The frame, fi, q, carries a shaft provided with two screw-threads, 17, each being clasped by a nut to which is attached a clamp, a. At each complete revolution of the shaft the clamps move laterally the distance of one turn of the screw-thread. The revolution of the screw is so effected by an escape-wheel that the clamp, a, is moved laterally one-eighth of a. millimetre at each oscillation of the lever, A, B. If a style be fixed in the clamp, a, so as to press by its elasticity against the surface of a sheet of paper laid upon the tablet X, and to leave a mark upon it, it is evident that by the oscillation of the frame, fi, q, and the lateral motion at right angles caused by the screw, vz,', the whole surface of the sheet would be covered with parallel-ruled lines one-eig-lith of a millimetre apart. The apparatus, however, is so arrang,,ed that the style is lifted from the paper while passing; in one direction over the tablet X, and in like manner upon the other tablet while passing in the opposite direction. The synchronism of the corresponding instruments is effected by a pendulum, a description of whose action will be unnecessaiy.
Ofieralion of Oise/11's Afifiorahrs.—The manner in which messages are transmitted and received by Caselli's apparatus will now be understood from the following-: At the sending station the message or any drawing or diagram to be transmitted is written in bold lines with ordinary ink on "silver paper," so called, which is prepared expressly for the purpose by coating the surface with tin. This is fastened by clips upon the surface of the tablet X, and through the clips the metallic surface is in connection with the negative pole of a main battery and with the earth, while the positive pole of the battery is attached to the clamp, a, and to the line. When the pendulum at the sending station is set in motion, the clamp and the style attached thereto begin to traverse the entire surface of the metal lic paper. As long as the style is in contact with the metallic surface the main battery current is shunted through a short circuit; but whenever this contact is broken by the style passing- over the insulating writing-, its current passes through the line and through the clamp, a, at the receiving station, and filially to the earth. The clamp holds an iron-wire style which traverses a sheet of paper laid on the tablet X, the paper being prepared by treating it with a solution of yellow prussiate of potash acidified by sul phuric acid.
As the pendulums at both receiving and sending- stations swing in exact time with each other, it is manifest that when the style at the sending sta tion passes over a line of the non-conducting writing a corresponding blue line will be drawn upon the chemical paper at the receiving station. After
half an oscillation of the pendulum has been completed, the styles are moved laterally one-eighth of a millimetre, but, as the clamp and the style are lifted from the paper during the last half-oscillation of the pendulum, this portion of time occupied in its motion is lost, but may nevertheless be ntilized for sending another message, in the opposite direction, by means of the duplicate portion of the apparatus:. After a complete oscillation, the styles at both stations will have moved laterally one-eiglith of a milli, metre. During the first half of the second oscillation there will appear upon the paper at the receiving station a second series of blue lines exactly parallel to the first and one-eighth of a millimetre distant from them. In this way the two instruments continue to operate until the style at the sending station has g-one over the whole surface of the metallic paper, and that at the receiving- station has produced in parallel-ruled lines a copy of the de spatch written upon the metallic paper. The Caselli system has been put in practical use, but to only a small extent. The objection made to it and to others devised for the same purpose is that they are very slow in operation.
To avoid the necessity of providing intermediate stations with two sets of apparatus, one to communicate in each direction, and also to avoid re peating at each of the intermediate stations a message sent from one ter minal station to the other, which would involve serious loss of time and multiply the chances of error in transmission, various arrangements have been devised by which communication may be established between any two stations inclnded in the line. The pan-telegraph was opened for public use on the Paris-Lyons Railway in 1865.
Telegraph Lines: Oz,,erhcad Wires ana' Snpporls.—The metallic con ductors which serve for the transmission of the current consist usually of iron wire of suitable gauge, which it is the practice to galvanize to protect against rusting. At the terminal stations these wires are soldered to cop per plates having about io square feet of surface, which are sunk in the earth. For supporting- the line wires on overground lines, wooden posts are used, from 25 to 3o feet long, on which the wires are strung upon cross braces from insulators. These posts are occasionally treated with antiseptic substances, as described on page 1.93. Their distance apart is, as a rule, 175 feet, corresponding to about thirty posts to the mile. Sometimes posts of iron are substituted for those of wood.