to the action of salt-water, was composed of vulcanized rubber. Between these two layers was a third, the composition of which was a trade secret. It was claimed that this layer prevented the sulphur of the outer layer from penetrating to the copper. I cannot learn that any rubber cables have been made for very many years. Previous to that time Hooper’s core was used for cables in the Persian Gulf, on the coast of China, for a length of two or three thousand miles on the coast of Brazil, and for torpedo purposes in the neighbourhood of the fortifications of Portsmouth.
3. The Jute Padding.—This need not detain us. Its use is simply to prevent the outside sheathing from injuring the core.
4. The Sheathing of Iron Wires.—This consists of a number of iron or steel wires bound round with hemp, which, are twisted spirally round the core. These wires are simply for the protection of the cable, and are not used in any way for the transmission of the current. There would be little need for this sheathing if it could be insured that the cable should lie on a smooth bed in water sufficiently deep to be removed from the influence of tides and currents. Unfortunately for submarine cable enterprise, the bottom of the ocean is as diversified as the land-surface. There are hills and dales, mountains and rugged crags, against which the cable may chafe until it is destroyed. There are precipices and narrow ravines across which the cable lies supported from the two sides and not sinking to the bottom. In such a position it is more liable to injury. A cable was broken in the Persian Gulf by a whale getting its tail entangled in it, while near the mouth of the Amazon parts of the cable have been raised with fish-teeth sticking in them. All these things show the need of a cable being well protected.
That the iron sheathing is very efficacious is shown by the fact of comparatively few cables having ceased to work while the sheathing has remained intact, although the insulation in many cases has been far from perfect.
From time to time proposals come up for what are known as “light cables”—that is cables in which the iron-wire sheathing is replaced by hemp or sheet copper—but experience does not serve to recommend them.
From what has been said it will be seen that cables are more liable to injury near the shore, hence those parts are enveloped in much stronger sheathing. Often a second or even a third spiral of stout wires is coiled on the part of the cable meant to be laid near the shore, each wire being covered with hemp and the whole passed into a bath of a preparation of tar as it is being coiled on the cable. The cable thus prepared is technically known as “shore end.” the main part of the cable being known as “deep sea.” From the time the insulator is placed on the copper the core is kept wet, and is stored in tanks, where it is covered with water. When the sheathing is put on the water penetrates it and reaches the insulator. This is found necessary, as otherwise, chemical changes may cause heating and consequent, destruction of the latter. It also insures the cable being stored under conditions similar to those when it is in actual use.
During the whole process, of manufacture the cable is subjected to electrical tests—continuous tests, where the instruments are observed every few minutes, and special tests two or three times a day. The least flaw is thus discovered at once, its locality sought for, and the faulty piece cut out. During the manufacture of the “shore end” the tests are more rigid still, as the removal of a faulty piece in that part involves the labour of cutting through a number of stout iron wires and a great loss of material.
III. The Laying of the Cable.
Telegraph cables are vastly more expensive than land-lines, in their first cost, in their maintenance, and especially if a line needs to be duplicated. If a cable has to be duplicated an entirely new one must be manufactured and laid, while a second land-line can be insulated on the poles that carry the first line. It is the object, therefore, of the telegraph engineer, in connecting places separated by the ocean, to lay the cable where the sea is narrowest. Hence communication between England and the Continent of Europe was first made by the cable across the Strait of Dover. Hence, too, the first Atlantic cables were laid from Ireland to the east coast of Newfoundland, communication from thence to the United States and Canada being afforded nearly all the way by land-lines. The bed of the Atlantic between the points named is very suitable for cables, being very free from steep declivities and ravines, of a moderate depth, sufficiently deep for the cables not to be exposed to the violence of storms, and not excessively deep in case it is necessary to lift the cables for repairs. Accordingly this part of the bed of the ocean is known as “Telegraph Plateau.”
