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Action of the Relay.

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communication with the other end of the Morse coil is so placed that the armature, A X T, bears against it whenever the end, A, of the latter is attracted. Thus every attraction of the armature of the relay will allow a local electric wave of short or long duration to pass through the Morse; and the result imprinted by s on the paper, P, will be exactly the same as if the line current had done the work directly. In the earliest Morse instruments, the marking style, S, was a pencil, but it required so often to be sharpened that this was given up for a simple metal point which indented the message in the paper. Modern instruments, however, actually mark the message on the paper in ink, and with greater accuracy and ease than it could be indented, an inking roller or wheel, placed at the end of the lever, replacing the points. Sometimes the recording apparatus of the Morse is removed altogether, and the message is read off by the clerk by the mere sound. Its advantage over the old needle system, which it resembles in being nonrecording, is that the ear is a more reliable interpreter in such cases than the eye, and the labour is very much less.

Besides recording the message, or, instead of recording the message, at the distant station, the Morse there may be turned into an automatic re-transmitter of the message to a second station farther on. In this way telegraph messages may be sent, without the labour of re-transmission, to almost any distance by means of relays. With delicate instruments of this class, messages are transmitted by the Messrs. Siemens from London to Teheran, a distance of nearly 4000 miles, without any re-transmission, five relays being interposed in the circuit.*

It is beyond the scope of the present work to describe the various forms of telegraphic apparatus which have been invented in recent years; there seems, indeed, to be almost no limit to the possible accomplishments of telegraphy. Wheatstone and Breguet have invented dial or clock-face telegraphs, so called from the letters of

* See Electricity and Magnetism,' by Professor Jenkin, p. 309 :— The following experiment will serve as a singular illustration of the velocity of the electric impulse as transmitted through metallic wires. On New Year's Day, 1845, a few seconds after the year had commenced, a message travelled from Paddington to Slough apparently in less than no time,' for it actually reached its destination in the year 1844. The difference of longitude makes the point of midnight at Slough a little after that at Paddington, so that a given instant which was after midnight at one station was before midnight at the other.

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Submarine Telegraphy.

the alphabet being marked round a dial; and, incredible as it might have appeared to the world of the last century, any untrained person may with one of these instruments actually spell or point out, letter by letter, his communication to a friend at any distance. Wheatstone's "Universal Telegraph" is of this description, and is now a common piece of office furniture in our larger mercantile houses and hotels. More wonderful still is the type-printer, invented by Hughes, of America, in 1859. It actually prints the message in Roman characters, on a long strip of paper, and is a most marvellous piece of ingenuity.

Lastly, by an invention of Casselli, named by him the Pantelegraph, a person writing a despatch, or even drawing a sketch, at one place, may have a fac-simile of his handiwork reproduced the same instant by telegraph at a distance of a hundred miles or

more.

Submarine Telegraphs.

1005. Surprising as are these achievements of telegraphy, they are rather triumphs of mechanics than of this science, for it is simply the electro-magnetic property of the current that is in every instance employed. The purely electrical triumphs of the last quarter of a century, accomplished by a thorough knowledge of the laws of this subtle agency, are no less important, though perhaps less striking. Until 1850, the problem of bridging the sea by telegraph had been unsolved; in that year the first ocean telegraph line between Dover and Calais was successfully laid; and it was considered a sufficient encouragement, though this cable lasted but a single day, to attack the problem with greater determination than ever. Every one now knows the success which has attended the indefatigable labours of electricians in connection with this problem. There are now many hundreds of telegraph cables, throughout the civilized world; and if joined end to end they would girdle the globe several times. The great feat of ocean telegraphy, the uniting of Europe and America, which, after many extraordinary difficulties and disappointments, was at last successfully accomplished, deserves special notice.

1006. The Atlantic Telegraph.-In 1857 the first attempt was made to lay an Atlantic telegraph from Valentia in Ireland, to Newfoundland; but after some 330 miles of cable had been submerged, it snapped. In the summer of the following year this cable was spliced, and, after three repeated failures by breaking of the cable,

The Atlantic Cables.

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the operation was at last successful, and Newfoundland was in momentary connection with Ireland. Several messages were sent ; the problem was solved; and, although the cable became useless in a few days, the failure was doubtless due to insufficient care in its manufacture.

After some years, the necessary funds for a renewal of the undertaking had been provided; a much better knowledge of the principles of submarine telegraphy had been acquired; and the Great Eastern was chartered for the undertaking. On the 23rd of July, 1865, a Company commenced laying it from Valentia ; they had paid out 1186 miles of cable, and were within 606 miles of Newfoundland, when it chafed against the bows of the Great Eastern, and broke in a depth of 2000 fathoms of ocean. Owing to the want of proper grappling apparatus, they failed to recover the lost cable. Next year, again, the Great Eastern started from Valentia with a new cable, which was laid without a hitch. Not only so, but a search for the lost cable of the former year resulted in success, and by the 8th of September, the two cables were in working order between the Old and the New World.

