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M. la Cour provides, by means of electro-magnets, that the fork shall be maintained constantly in a state of vibration. In either case the current sent into the line by depressing the signalling-key is interrupted by the fork an equal number of times per second that the fork vibrates. This intermittent current traverses the line to the distant station, where it is passed "to earth" through the receiving apparatus shown in Fig. 12. This consists of a fork similar to the first, and vibrating the same note. Each leg is surrounded by a helix of wire. Two other helices are placed upright, one on each side of the ends of the legs; they are fitted with iron cores, and adjustible poles, n, s, and are in fact electro-magnets. These four helices are joined up in series, so that the line-current passes through each in turn. It

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magnetises oppositely the legs of the fork, which are attracted by the contiguous poles of the electro-magnets. The legs of the fork are thus pulled apart, and let go with each pulsation of the intermittent current. In this way the fork is put into audible vibration.

These vibrations are also turned to the actual recording of the message in permanent marks, by means of a local circuit and Morse or other recorder. A fine metal point, P, is brought very near to one leg of the fork, so that when the fork vibrates this leg comes into contact with the point, and completes the local circuit through the fork, local battery, and recording instrument.

M. la Cour's first experiments were made in June, 1874, on a short line in the neighbourhood of Copenhagen, and in November of the same year he succeeded in working

successfully from Fredericia, in Jutland, to Copenhagen, a distance of 390 kilometres.

Mr. Elisha Gray's first English patent bears date of 1874, a few months before M. la Cour's; but in it there is no mention of multiplex telegraphy. Subsequent patents, in 1875 and 1876, describe the development of his system and its application to multiplex signalling. His principal methods of sending are represented in Figs. 13 and 14. In Fig. 13 the vibrator is a tuning-fork or reed, R, placed between two electro-magnets, A and B, which can be adjusted by the screws D and G to or from the reed, so as to have greater or less power over it. The reed is barbed with

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two short springs, which play against the adjustible contactpoints a and b, making contact with them alternately as the reed vibrates. The reed is perpetually maintained in vibration by means of a local battery and the electro-magnets in the following way:-The local battery is connected up, as shown, through the electro-magnets and the fixed end of the reed. When the reed vibrates so as to make contact, by means of its spring, with b, and break contact with a, the electro-magnet A is in circuit and actuated by the current, while the magnet в is cut out of circuit by the shortcircuit wire w. The magnet A therefore exercises a pull upon the reed R, which assists the reed in vibrating towards

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it, and breaking contact with b while making contact with a. This, while keeping the electro-magnet A still in circuit, also puts the magnet B in circuit. In this way the electromagnet B is alternately thrown in and out of circuit, and

Battery

Local

B

P

Fig.14.

To Line

the effect of this on the reed is to keep up its vibration. The reed, thus kept vibrating by the electro-magnets and local battery, is used to interrupt the line circuit and sending current. The sending battery is connected up, as shown, through the signalling-key to the reed, and by the reed

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spring and contact-point a to the line. At each vibration of the reed the contact between the spring and a is made and broken, and the circuit opened and closed. Whenever the signalling-key is depressed, therefore, an intermittent current enters the line.

Mr. Gray also procures the intermittent current by means of a vibrating bar or string, as in Fig. 14, where a is a thin steel bar stretched by the screw s, and carried by a fixed metal bar or frame, B. The bar A vibrates between upper and lower contact-points, c, d. M M are two electro-magnets whose poles are let through the bar B, so as to act upon the vibrating bar. PP are pillars supporting the whole. In this arrangement a local battery is also employed to set and maintain A in vibration: this is done, however, in the act of signalling. On closing the signalling-key the local current passes to the upper contact c, against which the bar A rests. From thence it passes, by means of the bar в and wire w, through the magnets м M, completing its circuit. These magnets then attract the bar downwards in its middle, plucking it away from contact with c, and bringing it into contact with d underneath. The local circuit being thus broken, however, the bar springs back again into contact with c, to be plucked down again as before. In this way the bar is started vibrating. The line circuit is made through the lower contact d, the bars в and A, and the pillar P. At every vibration of the steel bar A it is interrupted, and an intermittent current set up in the line.

In both of these methods the number of intermissions in the line current will correspond with the number of vibrations of the vibrators. By employing in the first method reeds of different pitch, and in the last bars or strings of different dimensions and tension, distinct intermittent currents will be produced.

The principal receivers for interpreting these currents into distinct audible sounds are represented in Figs. 15 and 16. In Fig. 15 M is a double electro-magnet supported over a resonance pipe closed at one end. The soft-iron armature of the electro-magnet, t T, is rigidly fixed to one pole at t, the other end being free to vibrate in front of the other pole at T. When the intermittent current from the line passes through the electro-magnet the free end of the armature or tongue is set into corresponding vibration, and the aircolumn in the resonance box, vibrating in unison with it, gives out an audible note. It is easy to see how, by employing a number of separate transmitters, such as described, to superimpose separate intermissions in the line current,

and passing the complex current so produced through as many separate receivers of this kind, that each receiver will only respond to its own particular set of intermissions. For the sounding-pipe of each receiver can be so constructed

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as to resound only to the vibrations set up in the tongue by a particular series of intermissions. In this way several distinct notes may be simultaneously telegraphed, each note being used for a separate message.

The receiver shown in Fig. 16 has been called the " physiological receiver," since it depends for its action on the

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contact of living animal tissue with a conducting surface. It is the most interesting of the two, because its action has not hitherto been explained. If the intermittent current from the line is passed through the tissue to the conductor,

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