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year, it will perhaps prove interesting to give a rapid historical outline of the attempts which have been made to apply the various indications of the electrical fluid as the medium of instantaneous communication between distant places. For several of the following references I am indebted to an article by Dr. Steinheils of Munich, translated in the May number of Sturgeon's Annals of Electricity.
1.- Telegraphs by common electricity. The first electrical telegraph on record was proposed by Winkler of Leipzic, in 1746. He employed a Leyden jar which was discharged through a single wire, a reach of the river Pleiss being included in the circuit. Le Monnier afterwards made a similar experiment in Paris, using a wire 12,789 feet long. In 1798, Betancourt laid a wire between Madrid and Aranjuez, 26 miles distant, to serve for the transmission of shocks by the Leyden phial. The pith ball electrometer was used by Lomand; and the sparks from tin-foil on glass surfaces by Reiser about the same period.
In 1826, Francis Ronalds, of Hammersmith, published a description of a plan in which two clocks were employed, one at each terminal station. Each clock had a moveable dial with twenty signals on its circumference. As the required signal letter presented itself, a spark passed at each station by the discharge of a Leyden phial. This plan, though comprising, as I will point out in the sequel, the true principle of a good system, was found useless in practice, as each sign was given but once in each revolution.
Such are the principal attempts hitherto made to effect the object in view, by means of frictional electricity. At the Meeting of the Asiatic Society of Bengal, of June 1839, M. Adolphe Bazin presented a project for effecting telegraphic correspondence by means of thirty insulated conductors passing between the terminal stations, each conductor representing a letter or number, so that by the rapid succession of sparks correspondence could be effectually carried on. With this M. Bazin connected an hydraulic apparatus for the conveyance of intelligence across rivers, and in other situations where frictional electricity might not be suitable.
M. Bazin's plans, although very ingenious, were altogether impracticable, and as we shall afterwards establish, demanded thirty conductors, where only one is actually requisite; moreover the impediments to the use of common electricity are absolutely insuperable in all countries (Bengal for example) visited by periodical rains or inundations.
M. Bazin indeed admitted this freely, when he found that not one of the electrical machines I placed at his disposal could by ordinary manipulation be made to evolve the least sign of excitement. But even effecting the excitement, which I have done by enclosing the machines within a glass case hermetically sealed, and supplied with air artificially dried, still it is impossible so to insulate the external conductors, as to prevent the dispersion of the excitement outside the apparatus.
$ 2.- Telegraphs by Chemical decomposition. In Steinheils' historical sketch we find that Soëmmering, in 1807, employed a voltaic battery provided with thirty-five conductors, each termi. nating in a gold pin set in a tube ; on completing the connexions the water is decomposed and the ascent of bubbles of gas indicates the signal. This system is, however, only available for very short distances, as the decomposing power of the termination of any pair of conductors, the diameter being the same, diminishes rapidly by lengthening the wire. The law of the diminution, Ritchie has attempted to establish, but his experiments are not considered to be conclusive; its rapidity may be shewn by an experiment I performed in 1839. A voltaic battery, the conductors of which were six feet long, decomposed water to the rate of forty cubic inches of oxygen and hydrogen gases in three minutes. Conductors of the same diameter, but thirty-six feet long were next employed; the battery then only evolved twenty-five cubic inches of the gases; with wires of 200 feet only eleven inches were obtained ; still the battery was constant in its action, for with the original conductors at the close of the experiments it still gave forty cubic inches. Again in the experiments at the Botanical Garden in 1839, no chemical decomposition—even of the most yielding of all compounds, the ioduret of potassium-could be performed at the termination of one and a half miles, whereas other manifestations of electrical action were readily procurable at the termination of twentyone miles of wires.
$ 3.- Telegraphs by volta-magnetic deflection. The next method employed is the deflection of the magnetic needle by voltaic or magnetic electricity. I may remind the general reader that whenever electrical vibrations occur in exceedingly rapid intervals in an insulated wire surrounding and in the same direction with a balanced magnetic needle, the needle is deflected, either east or west according to the order in which the ends of the surrounding coil are
connected with the source of electrical excitement. As I am now writing for popular readers I may be pardoned by the adept for illustrating this interesting fact by an explanatory diagram.
