In July, 1838, Mr. Edward Davy obtained a patent for improve ments in apparatus for making telegraphic signals by means of electric currents. The principal feature in this patent is a chemically marl-ing telegraph, in which a local circuit (completed by galvanometer needles acted on by the line circuit, and forming the basis of what is called the relay system) marks chemically prepared paper, which is moved forward by clockwork released by an electric magnet. Two line wires are used to convey the electric current and one return wire, with a battery and two galvanometers (deflecting opposite ways) to each wire; the battery to the return wire giving a preponderance to currents through that wire, twelve different signals can thus be produced. The action is as follows : on one or more of six keys being pressed down, either two or three of the galvanometers act; and as the three wires admit of the current proceeding in either direction through them, it can complete either two or three out of the six circuits of the local battery ; thus marking longitudinally properly prepared paper at two or three out of six places. The clockwork escapement for moving the paper a sufficient distance between the signals, consists of two levers, one of which carries the armature, and works on a horizontal axis carried by the other lever. When the armature is attracted, it releases a pin from a notched fly vane, and enables it to move half a revolu tion ; and when the current ceases, the pin is again removed, and replaced by lateral motion given to the second lever by a wheel pressing against a projection on it. Calico "impregnated with hydriodate of potash and muriato of lime," is preferably used to receive the marks or signa Is.
We may here state in more precise terms the principle of the chemical printing telegraph. A train of clockwork is used to keep a ribbon of paper constantly unwinding from a drum, and passing over a metal roller b, fig. 6. A small steel wire c, connected with the wire from the steel wire c, through the paper strip, to the roller b. This paper being chemically prepared with a colourless solution, is coloured when de composed by the passage of an electric current through it. In this way, by regulating the duration of the current, the paper can be stained in dots and lines, as in the symbol-printing arrangement.
An ingenious modification of the chemical printing telegraph was made in the writing telegraph. It is evident that if, instead of employing a long ribbon of paper, enough of it were taken to pass once round the roller b, and if this roller not only rotated on its axis, but was also by means of a screw made to advance slowly from side to side as it rotated, the steel wire would trace on the paper a spiral line. If also, instead of communicating by a key, there were at the other end another roller, exactly like b, and which by means of clockwork could be turned at precisely the same speed as b, and with the same rate of I shift endwise, then a tracer, resting on this second roller would, if the current were kept continually passing, trace precisely the same spiral as the wire on the roller b. But supposing the current only to be maintained by the pressure of this tracer against the metal roller, and if the surface of the roller were partly covered with a non-conducting material, such as shell-Lac or resin, the tracer in passing over this as the roller turned round, would cease to make its stained mark, but it would begin to mark again as soon as the tracer had come upon the metal. In using this machine the words of the message are written on a slip of tin-foil in a non-conducting varnish. As soon as this is dry, the tin-foil is wrapped round the tracer roller, and the tracer being made to bear upon it, the two rollers, one at the communicating and the other at the receiving station, are started at the same moment. Then, while the tracer rests on the tin-foil, the wire on the roller 6 will con tinue to make its stained spiral line on the prepared paper ; but whenever in its passage the tracer rests on a piece of the varnished writing, the current will be broken, and the spirals traced by the steel wire will be interrupted ; which interruptions will evidently coincide with the writing which has passed under the tracer ; so that ou taking the paper off the roller, there will be a Jac-simile of the original writing in clear spaces left free from the parallel lines traced close together over every other part of the paper. This facsimile will appear liko
fig. 7.
In January, 1840, and in July, 1841, Professor Wheatstone obtained two important patents, the first of which was in connection with Mr. Cooke. The electromagnetic telegraph may bo regarded as a variation of the alarum ; but instead of the detent, there are two light spring pieces, shown in fig. 8, one with a hook-shaped end, and the other an obtuse wedge or pushing piece (known as a clawler and driver), and instead of the catch-wheel of the alarum, there is a light escape line, prosaea lightly on the paper on this metal roller, so that any current sent through the wire shall, in its way to the earth, pane through the meta wheeL Under this arrangement each movement of the armature towards the magnet will pull the wheel round one tooth, and the motion of the armature from the magnet by means of a light spring, will push the wheel round another tooth. The axis of the wheel carries a light index hand, which points as the wheel turns round to tho various letters arranged near the edge of a diso. The currents which work this telegraph are sent by means of a communecutor, fig. 0, consisting of a brass disc turning freely on its centre, and marked with letters, as in the disc, fig. 8. Opposite each alternate letter, the brass is cut away, and a small notch of ivory inserted, so that on turning round the disc a metal spring, which bears against its edge, rests alternately on the conducting brass and the non conducting ivory, whereby the current is alternately transmitted and interrupted. A radiating arm proceeds from each letter for the con veuience of turning the dim. If the disc stand so that its stop-mark + be opposite toe fixed mark, the spring is resting on an ivory piece, and if at the same moment the index of the distant telegraph in connection with it be also pointing to the +, then on moving the communicator one step forward, so that the letter A comes to the fixed mark, the current will be transmitted along the wire, the electro-magnet, fig. 8, will draw down its armature so as to pull the wheel one tooth forward, and make its index also point to the letter A. On again moving the communicator so that the index points to e, the current is interrupthl, the armature of the distant telegraph falls back, pushing the wheel round another tooth, and bringing its index also to n. In this way the index may be moved step by step to different letters so as to spell out a word or message, which word or message will be repeated on the disc of the distant telegraph. A alight pause is made after each letter of the message. to insure certainty ; numerous modifications have been made in this machine, for increasing the rapidity of its action. An addition of printing mechanism to this instrument forma the type printing telegraph. In this the escapement axis, instead of carrying a fixed dine, has light radiating arms of steel, at the ends of which are printing-types a, figs. 10, H. In front of the types is a small roller or plate b, for receiving the blow of a printing-hammer c, which stands behind the type disc. The paper winch receives the impression is placed between the roller and the type disc, which paper is coiled on the roller d, and is drawn off by passing between two other rollers e, which hold the paper tightly and turn a little way round after each blow of the hammer.