Page:Popular Science Monthly Volume 3.djvu/432

418 be produced which will be the exact reproduction of the broken line of the dispatch traversed by the style. Accordingly, when the style of M has described a series of lines close together and covering the sheet, R will be covered with a series of points or lines forming a copy of the dispatch. The tracing point is carried by a lever turning about an axis near its lower end. To this lower end is attached, a connecting rod, jointed at its other end to the pendulum (Fig. 17). While the pendulum swings in one direction, the style traces a line in one direction on the sheet. At the end of this stroke, an action occurs which, besides advancing the style, raises it, so that it does not touch the sheet during the return-stroke.

The synchronism of the pendulums at the two stations, which is absolutely necessary for correct working, is obtained by means of two clocks which are separately regulated to a given rate, the clock-pendulums making two vibrations for one of the telegraphic pendulum. The bob of the latter consists of a mass of iron, and vibrates between two electro-magnets, which are made and unmade according to the position of the clock-pendulum, as the latter makes and breaks the circuit of a local battery. The mass of iron is thus alternately attracted by each of the two magnets as it comes near them, and is prevented from gaining or losing on the clock.

It is evident that the Caselli telegraph may be applied to copy not only letters, but a design of any kind; hence the name of pantelegraph which has been given it. Fig. 18 represents a copy thus obtained upon Bain's paper. Fig. 19 represents a copy, obtained at the same time upon a sheet of tin-foil, such as is usually placed beneath the paper. The current decomposes the moisture of the paper, and the hydrogen thus liberated reduces the oxide of tin, of which a small quantity is always present on the surface. If the foil be then treated with a mixture of nitric and pyrogallic acid, the traces are developed, and come out black.

The Caselli system has been used for some years on the telegraphs