In working the Morse instrument, the operator at B depresses the handle H´. Suppose that this handle is kept depressed by a spring, and that a long strip of paper passing uniformly between the two points at _a_ prevents contact. Then no current can pass. But if there is a hole in this paper, then when the hole reaches _a_ the two metal points at _a_ meet and the current passes. We have here the principle of the Bain telegraph. A long strip of paper is punched with round and long holes, corresponding to the dots and marks of a message by the Morse alphabet. As it passes between a metal wheel and a spring, both forming part of the circuit, it breaks the circuit until a hole allows the spring to touch the wheel, either for a short or longer time-interval, during which the current passes to the other station, where it sets a relay at work. In Bain’s system the message is received on a chemically prepared strip of paper, moving uniformly at the receiving station, and connected with the negative pole of the relay battery. When contact is made, the face of the paper is touched by a steel pointer connected with the positive pole, and the current which passes from the end of the pointer through the paper to the negative pole produces a blue mark on the chemically prepared paper.[31] We see that by Bain’s arrangement a paper is marked with dots and lines, corresponding to round and elongated holes, in a ribbon of paper. It is only a step from this to the production of facsimiles of writings or drawings.