| Letter. | Sign. |
| A | . — |
| B | — . . . |
| C | — . — . |
| D | — . . |
| E | . |
| F | . . — . |
| G | — — . |
| H | . . . . |
| I | . . |
| J | . — — — |
| K | — . — |
| L | . — . . |
| M | — — |
| N | — . |
| O | — — — |
| P | . — — . |
| Q | — — . — |
| R | . — . |
| S | . . . |
| T | — |
| U | . . — |
| V | . . . — |
| W | . — — |
| X | — . . — |
| Y | — . — — |
| Z | — — . . |
| Ch | — — — — |
| é (accented) | . . — . . |
| Understand | . . . — . |
| Wait | . — . . |
The numerals run:
| Numeral. | Sign. |
| 1 | . — — — — |
| 2 | . . — — — |
| 3 | . . . — — |
| 4 | . . . . — |
| 5 | . . . . . |
| 6 | — . . . . |
| 7 | — — . . . |
| 8 | — — — . . |
| 9 | — — — — . |
| 0 | — — — — — |
For other accented letters, fraction signs, punctuation, and official directions as to the disposal of the message, there are other signs, but the above are the essentials of the Morse Code. The long and short signs represent the long and short signals of the receiving instrument, produced by the long and short contacts of the sending key with the battery. It will be seen that the letter A is rendered by a short signal followed by a long one; the letter B by a long signal followed by three separate short ones; and so on. Hence, in order to telegraph the letter A to his colleague at the distant end of the line, the clerk, by depressing the lever of the sending instrument, makes contact between the cable and the battery, first for a short time, and then for a longer time. The long and short signals are widely employed in overland telegraphy; but in submarine telegraphy a saving of time is effected by signals of opposite kind. Thus, if a left deflection, or deflection of the indicator to the left, signifies a 'dot' or short signal, a deflection to the right will signify a 'dash' or long signal. In this case the sending instrument or key has two levers, a right and left one, corresponding to the distinct signal which each produces. By depressing the left lever of the key, a pole of the battery is applied to the cable, which produces a left-hand signal on the receiving instrument at the distant station; and by depressing the right-hand lever, a right-hand signal is produced. Proper rests or intervals are permitted between the separate words, letters, and full stops of a message.
The battery in common use for submarine telegraphy is either the sawdust Daniell or the Leclanché. The Daniell consists of a plate of zinc and a plate of copper brought into contact with each other by sawdust saturated with a solution of sulphate of zinc; and crystals of sulphate of copper (bluestone) are packed round the copper plate, so as to dissolve there in the solution of sulphate of zinc. The zinc plate forms the negative pole of the battery, and the copper plate the positive pole. When these two poles are connected together by a wire or other conducting circuit, such as that made up of the cable and the earth, a current of electricity—the voltaic current—flows from one to the other, and always in one direction, namely, from the copper or positive pole to the zinc or negative pole. Hence it is that by applying the one pole or other to the cable and the other to earth through the earth-plate, the direction of the current in the cable is reversed and opposite signals produced.
The earth-plate is usually a copper plate several feet square, sunk deep into the moist subsoil near the station, so as to make a good conducting contact with the mass of the earth.
The receiving instruments for working a submarine cable are different from those used in working land-lines. Inasmuch as the current travels full strength, like a bullet, through a land-line, and in the form of an undulation or wave through a cable, so is it necessary to have different kinds of receiving instruments for each. In a land-line powerful currents can be used with impunity, and these can be made, by means of electro-magnetism, to move comparatively heavy pieces of mechanism in giving signals. But in a cable the currents are prudently kept as low as possible, in case of damage to the insulator, and the receiving instrument must therefore be delicate. In land-lines the current passes in an instant, leaving the line clear for the next signal, so that the indications of the receiving instrument are abrupt and decided. But in a cable the electric current takes an appreciable time to flow from end to end, so that the separate signals in part coalesce, the beginning of one blending with the end of that preceding it, so that the signals become involved with each other. It is necessary, therefore, that time be allowed for each wave to clear itself of the cable before another wave is sent in, otherwise we would have the cable as it were choked with the message. A continuous current of electricity may be said to be flowing through it, and the ripples on the surface are the separate signals of the message. It is to take cognisance of these waves or ripples that the receiving instrument for cable-work must be designed; and as the quicker the message is sent into the cable the smaller these ripples will be, the more delicate should be the instrument.
