In April, 1895, a communication was made to the Russian Physical Society by Prof. A. Popoff, of the Torpedo School, Cronstadt, Russia, and appears in the _Journal_ of that Society for January, 1896. In this communication the use of an elevated wire and of a tapper-back worked through a relay by the coherer current are clearly described, and signalling was effected for a distance of 5 kilometres (3½ miles). An extract from this communication is given in _The Electrician_ for December, 1897, Vol. XL., page 235, and from it we reproduce Fig. 42, illustrating the tapping back arrangement. The following extracts from this paper may also be quoted:-- “On using a sensitive relay in the circuit with the coherer tube, and an ordinary electric bell in the other circuit of the relay, for sound signals and as an automatic tapper for the coherer, I obtain an apparatus which exactly answers every electric wave by a short ring, and by rhythmical strokes if electric vibrations be excited continuously.” “On connecting an electromagnetic recorder in parallel with the bell, tracing a straight line along the paper band which is moved by a 12-hour clockwork cylinder, I obtain an instrument registering by a cross line on the moving band every electric wave that reaches the coherer from across the atmosphere. Such an apparatus was placed at the Meteorological Observatory at St. Petersburg in July, 1895, one of the electrodes of the coherer being connected by an insulated wire with an ordinary lightning conductor, the other electrode of the tube-coherer being connected with the ground.” [Illustration: FIG. 42 (Fig. 2 on p. 235 of _The Electrician_, Vol. XL.).--Method of automatic tapping back by relay current employed for telegraphy by Prof. Popoff in 1895.] Prof. Popoff then goes on to say that his apparatus works well as a lightning recorder, and that he hopes it can be used for signalling to great distances. He says:-- “I can detect waves at the distance of one kilometre if I employ as sender a Hertz vibrator with 30 centimetre spheres, and if I use the ordinary Siemens relay; but with a Bjerknes vibrator 90 centimetres diameter, and a more sensitive relay, I reach five kilometres of good working.” Thus it is plain that Prof. Popoff employed the elevated wire as receiver in 1895, but did not employ it as sender. In 1897 Prof. Slaby, of Berlin, published (in German) a book called “Spark Telegraphy,” in which he described his success in signalling from 3 to 13 miles across land. From this book we take the following illustrations of the coherer and its connections:-- Fig. 43 shows the coherer tied on to a glass tube, by which it is supported. [Illustration: FIG. 43 (Fig. 7 of Slaby’s book).]