77 



TELEGRAPH, ELECTRIC. 



TELEGRAPH, ELECTRIC. 



78 



duced ; but if the action were continued for a longer time, a lengthene 

 depression was the result, as the paper was drawn on. The combina 

 tions of these two kinds of marks denoted the various letters an 

 figures. Thus, dot and dash . ia taken to signify a ; . . . t 

 signify 6 ; . . for c ; . . d ; . for ; and so on. This is calle 

 the dot and dash code. fig. 5 shows the arrangement of what i> 



Fig. 5. 



called the eymbol-printiHg telegraph : a is the armature attached t< 

 the lever which carries the steel point. The ribbon of paper is drawn 

 slowly off from its roller r, by a train of clockwork in the box c, wbicl 

 drives two gripping rollers y. In its passage the paper goes over a 

 brass roller b, containing the groove. If while the paper is thus bein| 

 drawn over the roller 6, the clerk at the communicating station were b' 

 means of a common telegraph key or handle to transmit the current fo' 

 an instant, the armature would be momentarily drawn down, and thi 

 steel point would mark the paper with a depressed dot. The spring 

 would raise the arm the moment the magnet had ceased to act. Bu. 

 if, instead of transmitting only a momentary current the communica 

 ting clerk were to continue the transmission for a short time, the arm 

 would be pulled down during that time, and the point would indent a 

 line in the paper. 



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 chtmically marking 

 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 tyttfm) marks chemically prepared paper, which is moved forward 

 by clockwork released by an electric magnet. Two line wires are use<J 

 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 clock-work 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 muriate of lime," is preferably used to receive the marks or 



We may here state in more precise terms the principle of the cftemical 

 printing telegraph. A train of clockwork is used to keep a ribbon 

 of P*P er constantly unwinding from a drum, and passing over a metal 

 roller b, fij. 6. A small steel wire c, connected with the wire from the 



linp, promts lightly on the paper on this metal roller, so that any current 

 lent through the wire shall, in its way to the earth, pass through 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 ifc 

 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 6, and with the same rate of 

 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 first roller i. 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 b 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 011 taking 

 the paper off the roller, there will be a f.ic-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 fac-simile will appear like 

 f'J. 1. 



Fig. 7. 



In January, 1840, and in July, 1841, Professor Wheatstone obtained 

 iwo important patents, the first of which was in connection with 

 Hr. Cooke. The dtftro-maynetic telegraph may be regarded as a 

 variation of the alarum ; but instead of the detent, there are two light 

 spring pieces, shown in Jig. 8, one with a hook-shaped end, and the other 

 an obtuse wedge or pushing piece (known as a dawler and driver), 

 and instead of the catch-wheel of the alarum, there is a light escape- 

 Fig. 0. 



Fig. 8. 



nent wheel. Under this arrangement each movement of the armature 

 awards the magnet will pull the wheel round one tooth, and the 

 motion of the armature from the mignet by means of a light spring, 

 will push the wheel round another tooth. The axis of the wheel 

 irries a light index hand, which points as the wheel turns rouurl to 

 >e various letters arranged near the edge of a disc. The currents 

 hich work this telegraph are sent by means of a communicator, 

 j. B, consisting of a brass disc turning freely on its centre, and 

 narked with letters, as in the disc, fig. 8. Opposite each alternate 

 itter, the brass is cut away, and a small notch of ivory inserted, so 

 lat on turning round the disc a metal spring, which bears against 

 edge, rests alternately on the conducting brass and the non- 