There are only two instruments in use on long cables, and both are the invention of Sir William Thomson, the distinguished Glasgow physicist and electrician. The mirror galvanometer has been already described in this Journal in a paper on the manufacture of submarine cables; and the 'mirror' or 'speaker,' the commonest of these receiving instruments, is but a modified form of the mirror galvanometer. It consists of a hollow coil of silk-covered wire, in the heart of which a tiny mirror, with several small magnets cemented to its back, is suspended by a single thread of floss-silk fibre. A beam of light from a lamp is thrown upon the mirror, and reflected from it on to a white screen, across which a vertical zero-line is drawn. When no current is passing through the coil, the reflected beam of light which makes an illuminated spot or gleam on the screen, remains steady at the zero-line. But when a current passes through the coil, the magnets in its heart are moved and the mirror with them, so that the beam of light is thrown off at a different angle, and the spot of light is seen to move from the zero-line along the screen to right or to left of the zero-line according as the current is made or reversed in the coil; so that as the key is manipulated at the sending station, so are right or left signals received by the clerk who sits watching the movements of this spot of light, and interpreting them to his fellow-clerk, who writes them down. In the form of instrument here described, and also in the other receiving instrument for submarine work, the zero is not fixed but movable. The vertical line on the screen is only the nominal zero. The continuous current underlying the ripples which form the message, deflects the spot from the zero-line; but this slow deflection can be disregarded by the clerk, for over and above it there are smaller quicker movements of the spot to right and left corresponding to the ripples, and these are the proper signals of the message. It requires long practice to make a good 'mirror' clerk, one who can follow the gleam with his eye through all its quick and intricate motions, and distinguish between those due to the shifting zero and those due to the various signals sent. Even this compound-ripple difficulty, however, is now got rid of by the use of an apparatus called a 'condenser,' the effects of which are that continuous currents are neutralised, and the pulsations of the signals sent are alone seen in the movements of the light upon the scale.
The other instrument is the siphon recorder, which permanently records in ink the signals which the 'mirror' only shews transiently. The principle of the siphon recorder is the converse of that of the mirror. In the mirror there is a large fixed coil and a light suspended magnet. In the siphon recorder there is a large fixed magnet and a light suspended coil. When the current passes through this coil, the latter moves much in the same way as the magnet moves in the 'mirror;' that is, it rocks to right or left according as the current flows. This rocking motion is communicated, by a system of levers and fibres, to a very fine glass capillary siphon, which dips into an ink-bottle and draws off ink upon a strip of running paper. The ink is highly electrified, so as to rush through the siphon and out upon the paper, marking a fine line upon it as it runs. When no current passes in the coil, this zero-line is straight; but when currents are passing, the line becomes zigzag and wavy; and the right and left waves across the paper constitute the message. Both of these instruments are very beautiful and ingenious applications of well-known electric, optical, and mechanical principles. The great merit of the recorder is that if a false signal is accidentally made by the sending clerk, the whole word need not always be lost by the receiving clerk, but may be made out from the rest of the word written down. Thus much repetition of messages is saved. There is some advantage too in having a written message for purposes of after reference.
A singularly ingenious system of telegraphy, termed the duplex, has recently been extended to long submarine cables, and is likely to become of general, if not universal application. It is effected by constructing an artificial line, in this case representing an artificial cable, which shall have the same influences on the current that the actual cable has. The signalling current from the battery is then split up at each station between the actual cable and the artificial cable, so that half flows into one and half into the other. And there is placed a receiving instrument in such a way between these two halves of the current that they exactly counterbalance each other's effect upon it; and so long as sending is going on from a station, the receiving instrument at that station is undisturbed. But the sending currents from the other station have the power to disturb this balance and cause signals to be made. Thus then, while the sending at a station does not affect the receiving instrument in connection with the cable there, the currents sent from the distant station cause it to mark the signals. Each station is thus enabled to send a message and receive one at the same time; and this is what is called duplex or double telegraphy.