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. The reproduction is, in photographic language, a negative, a somewhat unsatisfactory feature of the method. A simple modification, however, of the electrical connections will reverse that, so that the reproduction shall be a positive. There are two ways of cutting off a current from any particular circuit. One is to interpose a resistance, through which current cannot pass in an appreciable quantity, and the other is to provide a second path for the current so much easier than the first that practically all the current will pass that way, leaving the first circuit, to all intents and purposes, free. It is as if a farmer wished to stop people passing across a certain field. Two methods would be open to him: one to put up a high gate over which no one would dare to climb, and the other to provide a short cut so much more pleasant and convenient than the old path that no one having the choice of the two ways would think of going the old way. What the farmer would call a short cut the electrician calls a short circuit, and a short circuit is often a more convenient way of cutting off a current than a switch which interposes resistance. At all events, in a case like this, a short circuit enables that to be accomplished which would be very difficult by any other means. In the apparatus as already described the battery had to drive the current along a long wire, terminating at the distant receiving instrument, whence the current returned via the earth. The foil and pen, acting as a kind of electrical "tap," controlled this. When foil and pen touched, the tap was open and current flowed. When the line of non-conductive ink interposed itself, the tap was off and the flow ceased.