If the project of an Atlantic Telegraph be justly ascribed to the daring of an American, and its success to his courage and perseverance through years of struggle and disappointment; the solution of the scientific problem involved in it, is due to the genius of a Scotchman, whom the writer of this volume first knew (and it is a pleasant memory to have known such a man in the beginning of his splendid career) as Professor Thomson of the University of Glasgow, where his father had been professor before him, whom the son succeeded in the Department of Physics, which included the then little known science of Electricity, to which the young professor devoted himself with all the eagerness of scientific genius. The project of a telegraph across the ocean suggested new problems and new difficulties, to which he applied himself with characteristic ardor, the result of which is here given. When the second expedition of the Great Eastern (in 1866) was successful, the British Government at once recognized his eminent services; and the name of Sir William Thomson has since been recognized, among the leaders in scientific discovery, not only in England but all over the scientific world. The government has recently added a further dignity in making him a peer of the realm, an honor hitherto reserved generally for the leaders of armies, like Wellington. To confer it on a simple professor shows an advance of civilization in the respect paid to intellectual greatness. In conferring such a title, the government does not honor the man more than it honors itself. It is to the glory of England that such an honor should be paid to science in the person of Lord Kelvin, as was paid to literature in the person of Lord Tennyson.

The following, taken in substance from an English scientific review, will indicate briefly, but with sufficient clearness, the problem to be solved in signalling to great distances under the sea, and the instruments by which this is accomplished:—

The speed of signalling through a submarine cable depends upon its electrostatic capacity, which, unless it be very small, gives rise to "retardation."

In the Proceedings of the Royal Society for 1855, Sir William Thomson showed how the effect at the distant end of a cable, caused by the application of a battery at one end, could be calculated and represented graphically in what is called the "curve of arrival." After contact is first made at the sending end between the cable and one pole of the battery (the other pole being to earth), a certain interval of time elapses before any effect is felt at the distant end. This interval of time is denoted by the letter a. After the interval of time a has passed, a current begins to issue from the cable at the receiving end, and increases in strength very rapidly. After a further interval of 4a or after a period of 5a from the first application of the battery, it attains about half its maximum strength, and there is very little sensible increase in strength after a time equal to 10a has elapsed. The curve of arrival is drawn by taking distances along o x to represent intervals of time, and distances along o y to represent strengths of current. Curve No. I. shows the gradual increase in strength of the received current at one end of a cable when the battery is applied to and kept in contact with the other end. For a distance corresponding to the interval of time a, the curve does not sensibly deviate from the straight line o x; in other words, no effect is observable at the receiving end during this time.

If now, instead of being continuously applied to the battery at the sending end, the cable had been applied to it during a short interval of time, and then disconnected from the battery and connected to earth, the curve of arrival would be of the form shown by curve No. II. Curve No. II. shows the effect of applying the battery during a length of time equal to 4a, and then putting the cable to earth. It will be seen that a current gradually diminishing in strength continues to flow out of the cable at the distant end for a considerable time after the battery has been disconnected. This continued discharge is what gives rise to the difficulty experienced in reading the signals sent through long cables.

The instrument first used for receiving signals through a long submarine cable (the short-lived 1858 Atlantic cable) was the Mirror Galvanometer, which consisted of a small mirror with four light magnets attached to its back (weighing, in all, less than half-a-grain), suspended by means of a single silk fibre, in a proper position within the hollow of a bobbin of fine wire: a suitable controlling magnet being placed adjacent to the apparatus. The action of this instrument is as follows. On the passage of a current of electricity through the fine wire coil, the suspended magnets with the mirror attached, tend to take up a position at right angles to the plane of the coil, and are deflected to one side or the other according as the current is in one direction or the other.

Of various other forms of receiving instruments devised by Sir William Thomson, the next to be noticed is the Spark Recorder, both on account of the principles involved in its construction, and because it in some respects foreshadowed the more perfect instrument, the Siphon Recorder, which he introduced some years later. The action of the Spark Recorder was as follows. An indicator, suitably supported, was caused to take a to-and-fro motion, by means of the electro-magnetic action due to the electric currents constituting the signals. This indicator was connected to a Ruhmkorff coil or other equivalent apparatus, designed to cause a continual succession of sparks to pass between the indicator, and a metal plate situated beneath it and having a plane surface parallel to its line of motion. Over the surface of this plate and between it and the indicator, there was passed, at a regularly uniform speed in a direction perpendicular to the line of motion of the indicator, a material capable of being acted on physically by the sparks, either through their chemical action, their heat, or their perforating force. The record of the signals given by this instrument was an undulating line of fine perforations or spots, and the character and succession of the undulations were used to interpret the signals desired to be sent.

The latest form of receiving instrument for long submarine cables, is that of the Siphon Recorder, for which Sir William Thomson obtained his first patent in 1867. Within the three succeeding years he effected great improvements on it, and the instrument has, since that date, been exclusively employed in working most of the more important submarine cables of the world—indeed all except those on which the Mirror-Galvanometer method is still in use.

In the Siphon Recorder (a view of which is shown in Fig. 1), the indicator consists of a light rectangular signal-coil of fine wire, suspended between the poles of a powerful electro-magnet, so as to be free to move about its longer axis which is vertical, and so joined up that the electric currents constituting the signals through the cable, pass through it. A fine glass siphon-tube is suitably suspended, so as to have only one degree of freedom to move, and is connected to the signal-coil so as to move with it. The short leg of the siphon-tube dips into an insulated ink-bottle, which permits of the ink contained by it being electrified, while the long leg is situated so that its open end is at a very small distance from a brass table, placed with its surface parallel to the plane in which the mouth of this leg moves, and over which a slip of paper may be passed at a uniform rate as in the Spark Recorder. The effect of electrifying the ink is to cause it to be projected in very minute drops from the open end of the siphon-tube, towards the brass table or on the paper-slip passing over it. Thus when the signal-coil moves in obedience to the electric signal currents passed through it, the motion then communicated to the siphon, is recorded on the moving slip of paper by a wavy line of ink marks very close together. The interpretation of the signals is according to the Morse code; the dot and dash being represented by deflections of the line to one side or the other of the centre line of the paper.

Perfect as this instrument seemed, yet after further years of study and experiment, Sir William Thomson was able, at the close of 1883, to present to the world the Siphon Recorder, greatly improved, because in a very much simpler form. In this form of the instrument, instead of the electro-magnets, he used two bundles of long bar-magnets of square section and made up of square bars of glass-hard steel. The two bundles are supported vertically on a cast-iron socket, and on the upper end of each is fitted a soft iron shoe, so shaped as to concentrate the lines of force and thus produce a strong magnetic field in the space within which the signal-coil is suspended. He made instruments of this kind to work both with and without electrification of the ink. Without electrification the instrument, as shown in Fig. 2, is exceedingly simple and compact, and in this form is capable of doing good work on cables of lengths up to 500 or 600 miles. When constructed for electrification of the ink, as shown in Fig. 3, it is of course available for much longer lengths of cable, but for cables such as the Atlantic cables, the original form of the Siphon Recorder is that still chiefly used. The strongest magnetic field hitherto obtained by permanent magnets (of glass-hard steel) is about 3000 c. g. s. With the electro-magnets of the original form of Siphon Recorder as in ordinary use a magnetic field of about or over 5000 c. g. s. is easily attained. In Fig. 4 is shown a fac simile of part of a message received and recorded by a Siphon Recorder, such as is shown in Fig. 1, from one of the Eastern Telegraph Co.'s Cables of about 830 miles length.