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etc., to the other pole b. Now when this device is inserted in the circuit at each station, the running of the signals together in the cable is less noticeable on the receiver, and sharper, clearer signals are presented to the operator reading the message.

The mirror galvanometer, employed as a receiver on many cables, is represented in Fig. 27, where c is a

H

G

FIG. 27.

coil of fine insulated wire, surrounding a small magnetic needle hung by a silk fibre, and carrying a tiny mirror attached to it. The details of this needle are shown in the lower figure, where c c are sections through the coil, м is the magnet-needle, carrying in front of it a small mirror. This needle is enclosed in a small chamber, glazed by a lens G, and inserted in the hollow of the coil c. A curving magnet H is supported over the coil to adjust the position of the smaller magnet in the chamber. Now a ray of light from a lamp L in front of the galvanometer is thrown

upon the tiny mirror and reflected back upon a white screen or scale s. The coil c is connected between the end of the conductor of the cable and the earthplate, as in the land circuit; a condenser, however, being usually interposed between the cable and the galvanometer.

Then the signal currents in passing through the coil deflect the tiny magnet hung within it, and the mirror, being carried by the magnet, throws the beam of light off in a different direction. Positive, or "dot," currents are arranged to throw the spot of light toward the left side of the scale; and negative, or "dash," currents throw it to the right side. Thus the wandering of the spot of light on the screen, watchfully followed by the eye of the clerk, is interpreted by him as the message. Letter by letter he spells it out, and a fellow-clerk writes it down word for word.

This receiver, however, like the sounder, has the disadvantage of leaving no permanent record, and Sir William Thomson has therefore introduced his Siphon Recorder on several long cables-for instance, the Eastern Telegraph Company's lines to India, and the AngloAmerican Company's cables across the Atlantic. It is undoubtedly one of the finest inventions of the age, although it is not very well known, because only a few are required, and these are only to be seen abroad, unless one should be specially exhibited. A general view of the instrument is given in Fig. 28, and the leading parts are detailed in Fig. 29. The principle of its action is just the reverse of the mirror galvanometer. In that instrument a tiny magnet moved within a fixed coil of wire; in the siphon recorder a light coil of wire moves between the poles of a powerful magnet. The

signal currents pass through the suspended coil to earth, and in doing so the coil turns to left or right, according as the currents are positive or negative.

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These movements of the coil are communicated by a connecting thread to a fine glass siphon, which is constantly spurting ink upon a band of travelling paper, and hence the trace of the ink on the paper follows and

delineates the movements of the coil. In Fig. 28, s is the signal coil suspended between the poles of two powerful electro-magnets м M.

In Fig. 29, the coil s is shown connected by a

®

FIG. 29.

thread and a small multiplying lever u to the siphon t swung from i. The siphon is as fine as a bristle, and dips into the ink-box K with its shorter arm, while the longer arm presents its nozzle to the travelling paper. So fine is the bore of the siphon that the ink will not

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run unless it is electrified, and hence an ingenious machine o, Fig. 28, termed the "mouse-mill," is provided to generate electricity by "induction," after the manner indicated in Chapter III. pp. 24-25. The stream of electricity thus supplied is led from the mill to the ink by way of the strip of paper, or other semi-conductor F, and the receiving plate D. The particles of ink thus electrified rush through the fine bore of the siphon and rain upon the paper. The latter is pulled continuously past the siphon-point by the pulley a, which is driving the "mousemill." There are many adjustments and interesting actions about the siphon recorder, but it would require a whole chapter to describe them, and we will now conclude our notice of this, the most delicate and beautiful of all telegraph instruments, with a specimen of the message it delivers.

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