File:EB1911 Telegraph - Cable Working.jpg

English: The arrangement of the apparatus for working some of the most recent submarine telegraphic cables is shown. The cable is supposed to be worked duplex; but, if S, C₁, C₂, and AC are removed and the key connected directly with C₃, the arrangement for simplex working is obtained. The apparatus consists of a sending battery B, a reversing transmitting key K, a slide of small resistance, three condensers C₁, C₂, C₃, an artificial cable AC, the receiving instruments I and G, and one or more resistances R for adjusting the leakage current. The peculiar construction of AC has been already referred to. The conductor of the cable is practically insulated, as the condensers in the bridge have a very high resistance; hence no appreciable current ever flows into or out of the line. Two receiving instruments, a siphon recorder and a mirror galvanometer, are shown; one only is absolutely necessary, but it is convenient to have the galvanometer ready, so that in case of accident to the recorder it may be at once switched into circuit by the switch s. When one of the levers of K is depressed, the condenser C₁ and the cable, and the condenser C₂ and the artificial cable, are simultaneously charged in series; but, if the capacity of C₁ bears the same proportion to the capacity of the cable as the capacity of C2 bears to the capacity of the artificial cable, and if the other adjustments are properly made, no charge will be communicated to C₃. After a very short interval of time, the length of which depends on the inductive retardation of the cable, the condensers corresponding to C₁ and C₃ at the other end begin to be charged from the cable, and since the charge of C₃ passes through the receiving instrument I or G the signal is recorded. The charging of C₃ at the receiving end will take place, no matter what is the absolute potential of the condensers, consequently the incoming signals are not affected by those which are being transmitted from that end. In actual practice the receiving instrument is so sensitive that the difference of potential between the two coatings of the condenser C₃ produced by the incoming signal is only a very small fraction of the potential of the battery B. When the key is released the condensers and cables at once begin to return to zero potential, and if the key is depressed and released several times in rapid succession the cable is divided into sections of varying potential, which travel rapidly towards the receiving end, and indicate their arrival there by producing corresponding fluctuations in the charge of the condenser C₃. All cables of any great length are worked by reverse currents. A modification (known as the cable code) of the ordinary single needle alphabet is used; that is to say, currents in one direction indicate dots and in the other direction dashes.