About twenty years ago the Great Western Telegraph Company was formed to connect the United States with Europe by way of the West Indies, an entirely new route. After many hundreds of miles of cable had actually been manufactured for this route the company was induced to abandon the enterprise. The Anglo-American company who owned the existing Atlantic cables feared the effect of competition, and paid the Great Western Company a sum of money to take their cable to another part of the world. At that time Brazil was being connected with Europe by telegraph, a cable being laid from Lisbon to Pernambuco, the easternmost port of Brazil, in accordance with the principle I have mentioned of laying cables across the ocean where it is narrowest. But the example of North America in respect to the use of land telegraph-lines could not be followed in South America, where the circumstances are totally different. A tropical climate, with heavy rainfall, dense forest, where there are very few roads and very little settlement except near the river ports, necessitated telegraphic connection with the other large cities of South America being made by cable. Here, accordingly, the company resolved to transfer their cable, and so changed their title to the “Western and Brazilian Telegraph Company.” The cables of this company, two thousand miles in length, and extending from Rio de Janeiro to the mouth of the Amazon, were manufactured and successfully laid by the company known as “Hooper’s Telegraph Works (Limited),” the proprietors of Hooper’s core. Another company was formed to continue cable-communication southwards to Buenos Ayres, and a third company to continue it northward from the Amazon River to join the West Indian Company’s telegraph system. The cable for the latter company was also made by Hooper’s. The capital for all these enterprises was subscribed in England, not with altogether satisfactory results to those who invested their money, the twenty-pound shares of the Western and Brazilian Company being now quoted at £10 15s.
The firm that manufactured the cable for the second of the above companies was unfortunate enough to lose its cable-laying steamer in South American waters with a considerable length of the cable on board. More cable was. manufactured, and shipped on to another steamer named the “La Plata.” Still more unfortunately, this vessel was also wrecked by a dreadful storm in the English Channel, and all on board, including a number of electricians, the officers, and crew—two men only excepted—were drowned. This disaster was said to be due in great measure to the steamer having on deck some heavy machinery for raising the cable lost in the first vessel. The saving of two of the crew of the “La Plata” by a passing vessel after they had been some thirty-six hours in the rigging-top exposed to the rigours of a winter storm is one of the most romantic episodes in the saving of life from shipwreck.
About the same time a smaller steamer was sent out by the Hooper Company to lay about a hundred miles of the shore end and intermediate parts of the cable forming the northern end of the cables mentioned. The ocean is very shallow near that part of South America, hence the need of a great length of shore end.
This brings me to the actual laying of the cable, in which I was concerned. A week or two after the above occurrences the cable steamship “Hooper” set sail from London. This vessel, reckoned at the time almost the largest afloat next to the “Great Eastern,” was built for the Hooper Company by a firm on the Tyne in the short space of ninety days.
Instead of a hold she was fitted with three enormous cylindrical tanks, which reached from near the keel to 1 ft. above the deck. In laying ocean cables it often happens that a number of vessels carry portions of a cable, to be afterwards joined into one. In this case one large vessel carried about a dozen lengths of cable in her three tanks. There was nearly two thousand miles in all, to be laid in several different places. We left the Thames on the 14th December, 1874, and proceeded across the Atlantic Ocean to Cayenne. In this neighbourhood, quite out of sight of land, we found a large buoy in the sea, and attached to it was the end of the cable laid by the “Hooper” on the previous voyage. We then proceeded up the coast, and found a similar buoy supporting an end of cable. This was the hundred miles of shore end which had been laid by the steamer which left London a few weeks before ourselves, and which we had spoken with off Cayenne. We anchored close by this buoy, hauled the end of the cable on board, joined it to a section two or three hundred miles long in one of our tanks, and proceeded to lay the latter section down the coast to Cayenne. During the voyage out, as during manufacture, each length of the cable was subjected to one or two electrical tests daily, but during laying the cable was subjected to a very rigorous continuous test. In this way, if anything occurred to cause a flaw in the insulator, it would be discovered in a moment. At the same time an arrangement was made for a signal to be sent from the shore station at regular intervals of five minutes, to insure that there was no interruption of the copper conductor.
