During September, 1899, the Marconi method of signalling to long distances was demonstrated before the British Association at Dover. The chief feature of the installation was the elevated wire supported by a mast, and terminating at the top in a small conductor, which is usually made of wire netting, and is suspended from an insulating rod. The lower end of this elevated wire passed into the building through an aperture, and was connected to one terminal of the usual Ruhmkorff coil, the other terminal of which was earthed. The signalling key was of the simplest description, being nothing more than a well-insulated Morse key worked by hand and causing a make-and-break in the primary circuit of the coil. The ordinary trembling break of the induction coil was at work in the usual way, so that while the signalling key was depressed continuously there was a torrent of sparks between the knobs of the secondary. This method of signalling was identical with that employed by everyone since the time of Hertz, except that, instead of connecting the secondary terminals to two insulated plates, one was now connected to earth and the other to a small insulated conductor at considerable elevation.
Fig. 48
(Fig. 11 of Prof. Slaby’s book on “Spark Telegraphy”).
Fig. 49
(p. 762, The Electrician, Vol. XLIII.).—
Marconi Signalling Mast at Dover Town Hall.
Fig. 50
(Fig. 1, p. 7, The Electrician, Vol. XLIII.).—
Mast at South Foreland; from which Signals went to a similar Mast at Wimereux, near Boulogne.
From this mast in the town of Dover ([Fig. 49]) signals could be sent to another loftier mast at the South Foreland ([Fig. 50]), where it is itself elevated by chalk cliffs far above the sea. From this South Foreland station, which was similar in all essential respects to the Dover station, except that its elevation was greater, messages could be sent and received to and from a station near Boulogne, on the coast of France, and to and from the East Goodwins lightship. The signalling was slow, but appeared dependable, and the simplicity of all the arrangements was remarkable ([Fig. 51]). Concerning the receiving apparatus there is little to be said, since it is in essence the same as that which has already been described. It consists of a coherer of the plug tube pattern, something like that depicted on [page 23], but excessively reduced in size, the glass tube being the size of a quill, the two silver plugs close together separated only by a very few nickel filings. This tube is mounted so that it can be struck after each signal by a light electric hammer worked by a current from a local battery switched on by a Siemens’ polarised relay, which is itself actuated by the coherer current. Whenever the coherer receives a signal the same current that works the tapper works also the Morse instrument standing on the table alongside, and records a short or a long signal on the tape. The coherer with its tapper, the polarised relay, and the battery (a few dry cells) are all enclosed in one oblong iron box, through an aperture in which the lower end of the elevated wire can be inserted and brought into direct connection with the coherer.