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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
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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
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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.