To prevent this latter test from interfering with the former the signals are sent by what is known as an “induced current” The cable is connected to a condenser at each end—that is, one condenser is at the shore station, the other in the testing-room on the ship. The cable, therefore, is completely insulated. It is charged by a battery of about a hundred cells, and the amount of loss of charge in a given time determines the state of its insulation. Whether charged or not, if contact is made between the shore battery and the condenser a momentary current is induced in the cable, which works an instrument in the testing-room on the ship. I wish to direct attention to this point, as I shall have occasion to refer to it again in speaking of the work of signalling through the cables. The condensers employed consist of a number of sheets of tinfoil separated by paper soaked in paraffin. The tinfoil projects at the end of the paper one set, say numbers 1, 3, 5, &c, at one end, the other set, numbers 2, 4, 6, &c, at the other end. Each set is connected to a binding-screw on its own side of the condenser. In this way two large metallic surfaces separated by a thin dielectric are obtained, and one surface receiving the slightest charge induces a momentary current in the other surface, and in any conductor connected with it, though no current passes across the paraffin dielectric.
The actual laying the cable is simply letting it run out of the ship into the sea. As the ship goes on some of the cable is left behind. The running-out must be controlled by machinery to prevent it running out too fast, which would waste the material, as also to insure it being paid out fast enough, otherwise the ship might go on and the cable be broken. This machinery is also used for lifting the cable when a fault occurs or repairs are necessary. In laying the cable passes up to the top of the tank, then above the deck, round the drum of the paying-out machine, and over a wheel at the stern. A dial attached to the machine shows the amount of strain on the cable.
We went on paying out cable day and night till we again reached the buoyed end of the cable near Cayenne. This cable-end was brought on board the ship; the last message was sent to the shore station, Demerara; the end of the cable was hauled up from the testing-room on to the deck and joined to the one just hauled up from the buoy, and our work in that part was completed.
Before treating on some points in connection with laying the next section it may be in place here to say something about the testing-room. On the “Hooper” this was placed in one of the triangular spaces left between two of the cylindrical cable-tanks and the side of the vessel. It was immediately below the main deck. In fitting up the necessary apparatus on board ship two things that do not trouble the land experimenter have to be taken, into account—the rolling of the vessel, which would, disturb the liquids of the battery, and the mass of iron of which the ship is made, which, is acted on by the earth’s magnetism in a different manner every time the ship changes its course. This affects the needles of the galvanometers. The invention of apparatus specially designed, to overcome the difficulties here indicated is due to Sir William Thomson. Our batteries were of the form devised by that gentleman for testing the first Atlantic cable. With several modifications it is essentially a Daniell’s battery—that is, the metals used are zinc and copper, the latter metal being immersed in a solution of sulphate of copper. For telegraphic purposes this battery is now made without any porous cell, the copper plate with the solution of sulphate of copper being at the bottom of the cell, and the zinc in a solution of sulphate of zinc at the top. As the former solution is the heavier it remains at the bottom, and the two do not mix. A further modification for use on board ship is to have the cell packed with sawdust, so that the metals, instead of being in liquids, are in a sort of paste, but still in the proper solutions. The top of each cell was covered with solid paraffin, which kept everything tight in its place, and prevented evaporation. The use of sawdust is very effectual in preventing the solutions mixing, however much the ship may pitch and roll. It is stated in treatises that sawdust increases the internal resistance of the battery. This is of little importance in testing cables, but the resistance is much lessened by keeping the sawdust well moistened. We found some cells in which the resistance amounted to 40 ohms, after they had been kept in use for some months, when no precautions had been taken to prevent evaporation. But precisely similar cells in which the sawdust was thoroughly moist had no higher resistance than 8 ohms.
