The cylinder of the instrument is shown at a; b is the metal style connected by the wire g with one of the poles of the voltaic battery; o is the arm which holds the style and serves to insulate it from the rest of the apparatus; c is a fine screw on which that arm traverses as the cylinder revolves; d d are cog-wheels to turn the screw. The speed of the instrument is regulated by the fan e; f is the impelling weight, and h the wire connected with the distant instrument. The receiving and the transmitting instruments are alike, the only difference between them being that the style of the copying instrument is steel instead of brass wire.
As the cylinder a is connected by the wire h with the distant instrument, and through it with one of the poles of the voltaic battery, the electric circuit is completed by passing from g through the tin foil message, or through the paper placed on the cylinder. This will be the case whenever the style of the transmitting instrument is pressing on the metallic writing; and at those times the electro-chemical action of the voltaic current will produce a blue mark on the paper of the receiving instrument, by the deposition of iron and its combination with the prussiate of potass. The circuit will in like manner be interrupted whenever the point b presses on those parts of the message where the varnish is not removed; and thus, as the two cylinders revolve, there will be a succession of small blue marks on the parts where the writing allows the electric current to pass. As the arms that carry the points traverse on screws, they are drawn along as the cylinders rotate, so as to press on fresh parts of the message and of the paper at each revolution. The steel point would therefore draw a series of spiral lines on the paper, if the electric current were not interrupted; but the interposition of the varnish breaks those lines, and as the point passes over different portions of the letters at each revolution of the cylinder, the marks and the interruptions on the paper correspond exactly with the forms of the letters, and thus produce a copy of the writing placed upon the receiving cylinder, in blue characters on a yellowish ground. Or the message may be written on unprepared tin foil with a pen dipped in varnish; in which case the writing will be copied in white characters on a ground of dark lines, as in the accompanying specimen, A being the writing on tin foil, and B the message received.
It is essential to the perfect working of the copying telegraph that the corresponding instruments should rotate exactly together. This is effected by an electromagnetic regulator, which being put in action by one instrument, governs the movements of the distant instrument with the greatest exactness, as proved at a distance of 300 miles.
It might be supposed, as the points must traverse several times over the same line of writing to copy it, that the process is a slow one; but in consequence of the rapidity with which the cylinders revolve, this is not the case. The ordinary speed is one rotation in two seconds, and at that rate three lines of writing, containing sixty words, would be copied in one minute, which is three times as fast as an expeditious penman can write.
The advantages proposed to be gained by the copying telegraph, in addition to its increased rapidity of transmission, are the authentication of telegraphic correspondence by the signatures of the writers, freedom from the errors of transmission, and the maintenance of secrecy. As a special means of obtaining secrecy, the messages may be received on paper moistened with a solution of nitrate of soda alone, in which case they would be invisible until brushed over with a solution of prussiate of potass, to be applied by the person to whom the communication is addressed.
Professor Wheatstone has recently contrived an improvement in his index telegraph, which was described by Professor Faraday in a lecture at the Royal Institution in June last. Its chief merit, however, consists in the beauty of the mechanism, for it is essentially the same as the index telegraphs he and others have previously invented, with the substitution of magneto-electricity for the moving force.
Having now traced the history of the invention of the instruments by means of which messages may be transmitted, it becomes necessary to describe the methods employed for making the electrical connection from one place to another. This part of the electric telegraph system is, after all, the most essential to its efficient working, and bears the same relation to the transmitting instruments that the structure of a railroad does to locomotive engines in the system of railway conveyance.