SUBMARINE TELEGRAPHY History.—Although suggested as early as 1798 by the Spanish scientist Salva, and made the subject of several experiments in the early part of the 19th century by Morse, Wheatstone and others, which demonstrated the possibility of submarine teleg raphy, it was not until the introduction of gutta-percha as an insulator and the invention of a machine for applying it to wire, that the submarine cable became commercially attractive.
In 1850 a cable was laid between England and France, but it was broken shortly after communication was established because of its inherent physical weakness. The addition of galvanized iron armour wires, wound with a long spiral lay on a cushion of jute yarns around the central gutta-percha-covered copper con ductor, contributed the necessary strength when the next attempt was made in 1851, and this cable, between Dover and Calais, proved successful. In the next few years a number of cables were laid between England and adjacent shores, between Denmark and Sweden, and in the Mediterranean. The first attempt to lay a cable across the Atlantic in 1857 ended in failure when the cable broke at a depth of 2,000 fathoms. In the absence of any means of recovering the end the project had to be abandoned. In Aug. 1858 a cable was successfully laid between Valentia, Ireland, and Hearts Content, Newfoundland. This was operated for about three months, when it became interrupted in deep water and could not be repaired. In the years 1865 and 1866 two more cables were laid: the first broke when about two-thirds laid. but the second was successfully laid, and soon afterwards the end of the 1865 cable was picked up with difficulty and its laying was completed.
These cables, with which the names of the American financier, Cyrus Field, and the British scientist, Lord Kelvin, are closely associated, were operated by the Atlantic Telegraph Company without competition until 1869, when the French Atlantic Tele graph Company opened a cable for traffic. With the success of the Atlantic cables established, the growth of submarine cable sys tems was rapid until at present (1929) there are 21 cables across the Atlantic between North America and Europe alone, and there is a total of about 3,500 cables in the world, with an aggregate length of over 300,00o miles.
It was early realized, even before the first Atlantic cable was laid, that a very sensitive instrument would be needed for the reception of signals on long cables. The mirror galvanometer, invented by Prof. William Thomson, later Lord Kelvin, filled this need for many years. The syphon recorder invented in 1867, which had the advantage of leaving a written record, gradually replaced the mirror galvanometer, and in its improved form is still largely used on long cables. In 1871 the duplex system was intro
duced, permitting simultaneous transmission of messages in both directions, and the speed of operation has been gradually increased from the initial 15 letters per minute of 1858 to the successful transmission of upwards of 2,500 letters per minute in recent years, while the rate charged dropped from a minimum of about $100 per message to a maximum, between New York and London, of 25 cents per word.
23 shows the constructional details of a modern submarine cable. The conductor is made of copper, stranded or having a central wire surrounded by strips wound spirally. Over this is moulded the insulation. Some of the shorter cables have made use of india rubber as an insulator, but although progress has been and is still being made to develop substitutes, gutta-percha remains the best material for the purpose.
The operating characteristics of a cable depend upon the size of the conductor and the thickness of the insulating wall. Some of the very recent cables are loaded by means of a thin ribbon of thread of highly permeable nickel-iron alloy wound around the copper conductor, which has the effect of greatly increasing the speed at which the cable can be worked. To protect the insulation against injury by marine borers, such as the teredo worm, a brass tape is wound around the gutta-percha on all cables except those lying in very deep water. Next comes a filler of jute yarns which acts as a cushion for the armour or sheathing wires. These are wound on with a long spiral lay. For cables used in the deeper portions of the ocean it is the practice to use galvanized steel wires of high tensile strength, and as the depth of water decreases the steel wires are replaced by galvanized wrought iron wires of larger size. As the cable approaches the shore the armouring is made heavier until at the shore it is doubly sheathed. This added pro tection in shallow water is necessary to prevent damage by ships' anchors, trawling gear, rocks and, in northern latitudes, icebergs. An outer double wrapping of tarred jute yarn, or tape steeped in a compound of tar and pitch, preserves the armour from corro sion. So efficacious is this covering that lengths of cable have been picked up in deep water after more than 4o years of con tinuous submersion with the jute wrapping and compound still adhering to the armour wires, the latter being to all intents and purposes as good as ever. In addition, extra protection is now given to deep-sea armourings by taping each individual armour wire. The outside diameter of the ordinary deep-sea type of cable is about f in., and the weight in air about two tons per nautical mile. The shore-end types run as large as 31 in. in diameter and weigh as much as 3o tons per nautical mile.