TELEGRAPH (Gr. tele, far off, and graph°, to write) is a general name for any means of conveying intelligence other than by voice or writing. The idea of speed is also implied. Alarm-fires (see BEACON), the semaphore (q.v.), and the signals (q.v.) used at sea, are among the earlier forms of telegraph. But all other agents are now thrown into the shade by the electric telegraph. It has been reserved for our own day to develop into practical use the capabilities of electricity and magnetism as a means of distant communication; although in earlier times, the possibility of such a use of this natural power has been frequently suggested. The public use of the electric telegraph dates not earlier than 1846; but the idea that magnetism could be applied for distant communica tion is at least two centuries and a half old. Galileo, in one of his dialogues on the rival astronomical systems, written in 1632, puts in the mouth of one of his speakers, a refer ence to a secret art, by which, through the sympathy of a magnetic needle, it would be possible to converse across a space of two or three thousand miles. In 1753, a letter appeared in the Scots Magazine, bearing the initials C. M., and headed " An Expeditious Method of Conveying Intelligence," in which we must recognize the first perception of the uses to which telegraphy might be put. This interesting letter starts with the remark that it " is well known to all who are conversant in electric experiments, that the electric power may be propagated along a small wire from one place to another without being sensibly abated by the length of its progress;" and it goes on to describe an arrange ment of wires corresponding in number to " the letters of the alphabet, to be fixed in glass or jewelers' cement at intervals of 20 yards, and to convey, from an electric machine or rubber, a current which would lift each letter, "marked on bits of paper, or other substance that might be light enough" to rise to the electrified ball which formed the terminal of each wire. The apparatus proposed is crude and clumsy, yet we can hardly fail to recognize, in the letter of this Renfrew man, the full appreciation of what the electric telegraph might become. From a period shortly anterior to the date of the letter in the Seats Magazine, down to 1837, a large number of proposals, more or less ingen ious, are on record. The space at our command will not permit us to describe all those inventions, or to enter upon the much-vexed question as to who is entitled to the name of inventor of the electric telegraph. We must be content to furnish a description of the more important instruments in use, some statistics of the more recent history of telegraphy generally .in this country, and a notice of the progress of submarine teleg raphy. In our description of instruments, etc., we shall assume the reader to be famil iar with the chief facts of galvanism (q.v.) and electromagnetism (see MAGNETISM).
Telegraph instruments may be classed under two heads, namely, those which record the signals, and those which only give passing signals to the observer or listener. Among the former are several kinds, namely, those giving a record in arbitrary signs (i.e., in the dots and dashes of the Morse alphabet); those which print in ordinary char acters, such as the Hughes type-printing instrument; and lastly, a class of instruments giving a facsimile of the message. The latter two classes are not much used, and the
number of Hughes instruments in use in this country has somewhat decreased. The great bulk of the telegraphing of the world is done either by the Morse printer, or by the non-recording instruments; and to those we shall therefore devote the major part of our description.
The Morse.—The leading principle in the Morse and other allied instruments is, that by the depression of a key or other method, an electric circuit is " closed" or completed, and a signal is transmitted along the wire to the distant station, where, on its arrival, it reproduces the signal by the action of an electromagnet or otherwise. Electrically, the Morse consists of the transmitting key, (fig. 2) and the electromagnet and armature Fig.1); while mechanically, it consists of a lever. with circular wheel or disk, attached to the armature, and a clock-work ment by which the paper tape to be printed upon is carried forward under the disk. Fig. 1 represents the magnet and armature by which signals are received. On the current being received from the distant station, it traverses the coils of the electromagnet E, and the armature F is drawn down by the action of the current. A and B are screws for regulating the play of the armature; and of the inking-disk I, at the other end of the lever. The ment clerk is first required to adjust B so that the upper edge of the disk shall press gently against the paper tape (which is not shown in the engraving) and the screw A, so that the under surface of the disk shall dip into the ink-well or reservoir (likewise omitted in the diagram). B haying been adjusted, the screw D is turned so that when the brass stopf rests upon the stud b, the poles of the magnet shall clear the armature without actu ally touching it. A thin streak of light should be seen between the armature and the poles. Screw C is used to adjust the spiral spring above, by which, on the cessation of the electric current, the armature is drawn back and the disk to mark the paper. In the first Morse instruments the marks were made on the paper with a pointed style (the instrument being thus known as the embosser); but by the invention of the ink-writing arrangement of Siemens, which we are now consider ing, the legibility and permanence of the record were secured, besides the advantage that a very light current will serve to make the marks. The case containing the clock-work, the arrangement of wheels by which the paper tape is unwound and carried forward, and the switch, by which the running of the tape is stopped, are not shown in the figure. The passage of a current draws down the armature, and elevates the disk, causing a straight mark on the tape so long as the current flows. When it ceases, the spring S draws back the armature as already described, and the mark is discontinued. Thus the duration of the current determines the nature of the mark, a momentary passage causing a dot, a longer depression of the key a dash; and as the alphabet invented by prof. Morse consists of dots or dashes, or a combination of the two, the above figure and explanation disclose the whole mystery of this system of telegraphy to those who have mastered the phenomena of electricity.