In these two instruments no use was made of the power which exists of determining the deflection of the needle to either side, by merely reversing the connections of the battery.
We have thus traced the history of the telegraph up to the point at which it first assumed the practical form in Cooke and Wheatstone's inventions ; but what had been accomplished remained either unknown or was known only to a few leading men of science, until the unex pected development of the electric telegraph in the hands of those gentlemen led each one who was in possession of any title to the merit of having believed in, and experimented upon, its possibility, to produce his title, or to have it eagerly put forward by his friends and fellow-countrymen.
In June, IS37, the experiments of Messrs. Cooke and Wheatstone, which had been progressing for more than a twelvemonth, appeared so far successful as to induce them to apply for a patent for their inven tions. The principal points of novelty in this patent were the use of a much smaller number of needles to denote all the required signals, the employment of the temporary magnetism excited by the current in soft iron, to ring an alarum ; and the reciprocal arrangement by which the invention was rendered practically applicable to a long line of com munication. In explaining the invention, Mr. Cooke (` Telegraphic Railways') says :—" If a magnetic needle were placed parallel and near to any part of a conducting wire, which we will suppose to be laid down between London and Blackwall, the transmission of an electric current from a voltaic battery would cause the needle to change its position, ao as to stand during the continuance of the current at right angles to the wire, being turned in one direction or the other according to the course of the current. If this deflexion of the needle were limited by two fixed stops placed respectively at the two sides of one of its poles, the motion of that pole to one stop might evidently consti tute one signal, and its motion to the other stop another signal." Such an apparatus is shown in fig. 1, the dial upon which the signals
are represented being removed. In this cut a may be supposed to represent the battery, and I, Z, the conducting wire, which is formed behind the dial into a coil c: d d is the front or index needle, mounted upon an axis passing through the coil, another needle on the same axis being within the e.oil. The front needle carries upon its extremity, which comes through the dial, an index or pointer e. The arrows indicate the direction of the current required to deflect the magnet to the position indicated in the figure ; and a current in the opposite direction would produce a deflexion towards the opposite side. While no current passes through the wire, the maguet and pointer remain vertical. The next cut (fig. 2) represents three such instruments complete, and connected together by wires enclosed in tubes, which may be of any required length. One of these may be supposed to be at the 31inories, the next at an intermediate station, and the third at Blackwall ; and as each is provided with a battery, and a handle (beneath the dial) by which the conducting wire may be connected with it at pleasure, the attendant at every station at which such an instrument is placed can instantaneously communicate the signal to "stop" or to "go on" to all the other stations; attention being previously engaged by ringing a bell, placed above the dial. Fig. 3 represents a dial, in which, by the combination of four such magnets and pointers, all the lettere of the alphabet may be expressed by pointing one or two needles towards them ; and of course a larger or smaller ntunber of signals might be made on the same principle if necessary. A telegraph with two pointers, showing eight signals, is considered by Mr. Cooke to be sufficient for all ordinary purposes. The wires, where several arc used, are covered with some insulating material (such as a mixture of cotton and caoutehouc), and combined into a rope and enclosed in an iron tube, which may be either buried beneath the surface of the earth or supported above it.