The effect of the movements of the ship on the galvanometer needles was overcome by the invention of Thomson’s marine galvanometer. This is a modification again of that gentleman’s mirror galvanometer. The latter instrument is so sensitive that it will indicate a current if one simply presses separate fingers on the two terminals. I have also obtained a very sensible deflection on an ordinary instrument of this description by working a small frictional machine for a few minutes and collecting electricity from the air of the room. In this case, although the electricity generated is at a high potential, the current obtained from the air is naturally an extremely feeble one, but the mirror galvanometer is sufficiently sensitive to indicate a current flowing for some seconds, or even a minute or two. Everything possible is done to make this instrument sensitive. It is composed of coils consisting of thousands of turns of copper wire, and the magnetic intensity is reduced to a minimum by having several magnets, one-half the number having their north poles immediately over the south poles of the other half. But what more than anything else makes the instrument extremely sensitive is the employment of a small mirror attached to the magnets to reflect a beam of light on to a scale. This arrangement gives virtually a pointer of a foot or two in length, but absolutely without weight.
A similar course is followed with the marine galvanometer. The galvanic needles are rendered astatic in the same way; they are surrounded by coils containing thousands of turns of wire, and the deflection is shown by the movement of a beam of light. In the land instrument it is sufficient to suspend the mirror with the magnetic needles attached by a single fibre, and they are allowed to hang and swing freely, because the instrument can be kept in a vertical position. In the marine instrument the vertical position cannot be maintained on account of the ship pitching and rolling, and it is therefore necessary to use several fibres, and to attach them at the bottom as well as at the top. In this way a much stronger current is necessary to deflect the needles. Again, it is necessary to minimise the effect of the earth’s and the ship’s magnetism on the instrument, otherwise it would be more sensitive with the ship in one position than another, and it would fail to indicate the principal thing required of it—the real condition of the cable. For this purpose several powerful magnets are placed in the instrument, and the whole is enclosed in a thick iron case. The desired end is thus attained, but with the effect of still further reducing the sensibility of the instrument.
The other instruments in our testing-room consisted of coils of fine wire of known resistances, enclosed in suitable cases, on which the resistance of the coils was marked. These resistance - coils were of an alloy of platinum and silver, being a substance whose electrical resistance does not vary so much as that of many metals with a difference of temperature. German is another alloy used for resistance-coils for the same reason. The resistance of the coils being known, and practically unaltered from day to day, the insulation resistance of the cables can at any time be compared with them; if this insulation resistance fall below a certain standard it is assumed there is a defective place in some part of the cable. The defect is accordingly searched for, and the faulty length cut out.
A voyage of some hundreds of miles from the Cayenne coast brought us to the Island of Trinidad, in the West Indies. Our next length of cable was to be laid from here five hundred miles northward to the Danish Island of Santa Cruz. On this occasion the whole length of the cable was laid from our ship, so that I had an opportunity of seeing the shore end landed. The place selected for the starting-point was a small cove on the northern shore of the island. At the head of this cove is a strip of sandy beach only some 20 yards long; everywhere else the sea washes up to the base of the steep forest-covered cliffs. A small structure of galvanised iron stood on the beach. This was to contain the electrical apparatus for testing the cable while being laid. Behind this hut a path led up the hills, by the side of which could be seen the poles and wires of the land telegraph-line that crosses the high lands of northern Trinidad, and was to link our line with the town of Port of Spain, the capital of the island. It was late in the afternoon when our great vessel reached the mouth of the cove. We immediately set to work. The ship’s boats were got out, and a raft was made by fastening boards across two of the boats. The heavy shore-end cable was coiled out of one of the tanks on to this raft, leaving, however, one end on the ship joined to the main cable; the raft was towed to the shore, paying out cable as it went; the end of the cable was hauled on to the beach, a spare piece was cut off, leaving the proper length to be taken into the hut.
