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 pa.s.sage 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 const.i.tuting the signals. This indicator was connected to a Ruhmkorff coil or other equivalent apparatus, designed to cause a continual succession of sparks to pa.s.s 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 pa.s.sed, 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.

[Ill.u.s.tration: FIG. 1.]

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 const.i.tuting the signals through the cable, pa.s.s through it. A fine gla.s.s 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 bra.s.s 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 pa.s.sed 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 bra.s.s table or on the paper-slip pa.s.sing over it. Thus when the signal-coil moves in obedience to the electric signal currents pa.s.sed 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.

[Ill.u.s.tration: FIG. 2.]

[Ill.u.s.tration: FIG. 3.]

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 gla.s.s-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 s.p.a.ce 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 gla.s.s-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.

[Ill.u.s.tration: FIG. 4.]