Many of our readers will be interested in understanding how it is possible to transmit messages over the same wire in opposite directions at the same time. Th;; following from the London TelegrapJdc Journal will explain one of the ways in wliich it can be done : The transmission of messages over a single wire in opposite directions at the same instant, had occupied the attention of the scientific, both in Europe and America; and the problem has been solved, in as many different ways, by no less than five individuals. The following drawing illustrates the method devised by Dr. Gintl, of Germany, which seems to be very simple, and proves, upon trial, to work with entire success. The apparatus used is that of Professor Morse. The arrangement of the circuit is that teclmically known as the open circuit. Let mo premise that in transmitting a dispatch by this system, the electro-magnet of the transmitting station does not work—only that of the receiving station is operated by the current. When the key, or transmitter, is at rest, a spring closes the connecting point at the bade end, and when it Is pressed down by the operator in transmitting a message, the bn.clr cnrmef'tinn is lirokeTi nnfl the front one estn.hlishfri. I have represented a section of line between London and Livorpocd, A A', are two rheostats in the oflces of London and Liverpool, -which represent, each of ihem, the exact resistance of the line wire between these two points. B B' are electro-magnets of peculiar construction, being so arranged that a current may traverse either half or the whole ot the coils, or may traverse one coil in one direction, and the other coil in the opposite direction. C C are the batteries; E E', the keys; and P P' the groimd plates. Let us now suppose that London wishes to send to Liverpool. The operator at London presses down his key, and the current from the battery, C, passes through the key to the main wire, and thence down the branch wire, D', through the key, E', to magnet, B', thence through the ground plates, P' and P, to the magnet, B', and thus back to its starting point in the battery at C. When the current passes through the coil, B', at Liverpool, it operates the apparatus there in the usual masmer. But I have not described the entire course of the current. When it reached the junction, D, one half of it passed through the rheostat. A, througli the upper half of the magnet, B,and thence to its starting point at the battery. It will thus be seen that one half of the current having passed in one direction through one of the coils, B, and the other half iu the opposite dirostion through the other coil, B, C, that its effect is neutralized and that no action takes place in the magnet at the transmitting station. Now let us suppose that London and Liverpool both press their keys down at the same moment, each sending to the other. The current from the batteries, C and C, would meet at the junction D and D', and neutralize each other, and consequently, no current would pass over tiie wire. It would, in fact, be the same as if the wire were actually broken between these points during the time that both keys were pressed down. Under these circumstances the current from the battery, C, returns through the rheostat. A, through one half of the coil, B, and thence back to the battery, C. What takes place at London, of course occurs at Liverpool under the same conditions. Thus the writing upon the London and Liverpool instruments is actually performed by their own respective batteries, but as this record depends upon the closing of the key at the distant station, it amounts to the same as if done by the battery of the other. Having now shown how the record is made while the receiving station has his key in its ordinary position of rest, as well as where it is pressed down in the act of transmitting, let us now consider what will bo the course of the current when it is in neither of these positions—that is to say, when the back connection has been broken by pressing the lever to make a letter, but before the front contact has been established. We will consider that Liverpool's key is in this position, and that London is writing. In this case the current, on arriving at D', does not pass down the branch wire, as there is no outlet for it, but passes on through the rheostat. A', thence through -both coils, B', to the ground plate, P'. The current in this case passes not only along the line between London and Liverpool, but also encounters a resistance at A' of equal extent; but this is equalized bypassingthrough both coils of the electro-magnet, B, so that the adjustment of the instrument remains the same throughout. If this apparatus has not been generally used, it does not arise from its inutility. With a line well constructed and properly insulated, there would be no difiiculty in working it. It could not be relied upon where there is heavy escape, and to have entire success the resistance coils should exactly equal the resistance of the line wire, and the magnets be well constructed.
This article was originally published with the title "The Simultaneous Transmission of Messages over a Single Wire in Opposite Directions" in Scientific American 21, 4, 53 (July 1869)