The signals sent over the wires are simply the audible sounds made by the armature, as it moves to and from the magnets. MECHANISM IN TELEGRAPH CIRCUITS.--A telegraph circuit requires three pieces of mechanism at each station, namely, a key used by the sender, a sounder for the receiver, and a battery. THE SENDING KEY.--The base of the sending instrument is six inches long, four inches wide, and three-quarters of an inch thick, made of wood, or any suitable non-conducting material. The key (A) is a piece of brass three-eighths by one-half inch in thickness and six inches long. Midway between its ends is a cross hole, to receive the pivot pin (B), which also passes through a pair of metal brackets (C, D), the bracket C having a screw to hold one of the line wires, and the other bracket having a metal switch (E) hinged thereto. This switch bar, like the brackets, is made of brass, one-half inch wide by one-sixteenth of an inch thick. Below the forward end of the key (A) is a cross bar of brass (F), screwed to the base by a screw at one end, to receive the other line wire. Directly below the key (A) is a screw (G), so that the key will strike it when moved downwardly. The other end of the bar (F) contacts with the forward end of the switch bar (E) when the latter is moved inwardly. [Illustration: _Fig. 67._ TELEGRAPH SENDING KEY] The forward end of the key (A) has a knob (H) for the fingers, and the rear end has an elastic (I) attached thereto which is secured to the end of the base, so that, normally, the rear end is held against the base and away from the screw head (G). The head (J) of a screw projects from the base at its rear end. Key A contacts with it. When the key A contacts with the screw heads G, J, a click is produced, one when the key is pressed down and the other when the key is released. You will notice that the two plates C, F are connected up in circuit with the battery, so that, as the switch E is thrown, so as to be out of contact, the circuit is open, and may be closed either by the key A or the switch E. The use of the switch will be illustrated in connection with the sounder. [Illustration: _Fig. 68._ TELEGRAPH SOUNDER] When the key A is depressed, the circuit of course goes through plate C, key A and plate F to the station signalled. THE SOUNDER.--The sounder is the instrument which carries the electro-magnet. In Fig. 68 this is shown in perspective. The base is six inches long and four inches wide, being made, preferably, of wood. Near the forward end is mounted a pair of electro-magnets (A, A), with their terminal wires connected up with plates B, B', to which the line wires are attached. Midway between the magnets and the rear end of the base is a pair of upwardly projecting brackets (C). Between these are pivoted a bar (D), the forward end of which rests between the magnets and carries, thereon, a cross bar (E) which is directly above the magnets, and serves as the armature.