The receiving mechanism is of the electro-chemical type which Caselli used. The current passes from the receiving stylus to the paper, and there makes its mark in a way that will be understood from the description of the earlier apparatus.
The supreme advantage of this method of working, over that of Professor Korn, is that the operator can see what he is doing. To obtain good results, a number of electrical adjustments have to be made, and whether he has got them right or wrong can be seen as soon as the picture begins to grow upon the receiving paper. If a little readjustment be needed the operator sees it and can set things right before the really important part of the picture begins to appear, whereas with the Korn apparatus he does not know what is happening at all, since he can see nothing until the picture is finished and the photographic paper has been developed.
It will be apparent, too, to anyone who has carefully considered the wireless telegraphy chapters, that it ought to be possible to make the sending stylus or its equivalent control a wireless transmitter and a wireless receiver to operate the receiving stylus, so as to be able to send pictures by "wireless." Experiments to this end have been made with some measure of success, and sooner or later we are almost sure to hear that the difficulties, which are by no means small, have been overcome.
But we cannot conclude this chapter without a fuller reference to that marvellous invention, the telewriter.
In this a man makes a sketch with a pen on a piece of paper, or maybe he writes a message, and simultaneously a pen, hundreds of miles away if need be, does precisely the same thing. The receiving instrument draws the sketch line by line, or it transcribes the message in the actual handwriting of the sender. A little touch, almost weird in its naturalness, is that every now and then the receiving pen leaves the paper and dips itself into a bottle of ink, after which it resumes its work at the very spot where it left off.
Now how the complicated lines and curves, the strokes and dots which make up a written language, even the little shakes and defects which give each man's writing a personality of its own, how all these can be sent over a wire is at first sight very difficult to understand. The inventor of this apparatus has discovered an extremely simple way of doing it.
But even he does not attempt to do it with one wire, it should be said, for he uses two. This is no drawback when, as is often the case, it is used in conjunction with a telephone, for the latter, to be effective, also requires two wires. Years ago single wires were employed for telephones as for telegraphs, the circuit being completed through the earth. But the difficulty arose that every wire through which currents flow is apt to induce currents in neighbouring wires—the induction coil is based upon that fact—and so messages in one wire were overheard on others, or, what was perhaps more annoying still, the dots and dashes passing in a telegraph wire would produce loud noises in a telephone wire that happened to be near. The use of two wires, however, entirely removes that trouble, for the neighbouring current then induces two currents instead of one, one in each, and it so happens that these are opposed to each other, so that they neutralise each other. So every telephone wire now is double and therefore is ready, as it were, to have the telewriter fitted to it.
But even with two wires the difficulty seems insuperable until we remember that the most complex of curves can be resolved into two simple movements.
The sending pen, with which the original writing or drawing is done, is attached to the junction of two light rods. The farther end of each rod is attached to the end of a light crank fixed so that it can rotate or oscillate, after the manner of cranks, in the plane of the desk upon which the paper lies. All the joints mentioned are of the hinge nature, so that as the pen is moved about the rods turn, more or less, one way or the other, the two cranks. This simple mechanism, it will be observed, carries out very effectively the principle just mentioned, for it resolves the motion of the pen, no matter how complicated it may be, into a simple rotating motion of the two cranks.
So the cranks turn this way or that as the draughtsman makes his picture, and it is very easy to arrange that their movement shall vary the strength of two electric currents, whereby we obtain electric currents varying in accordance with the movement of the cranks.