Fig. 75 shows the simplest form of coherer. The posts (A) are firmly affixed to the base (B), each post having an adjusting screw (C) in its upper end, and these screw downwardly against and serve to bind a pair of horizontal rods (D), the inner ends of which closely approach each other. These may be adjusted so as to be as near together or as far apart as desired. E is a glass tube in which the ends of the rods (D) rest, and between the separated ends of the rods (D) the iron filings (F) are placed.
The Decoherers.—For the purpose of causing the metal filings to fall apart, or decohere, the tube is tapped lightly, and this is done by a little object like the clapper of an electric bell.
In practice, the coils and the parts directly connected with it are put together on one base.
The Sending Apparatus.—Fig. [76] shows a section of a coil with its connection in the sending station. The spark gap rods (A) may be swung so as to bring them closer together or farther apart, but they must not at any time contact with each other.
The induction coil has one terminal of the primary coil connected up by a wire (B) with one post of a telegraph key, and the other post of[p. 107] the key has a wire connection (C), with one side of a storage battery. The other side of the battery has a wire (D) running to the other terminal of the primary.
The secondary coil has one of its terminals connected with a binding post (E). This binding post has an adjustable rod with a knob (F) on its end, and the other binding post (G), which is connected up with the other terminal of the[p. 108] secondary coil, carries a similar adjusting rod with a knob (H).
From the post (E) is a wire (I), which extends upwardly, and is called the aerial wire, or wire for the antennæ, and this wire also connects with one side of the condenser by a conductor (J). The ground wire (K) connects with the other binding post (G), and a branch wire (L) also connects the ground wire (K) with one end of the condenser.