A fuller account of this place and the laying of this section of the cable was written by one of our electricians for the Leisure Hour in 1877, the article bearing the title of “Cable-laying in the Tropics.” We were accompanied on this part of the voyage by a British man-of-war, which was engaged in taking soundings for us. One of their midshipmen joined us at Trinidad to receive their signals and interpret them to our captain, I suppose on account of naval signals not being known in the merchant service. The sailors having returned to the ship, we proceeded, on our voyage, paying out cable day and night as before.
On the second or third day the electrical staff reported a flaw in the cable. The captain called out, “Full speed astern.” As soon as possible the paying-out was stopped, the cable was cut, and it was found the faulty piece had not left the ship. It took only an hour or two to cut out the faulty piece, join up, and go on as before.
This expeditious manner of dealing with faulty cables is only possible on steamers. In the early days of cable enterprise they endeavoured to use sailing-ships towed by steamers or tugs to lay cables. If our ship had been a sailing-vessel she could not have been stopped in time, and the faulty piece of cable must inevitably have left the ship. The result would have been considerable loss of time in hauling the cable up again from the bottom of the ocean. This incident, then, illustrates one of several reasons why sailing-ships are not now used in cable-laying
In the course of a few days we came in sight of our destination, Santa Cruz. On arriving the ship anchored, and next morning the shore end was landed as at Trinidad. Here I left the “Hooper” to take charge of the shore station, as two other cables had to be laid from this island. We had a more comfortable structure to work in than had our fellow-electricians in Trinidad, as there was a good-sized stone hut erected on this beach.
This brings me to notice that the cable electrician at the shore station may have certain disagreeable experiences like most other people, though I have none of my own to chronicle. Some electricians, however, that we knew, who spent some months at a cable hut in a lonely place in southern Brazil, told us they used to fire off guns at night to intimidate the wild animals prowling about. The work of the shore electricians during cable-laying is to send five-minute signals to the ship, which insures that the copper wire has not been broken. They also have to hold themselves in readiness to receive any instructions from the ship, and, if necessary, to send any important message to the ship. They do not require to test the cable till the work of laying is finished.
No incident worth remark occurred while we were at Santa Cruz, except the rupture of the copper conductor mentioned in the first part of this paper. The manufacturing company is responsible for the cable for thirty days after the completion of laying. We were instructed, however, to allow the telegraph company the use of the cable for that period if it were protected against lightning. No special precautions for this are required while the cable is on the ship or during laying, but in the practical working a new element of danger is introduced. It arises in this manner: The most convenient place for landing a cable is often at a considerable distance from a town in which the telegraph-office is situate, so the cable is continued by a land-line to the office. This land-line may be struck by lightning, when, if the charge of electricity conveyed by it were allowed to enter the cable, it would probably inflict considerable damage. The place where a land-line joins a cable is accordingly fitted with what is termed a lightning-guard. These guards are of two kinds. The first consists of a short length of thin wire inserted between the land-line and the cable; this is fused by the heat developed by any powerful charge such as produces lightning. The second kind consists of a brass plate in contact with the end of the land-line; this plate is insulated from a similar plate in contact with an earth-wire by a thin layer of some material such as paper soaked in melted paraffin. In the latter case a powerful charge of electricity ruptures the paper, and the lightning passes harmlessly to the ground. In either case the lightning is prevented from entering the cable.
I waited at Santa Cruz the thirty days. A representative of the telegraph company took another test of the cables. I, as representing the manufacturers, took final tests, and, all these tests being satisfactory, we left.
IV. The Working of the Cable.
The instruments used by the telegraph-cable companies for signalling on their submarine lines are usually of a different character from those used on land-lines. The principal reason for this is that guttapercha and rubber are only indifferent substitutes as dielectrics for the dielectric of land-lines—atmospheric air. The consequence is that the passage of the current is retarded, and especially on cables of considerable length.
