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We have, in the next place, to explain how the signals sent out are recorded.
Fig. 83.—Marconi receiving arrangement for wireless telegraphy.
At the receiving station is erected a second insulated aerial, antenna, or long vertical rod, A ([see Fig. 83]), and the lower end is connected to the earth through a coil of fine insulated wire, P, which forms one circuit of an oscillation-transformer. The secondary circuit, S, of this oscillation-transformer, which is called a jigger, is cut in the middle and has a small condenser, C1, inserted, consisting of two sheets of tinfoil separated by waxed paper ([see Fig. 83]), and to the ends of this circuit is connected the coherer, or metallic filings tube, T, which acts as a sensitive receiver. The Marconi sensitive tube ([see Fig. 84]) is made as follows. A glass tube about ¹⁄₄ inch in diameter and 2 inches long has two silver plugs put in it, and these are soldered to two platinum wires which are sealed into the closed ends of the tube. The ends of the plugs are cut in a slanting fashion and made very smooth. These ends very nearly touch each other. A very small quantity of very fine metallic powder consisting of nineteen parts nickel and one part silver is then placed between the plugs. The quantity of this powder is scarcely more than could be taken up on the head of a large pin. The glass tube is then exhausted of its air and sealed. The tube is attached to a bone rod by means of which it is held in a clip.
Fig. 84.—Marconi coherer.
To the two sides of the above-mentioned condenser are connected two wires which lead to a circuit including a single voltaic cell, V, and a relay, E. The relay is connected to another circuit which includes a battery, B, and a piece of apparatus called a Morse printer, M, for marking dots and dashes on a strip of paper.