In this diagram, 1 represents the voltaic couple ; z zinc; and c copper; 2 shews the magnetic needle on its stand in the magnetic meridian, with the surrounding coil of wire, with its terminations a and b. In the first the wires cross, or that from z proceeds to b, that from c to a, and the deflection accordingly is from north to west. In the second the wire from z proceeds to a, that from c to b, and deflection of the needle is from north to east.
Thus with two wires we can obtain two signals only, but one wire may belong, or be common to any number of galvanometers, so that from three wires we can obtain four signals ; from four wires six signals; from five wires eight signals; from six wires ten signals ; eight wires fourteen signals; ten wires eighteen signals ; twelve wires twenty-two signals; fourteen wires twenty-six signals, or the alphabet.
In the following diagram six galvanometers are represented connected with seven wires, one being common to all. The six wires run any distance in a bundle, and are best insulated by silk or resin from each other. The ends of the wires then proceed to little cisterns of mercury, disposed in a circle. From the centre of the circle a moveable wire proceeds as a radius, which may be moved to any of the cisterns 1, 2, 3, 4, 5, 6. To this centre proceeds one of the poles (z) of the voltaic couple-and to the termination of the common wire, proceeds the second pole of the couple c.
In the diagram the connexion is made with No. 2, and the dotted line shews the deflection of the needle--and this deflection may be reversed by crossing the course of the battery wires, as shewn at R. The five parallel lines at D shew the conductors, which may be indefinitely prolonged.
Thus by a move of the radius wire to any of the cisterns we can deflect the needle at the corresponding galvanometer; and by a move of the cross wires we can reverse the deflection at our pleasure.
We have here a combination which affords sufficient numbers for spelling, numbering, dictionary and cypher signals. Even four galvanometers which can be worked by five wires, will afford the necessary combinations for every description of signals.*
This telegraph has been actually laid dow en London and Drayton, and is to be carried on to Bristol. Though extremely ingenious, I shall presently prove that the railway itself without any special conductors, or at most with one wire, is a perfect telegraphic line.
In Davy's telegraph the needles carry slight screens which conceal illuminated letters or numbers—on deflecting the needle the signal is disclosed.
Soon after the discovery of the deflection of the needle, several attempts were made to establish by its use, the laws of action of the battery. Ritchie attempted to prove that the deflection was in the direct ratio to the surface of zinc acted on in the battery. Thus supposing the conductors unchanged, and that by the corrosion of one superficial inch of zine a deflection, say of 5° be obtainable, the corro. sion of two superficial inches will give a deflection of 10°. Were this assertion supported, a single galvanometer would give us all the signals we could require. It is now however proved that the supposed law by no means holds good. It is quite true that we may double or treble a given deflection, or that we may by direct experiment proportion the voltaic force to the deflection required, but such experiments are only fit for performance in the closet or laboratory,-require such careful adjustment and observation-and are, moreover, so exceedingly delicate, and take so much time in recording, that they become quite unsuitable for the rapid transmission of telegraphic signals.
In the preceding arrangements in which several galvanometers were used, we have manifestly all that we require within the distances to which experiment has yet reached. But the expense of wire next presents itself as a motive for endeavouring to improve the system by diminishing the number of the wires. To render this intelligible, of the copper bell wire best suited for these experiments, each mile costs 276 rupees.
Steinheils of Munich, the most recent writer on this subject, proposes either of two very ingenious methods. The first is causing the galvanometrical needle to terminate in a fountain pen, the tip of which touches and marks a strip of paper revolving by clockwork ;-according to the number of dots a letter or numerical signal can be obtained. The second plan is the employment of the tip of the needle to strike a bell, when the number of strokes in a given time afford the requisite signal.
The galvanometer moreover has been rendered so exceedingly delicate in its indications, that very feeble electrical forces will succeed in producing deflections. The electricity evolved by holding up the hand before a disk composed of bismuth and antimony, caused in an instrument contrived by Dr. Page, of Baltimore, a deviation of fifty degrees. In a galvanometer in my possession, constructed by Messrs. Watkins and Hill, the action of a drop of acidulated water on a zinc wire the size of a pin, and opposed to a copper element of equal size, urges the needle through a quarter of a circle. Moreover the differeu.