To show the remarkable perfection of the insulation of the two cables, Mr. Latimer Clark joined the ends of their two conducting wires in Newfoundland so as to form an immense circuit of 3700 miles, and then, by means of a battery formed of a lady's thimble, a strip of zinc, and some acid, he succeeded in signalling through the whole of this enormous length of wire.

The Atlantic cables consist of (1), a central conducting strand or seven copper wires; (2), an insulating coating of gutta-percha over this; (3), a layer of five strands of hemp, soaked in a tarry compound to protect this insulation from injury; and (4), an outside protecting sheath of eighteen strands of iron wire, the whole being slightly over an inch in diameter, and weighing three quarters of a ton per nautical mile.

Since the completion of the English Atlantic cable in 1866, a French cable has been laid from Brest to the Island of St. Pierre, which very much resembles the former in construction, being, however, of a somewhat lighter make.

The indicating apparatus used with the Atlantic telegraph is a Thomson's reflecting galvanometer (Art. 983), which instrument alone is sensitive enough for the purpose. Right and left deflections of the spot of light correspond to dots and dashes in the Morse alphabet; and but for this wondrously delicate contrivance, it is

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Duplex Telegraphy.

questionable whether the Atlantic cable, when laid, would have proved practically successful.

It is remarkable that short signals entering this cable, owing to the retarding effects of induction, ooze out at the other end as long signals, and but for the laborious investigations of the best electricians of the day, this difficulty alone would have made the cable anything but a commercial success, as the transmission of a message would have occupied too long a time. In order to give some idea of how these difficulties have been overcome, it may be stated that it has been ascertained that for about one-third of a second after an electric wave or current has set out from Valentia, there is no sensible effect in Newfoundland; after two-fifths of a second the effect is very feeble, only about seven per cent. of the maximum effect which is perceived after the lapse of three seconds. If no means had existed of overcoming the delay consequent on this drawling out of the current, it would have taken about two minutes to transmit an average word to America; whereas a speed of about fifteen words a minute has been actually attained—a speed perfectly marvellous when the difficulties are fully appreciated.

1007. Duplex Telegraphy.-Further improvements, by the study of the laws of electricity, have been quite recently effected in telegraphy, whereby, a single wire may be made to carry simultaneously two or more messages in the same or different directions, without any interference or confusion resulting. The explanation of the principles, however, on which these operations are based, is beyond the limits of such a work as the present; it may be readily obtained in the technical manuals on the subject which are now easily accessible.

1008. In connection with the subject of telegraphy, and as exhibiting the world which it has discovered for human ingenuity to conquer, we may mention a rather remarkable invention recently made by a Spanish gentleman.

Its object is to place a series of stations—such as all the houses in a town or district-in communication with a head central office, so that on any sudden emergency a message might be despatched giving the locality of the station requiring assistance, and the nature of such assistance. Without entering into technical details we may say, that, in the case of ordinary telegraphic stations mutually connected, the power of sending a message from one station may be interfered with by an intermediate station, and much delay occasioned thereby. By this newly-invented arrangemert, how.

Rapid Transmission of Messages.

769 ever, every station may secure for itself a hearing at head-quarters until its message is completed, when the rext in order of time may obtain an audience in its turn. The essence of the system lies in employing two separate wires with each of which each station has connection. The one wire is for sending notice to the central station, which it does by transmitting a current, to throw the central "Morse" into connection with the sending, or line-wire, and by an ingenious contrivance completing the sending circuit so as to allow of the message being sent only from the particular station which "started" the signal. Nobody can send a message until he has first signalled along the starting-wire; and then no one can interfere with the delivery of the message until it is complete. As soon, however, as the message is complete, the Morse, by an automatic arrangement, returns to its first connection with the starting-wire, so as to be at the beck of the next caller. It appears very suitable for hotels, hospitals, and large offices, where telegraphic communication with a single head office is in operation.

1009. This account of the electric telegraph would be incomplete without some illustrations of its extraordinary effect on time. It appears on some occasions to overthrow all our ordinary notions of time, as measured by the sun. One instance of this has been already given in a note at page 765, and the two following instances furnish a wonderful proof of the velocity with which the electric current may be made to pass by land and water from one station to another however remote :

On a Thursday night at 9.8 P.M. a London mercantile firm received a message which had been sent via Teheran from Kurrachee, India, on Friday morning at 12.43 A.M. The message was therefore received in London the day before it was sent from India. The time actually occupied by the message in transmission was fifty minutes. The sun would require four hours and twenty-six minutes to travel the same distance, and as the message was sent soon after midnight, the extraordinary result was produced of its arriving at its destination on the previous evening.

On another occasion, a direct communication was made by uniting the telegraph wires between Valentia, in Ireland, and San Francisco, in California. A message was sent from Valentia at 7.21 A.M. on February 1, and after traversing the bed of the North Atlantic and the whole continent of North America, the acknowledgment of it was received in Ireland at 7.23 A.M. The San

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