Currents flow in varying intensity away to the coils TT’ and work the arms DD’, the wires FF’, and the pen, a tiny glass tube.
In the perfectly regulated telautograph the arms AA’ and the arms DD’ will move in unison, and consequently the position of the pen must be the same from moment to moment as that of the pencil.
Mr. Foster Ritchie, the clever inventor of this telautograph, had to provide for many things besides mere slavish imitation of movement. As has been stated above, the pen must record only those movements of the pencil which are essential. Evidently, if while the pencil returns to dot an i a long line were registered by the pen corresponding to the path of the pencil, confusion would soon ensue on the receiver; and instead of a neatly-written message we should have an illegible and puzzling maze of lines. Mr. Ritchie has therefore taken ingenious precautions against any such mishap. The platen P on being depressed by the pencil touches a contact, X, which closes an electric circuit through the long-distance wires and excites a magnet at the receiving end. That attracts a little arm and breaks another circuit, allowing the bar Y to fall close to the paper. The wires FF’ and the pen are now able to rest on the paper and trace characters. But as soon as the platen P rises, on the removal of the pencil from the transmitting paper, the contact at X is broken, the magnet at the receiver ceases to act, the arm it attracted falls back and sets up a circuit which causes the bar to spring up again and lift the pen. So that unless you are actually pressing the paper with your pencil, the pen is not marking, though it may be moving.
As soon as a line is finished a fresh surface of paper is required at both ends. The operator pushes the lever S sideways, and effects the change mechanically at his end. At the same time a circuit is formed which excites certain magnets at the receiver and causes the shifting forward there also of the paper, and also breaks the writing current, so that the pen returns for a moment to its normal position of rest in the inkpot.
It may be asked: If the wires are passing currents to work the writing apparatus, how can they simultaneously affect the lifting-bar, Y? The answer is that currents of two different kinds are used, a direct current for writing, a vibratory current for depressing the lifting-bar. The direct current passes from the battery B through the rheostats RR’ along the wires, through the coils working the arms DD’ and into the earth at the far end; but the vibratory current, changing its direction many times a second and so neutralising itself, passes up one wire and back down the other through the lifting-bar connection without interfering with the direct current.
The message finished, the operator depresses with the point of his pencil the little push-key, K, and connects his receiver with the distant transmitter in readiness for an answer.
The working speed of the telautograph is that of the writer. If shorthand be employed, messages can be transmitted at the rate of over 100 words per minute. As regards the range of transmission, successful tests have been made by the postal authorities between Paris and London, and also between Paris and Lyons. In the latter case the messages were sent from Paris to Lyons and back directly to Paris, the lines being connected at Lyons, to give a total distance of over 650 miles. There is no reason why much greater length of line should not be employed.
The telautograph in its earlier and imperfect form was the work of Professor Elisha Gray, who invented the telephone almost simultaneously with Professor Graham Bell. His telautograph worked on what is known as the step-by-step principle, and was defective in that its speed was very limited. If the operator wrote too fast the receiving pen lagged behind the transmitting pencil, and confusion resulted. Accordingly this method, though ingenious, was abandoned, and Mr. Ritchie in his experiments looked about for some preferable system, which should be simpler and at the same time much speedier in its action. After four years of hard work he has brought the rheostat system, explained above, to a pitch of perfection which will be at once appreciated by any one who has seen the writing done by the instrument.
The advantages of the Telautograph over the ordinary telegraphy may be briefly summed up as follows:—