Suppose that we connect the foil to one pole of a battery, and the other pole by a flexible wire to a metal pen or stylus. If we place the point of the pen in contact with the foil, we make a complete circuit, through which, of course, current will flow. But if, with it, we touch one of the non-conductive lines, there will be no current.
Fig. 13
Fig. 14
Taking a ruler, then, let us draw the point of the stylus across the foil in a series of parallel straight lines. It is these excursions of the stylus which the dotted lines are intended to represent. For nearly the whole of the time current will be flowing; but whenever the stylus is crossing one of the lines of non-conductive ink there will be a momentary cessation. Thus, the reader will begin to perceive, we obtain what we may call an electrical representation of the figure drawn upon the foil.
And now let us turn to Fig. 14. There, too, is a square, but in this case it is not foil, but paper which has been soaked in prussiate of potash. The reason for introducing this chemical is that it is susceptible to electrical action. Wherever current passes through it, it becomes changed into Prussian blue, so that if we place the point of a pen upon the paper, and cause current to flow out of that point through the paper, there we get a blue dot. If, while the current is flowing, we draw the pen along, we get a blue line.
Fig. 13 therefore represents in principle the sending apparatus of Caselli's writing telegraph, while Fig. 14 represents the receiving instrument. The two pens are connected together by the main wire, in such a manner that, when the point of the one is in contact with the bare foil current flows out of the other and into the paper; but as the former crosses an ink line all current ceases.
If, then, while the sending pen is drawn line by line across the foil, the other is drawn at the same speed, line by line, across the chemically prepared paper, we shall get on the latter a series of lines as shown in Fig. 14 almost continuous, but broken here and there. Each breakage represents a passage of the sending pen across a line, and taken together, as will be seen, they constitute a reproduction of the geometrical figure drawn upon the foil. As shown, the lines are rather far apart, and so the reproduction is not a very good one. They are only drawn so, however, in order that the principle may be shown the more clearly. They may be drawn so that they overlap, and then the effect is very much better, the received picture being almost an exact reproduction of the other.
It will be noticed that an essential to the success of this method is that the two pens should move in perfect unison, and that was the great difficulty. Caselli used an arrangement of pendulums, the best thing available at the time.