The instrument which was brought into use on the Great Western Railway shortly after the date of the patent, contained five. needles, arranged with their axes in a horizontal line. The needles when at rest hung vertically, by reason of a slight preponderance given to their lower ends. Each coil was-connected with one of the long conducting wires at one end, and was united at the other with a rod of metal, which joined together the similar ends of all the coils. Tho current was transmitted from the opposite end of the wires (where a set of ,five pairs of finger-keys, for making the connections with the battery, was placed) through two of the wires at once. That is to say, one of the wires, of which one key was pressed down, served to convey the current from one pole of the battery to the distant instrument, while the key of a second wire being brought into contact with the other pole, the current returned by the rod of metal con necting the coils and the second wire to the battery again. Two needles were in this manner deflected at once, and it will be obvious that the current would pass in opposite directions around their coils, and consequently that the deflections must be in contrary directions. The needles would therefore converge, either above or below their line of centres, as one or other of the pair of keys belonging to each wire was depressed. Fixed stops were so placed on each side of the needles as to limit their motion, and when resting against them, the needles were parallel to two converging lines, at the point of interseetiou of which a letter was placed. (Fig..3.) This was the signal indicated by the movement of the needles. In a similar manner, as lines were drawn diverging from the centre of each axle, mutually crossing one another, a number of points of intersection were formed, at each of which was a letter or signal. Any of these letters could be indicated by the simultaneous movement of two needles, so that a communication could be carried on with certainty and tolerable rapidity. At the same time a plan was :recognised, by which the number of wires requisite for maintaining a communication might be reduced, by using one of them at times as a return wire only, there being no needle in connection with this one. One needle could by the use of this wire be deflected by itself either to the right or left, and thus of course each would furnish two signals, in addition to those formed by its simultaneous deflection with any other. The instruments at the two stations were always rendered reciprocating; that is, at each end of the line were placed en instrument, a set of finger-keys, and a voltaic battery, so that either station could transmit or receive a signal. By an ingenious arrangement, the keys, on being released after depression, were made to resume by themselves the position necessary to enable that which had been the signalling station to become the recipient. By this means rnessaages and answers, or words and their acknowledgments, could follow one another without the necessity for any intervening adjustment of the instruments.
The bell or alarum which was to be rung, when the attention of the a detent or catch from a train of clock-work driven by a spring or weight This clock-work, by the intervention of a acape-wheel and pallets, rang the bell as in a common alarum.
In April, 1S3S, Mr. Cooke obtained a patent for some further im provements in this apparatus. Of these the most prominent was the mode of introduction of the intermediate apparatus. Before the date of these patents, the two stations at the extremities of a line of tele graph had alone been put in communication with each other ; but means were now devised by which any number of intermediate in struments might be introduced between the two terminals, and any intelligences rendered simultaneously visible iu all or in any of them, as required. Furthermore any one of these instruments could be put in communication with the rest, either generally or in part only; and the same mechanical adjustment which limited the connection of any intermediate instrument to one part of the brie, placed its bell in the circuit of the other part. Thus if, while intelligence was being trans mitted in one direction from an intermediate station, some message of importance were required to be sent from the terminus, or any other station on the excluded side, the ringing of the bell at the communi cating station would warn the attendant to restore his instrument to its intermediate position, and thus leave the line clear throughout.
in the same patent were included some improvements on the mode of protecting and insulating the mires, which were to be laid beneath the earth, in tubes or troughs of wood, iron, earthenware, or other material ; and also in the expedients for detecting the exact position of any accident or derangement, without the necessity of uncovering the whole length. Two needles were also shown to he sufficient for carrying on a complete communication with ease and rapidity.
In the course of the ensuing year (1839) Messrs. Cooke and Wheat stone's telegraph was brought into actual operation upon the Great Western Railway, where its capabilities were tested. The results of this trial demonstrated that the undertaking was thus far successful, and that the question of the practicability of the electric telegraph, so long at issue among scientific men, was set at rest.
We must here go back a little, to notice Dr. Steinheil's telegraph, which was erected between Munich and Bogenhausen in 1837. In this instrument, two needles or magnetic bars were placed within an elongated coil of fine wire. These bars were suspended on axes passing transversely across the coil, and in their quiescent position lay parallel to one another and to the sides of the coil. They had their poles placed the same way, so that when a current was transmitted along the wire, they had a tendency to move in the same direction, remain ing still parallel to each other. Against the outer end of each needle or bar a atop was placed, which checked its motion on one side, but left it free to turn to the other. The opposite poles of the two bars were therefore prevented from moving out from the coil, under the influence of the deflecting current ; and the effect of this arrangement Was, that the two bars could not move simultaneously, but only alter nately. Roth were acted upon alike, but when the inner end of one was free to move outward, the other bar remained pressed against its atop, and was fixed ; and on reversing the current, the effects upon the two needle* were also reversed; that which was before stationary, now moved forward, while the other was fixed. lu order to bring hack the needles to their ordinary ponition, a permanent magnet was fixed near to each at, the back of the coil, by tho attraction of which the needles were again rendered parallel after the cessation of the deflecting power of the current. The inner ends of these bars canied each a light braes arm, terminating in a cup furnished with a small per forated beak or spout.. These cups were filled with printing-ink, which oozed through the beak, and formed a minute bead or drop at its point, which, from its viscid consistence. did not drop of These beaks were arranged so u to be in the same horizontal line, and at a distance from each other a little less than the width of a strip of paper, which was placed before them. if then a galvanic current were passed through the coil, so that the right-hand needle tended to pees, with the end bearing the cup, out from the coil towards the paper (the cup on the other needle receding as far as the stop would allow, and then re maining fixed), the little beak would just touch the paper, and leave a minute dot of Ink on its surface. By reversing the current the other needle would approach and leave a point of ink on the opposite edge of the strip of paper. By the varied number and arrangement of these dots, on one or both edge. of the paper, the various letters of any communication were denoted. The paper used in this apparatus, being obtained in a long strip or ribbon. and coiled upon a roller, was made to pass slowly lengthwise before the printing points by a simple application of a weight, and cord, which as the printing was effected gradually wound the ribbon upon a second drum or reel. This instru ment was also adapted to give audible signals, by the substitution of small knobs for the ink-cups, and of two bells of different tones fur the ribbon of paper. One bell being so placed as to be sounded by the first needle, and the other by the second, the pre-concerted combina tions of their sounds might indicate various letters to a listener.