Attempts have been made to discover the velocity of electricity, but very discordant results have been obtained, as retarding influences cannot be got rid of. On land-lines, however long they may be, the transmission of the current is practically instantaneous. On the Ireland-Newfoundland cables, on the other hand, two-tenths of a second must elapse after sending before a sufficient current arrives at the receiving-station to work the most delicate instrument. The longer the cable the less the speed of the current, which varies inversely as the square of the length, so that a cable twice the length of another has only one-fourth the speed. Worse than this is the fact that when a current arrives it does so gradually: a small gradually increasing current is what is really received, so that with any rapid signalling the several signals run into one another, and a great difficulty is experienced in reading them. The reason of this retardation is twofold—first, a part of the charge enters the insulator; and, secondly, the cable being surrounded by water, which is a conductor of electricity, the current induces another current in the water near it, and these currents act on each other. The fraction of time named—two-tenths of a second—as requisite for the front part of a current to traverse the Atlantic is not to be looked on as inconsiderable in telegraphy. Expert telegraph clerks have succeeded in sending forty words a minute, while an automatic sender can send one hundred words in that time. Each word may be reckoned on an average to consist of five letters, and each letter of two signals, which would make four hundred signals a minute in the first case and one thousand a minute in the second case.
As at first only a small fraction of the current arrives at the distant end of the cable, it is obvious that very sensitive instruments—that is, those worked with very feeble currents—are most suitable for cable work. So it was that Sir William Thomson’s invention of the mirror galvanometer made Atlantic telegraphy practicable. Hence, too, it is seen that such instruments as the Morse and the Sounder, which are in use at the present time in our telegraph-offices, are not suitable for long cables, they being far less sensitive than galvanometers. On the cable from Santa Cruz to St. Thomas, however, only some forty miles long, I obtained clearer signals with a Morse instrument than with a galvanometer. It is only where a cable is of considerable length that difficulties as to speed of working have to be taken into account. In a cable that can be just conveniently worked with a galvanometer, not a twelfth part of the signalling could be got through in the same time with a Morse instrument.
The use of the mirror galvanometer for cables is now superseded by that of another instrument known as the “siphon recorder.” This, as its name implies, is a recording instrument, and, like the Morse, is an electro-magnetic one—that is to say, the current on arrival temporarily converts a piece of soft iron into a magnet, which attracts suitable apparatus, so that a mark is left on paper. Unlike the Morse, it is almost as sensitive to feeble currents as the mirror instrument. The invention of this instrument is also due to Sir William Thomson. The use of the siphon recorder necessitates the cable being fitted with a large condenser at each end similar to those used in laying cables. This arrangement has the advantage that the induced current received is instantaneous instead of being a gradually increasing amount, as when no condenser is used.
There are other advantages arising from the use of condensers. They prevent earth currents from entering the cable. One part of the earth’s surface may differ very considerably from another part in regard to the amount or potential of the electricity there. If these places are connected by a wire an electric current will traverse it. These currents sometimes interfere with the working of the telegraph where the line is not fitted with condensers. A faulty cable may be used for a long period by being insulated with condensers. If the fault is considerable enough to allow sea-water to penetrate to the copper, we have all the essentials of a galvanic cell—two metals (copper and the iron sheathing) in a saline solution. The effect is to corrode the copper, and thereby make the cable useless. When condensers are used the faulty cable may be permanently connected to the zinc pole of a battery at the shore station, whereby a negative current passes out at the fault. The water there is decomposed, hydrogen gas being evolved, and the cable is preserved from injury.
Ruptures of the cables across Cook Strait and the fitting of a cable-repairing ship by our Government remind us of the expense the proprietors of telegraph cables may be put to in keeping their lines in proper repair. On the whole, while our subject is of the keenest interest to the student and the scientist, the results of the various enterprises are often unsatisfactory to those who have invested their money. It has been proposed to lay a cable across the Pacific to unite these Australasian Colonies with British North America. I venture to think this most desirable in the interests of the British Empire as a whole.
Text courtesy of the National Library of New Zealand
Last revised: 28 February, 2017