Page:The Granite Monthly Volume 1.djvu/114

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��AN OCEAN CABLE.

��ing motion of the armature at the other end, whether a foot or ten thousand miles apart. An ocean cable differs from a land telegraph: 1 — in its construction, 2 — in the medium throngh which it passes, 3 — in its operation. Its construc- tion is similar in only one respect, and that is that in both cases a single wire conveys the message. On land the wire is hung up on poles and run through glass "insulators" to prevent the current from passing down the poles into the earth. The cable wire must be "insulat- ed" the whole length, for if it comes in contact with the water it is entirely worthless. Water is the best known con- ductor of electricity, and its presence is fatal to the passage of the current. How, then, to lay a wire through three thou- sand miles of water and not touch it is the first problem to be solved in ocean telegraphy. A glass tube would answer the purpose, but is out of the question. A more pliable substance (gutta percha) has been found, which is a non-conductor of electricity, and in which the conduct- ing wire is embedded. Other wires are wound around to strengthen it, and more gutta percha added, to make assurance doubly sure that no treacherous drop of water shall penetrate and destroy the ca- ble. The thrusting into the central wire of a penknife blade, or the scraping off a piece of the gutta percha as large as a pea, would be as effectual in destroying its working power as to cut out a hun- dred miles of cable and carry it ashore. Absolute perfection, and nothing less, ensures the transmission of a current of electricity through any submarine cable. A single wire thoroughly covered with only a very thin coating of any non-conducting substance would an- swer every purpose for the transmission of messages, but it would soon be broken in the uneven depths of the ocean. The additional wires are put on to lessen its chances of injury, as well as to render it impervious to water. In deep water, where the surface action caused by wind and tides is not felt, the cable is about an inch and a quarter in diameter, while the twenty miles of shore end, which is ex- posed to the heavy seas and ground swell, is as large as a man's wrist, and

��completely encased in a net-work of heavy wires — the whole weighing some twenty tons to the mile.

The cable having been laid, and every- thing in working order between Rye Beach and Torbay, let us enter the room where the operator sits, and observe the method by which he communicates with the operator at the other end of the cable. The first glance dispels all ideas of pon- derous machinery. All the instruments used in telegraphing and testing are placed upon a table five feet long and two feet wide. The instruments used in merely sending and receiving messages could all be put in a little box and car- ried in one hand by a child five years old. All are specimens of the most mag- nificent and delicate workmanship, and are very costly. The room is darkened, and as you enter you see a small lamp so screened as to throw the light in a cer- tain direction. All being in readiness, the operator sits down to send a message. He taps the "key" as in the land tele- graph, only it is a double key. It has two levers and knobs instead of one. The alphabet is substantially like the Morse alphabet; that is, the letters are represented by dashes and dots. For in- stance : Suppose you wish to write the word "Boy." It would read like this :

. B is one dash

and three dots. O is three dashes. Y is one dash, one dot and two dashes. Now in the land telegraph the dots and dashes would appear on the strip of paper which is perforated by a pin at the end of the bar connected with the armature.

If the operator could read by sound, he would dispense with the strip of paper, and read the message by the "click" of the armature as it is pulled down and let go by the electro- nagnet. But the cable operator has neither of these ad- vantages. There is no paper to perfor- ate, no click of the armature, no arma- ture to click. The message is read by means of a moving Hash of light upon a polished mirror, produced by the "deflec- tion" of a very small mirror which is placed within a "galvanometer."

I must here digress again and explain what is meant by a "mirror galvanome- ter." It is a small brass cylinder, two

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