In Messrs. Cooke and Wheatstone's firs apparatus there were five needles, arrange( with their axes in a horizontal line. Th -needles when at rest bung vertically, 1); reason of a slight preponderance given to thei lower ends. Each electromagnetic coil we connected with one of the long conductin wires at one end, and was united at the othei with a common rod of metal, which joinec together the similar ends of all the coils The current was transmitted from the oppo. alto end of the wires (where an appropriate set of five pairs of finger-keys, for making the connections with the battery, was placed) through two of the wires at once. In accor dance with the keys which were pressed down, the needles assumed various positions with respect to each other ; and these were made to indicate signals according to the entries in a signal book. The instruments at the two stations were always rendered reciprocating ; that is, at each end of the line were placed an instrument, a set of finger keys, and a voltaic battery, so that either station could transmit or receive a signal. BY a beautiful arrange ment, a bell or alarum could be rung, when the attention of the clerk at the diStant terminus was required. In 1838 Mr, Cooke obtained a patent for some further improve Ments of this apparatus.
Dr. Steinheil cohstructed an electric telegraph between Munich and Bogenhausen in 1837. In his telegraph he availed himself of the conducting power of the earth, whereby ho was enabled to reduce the cost of erection. The earth in fact occupied the place of the return wire. All that is necessary to enable this to be effected, is that the wire which connects the two ends of the metallic conductor with the earth, shall be carried to a sufficient depth below the surface to be always in contact with moist earth or with water ; and that it shall be at this point attached to a plate or piece of metal, of about two or three feet superficial. The electric telegraph invented by Professor Morse, of America, in 1837, was essentially a registering instrument, the various signals being traced on a strip of paper. An electro magnet was so placed as to be within attracting distance of an armature fixed to the shorter arm of a lever, of which the longer end carried a pencil projecting sideways from it and pressed lightly against a sheet of paper. This paper was made to travel slowly beneath the pencil. So long as no attractive power was exerted by the electromagnet, the pencil would continue to trace a straight live as the it paper moved onwards ; but on momentarily ) making the circuit with the battery, the arma ture was drawn to the electromagnet, and the 1, pencil, carried by the arm of the lover upwards, 1 made an angular mark, like the letter V re e versed, on the paper'. These angles might y either be joined in groups, by rapidly sue r ceeding completions of the circuit, or they 3 might be separated by longer or shorter spaces ; of straight line. In the telegraph constructed
r by Morse in 1814, between Baltimore and 1 Washington, a different mode of recording the . signals was adopted. The use of the pencil - was found objectionable, from its so frequently ; requiring fresh pointing, and from the risk of breakage. The same arrangements were retained in regard to the paper, but it was made in its course to pass under a roller , having a groove around it. The long arm of the lever carried a blunt steel point, standing out from its upper surface, vertically under the groove in the roller. • When therefore the arm of the lever was elevated, by the attraction of the magnet upon the armature, the steel point pressed the paper into the groove and produced an indentation. If the attraction were momentary, a depressed point was produced ; but if the action were continued for a longer time, a lengthened depression was the result, tis the paper was drawn on. The combinations of these two kinds of marks, denoted the various letters and figures.
In the year 1837 Mr. Davy of London obtained a patent for an electric telegraph, the chief peculiarity of which consisted in the method of registering or recording the various communications, by causing the current of a supplementary battery to pass through a riband steeped in a solution of iodide of potas sium and starch. The salt being decomposed by the current, a blue spot was produced by the combination of the iodide with the starch, and the position of one or more of these spots across the breadth of the riband determined the nature of the signal transmitted.
In 1840 Professor Wheatstone patented his electromagnetic telegraph, in which the indi cating power was the magnetisation of soft iron by the electric current, insteacl of the employment of a real or permanent magnet. One part of the apparatus is the Communi a wheel or disc, on the edge of which are alternately arranged pieces of iron and of ivory, the one a conductor and the other a non-conductor of electricity. A metal spring presses on this edge while the wheel revolves, and is connected also with the galvanic appa ratus, in such a way that when the spring is in contact with the iron, a galvauie circuit is completed, but when in contact with the ivory the circuit becomes broken. Thus alterna tions of action are produced, which by ingenious mechanism are made the means of transmitting impulses or symbols through the telegraphic wire. This telegraph requires only a single wire for its use, the return of the current being provided by the earth. One application to which Professor Wheatstone found his invention suitable was that of an Electric Clock, by adapting his wheel and galvanic apparatus to the action of the escape ment wheel of a common clock.