=109. Telegraph Keys= are merely instruments by which the circuit can be conveniently and rapidly opened or closed at the will of the operator. An ordinary push-button may be used to turn the current off and on, but it is not so convenient as a key. Fig. 106 shows a side view of a simple key which can be put anywhere in the circuit, one end of the cut wire being attached to X and the other to Y. By moving the lever C up and down according to a previously arranged set of signals, a current will be allowed to pass to a distant station. As X and Y are insulated from each other, the current can pass only when C presses against Y. Fig. 107 shows a regular key, with switch, which is used to allow the current to pass through the instrument when receiving a message. [Illustration: Fig. 107.] =110. Telegraph Sounders= receive the current from some distant station, and with its electromagnet produce sounds that can be translated into messages. [Illustration: Fig. 108.] Fig. 108 shows simply an electromagnet H, the coil being connected in series with a key K and a cell D C. The key and D C are shown by a top view. The lever of K does not touch the other metal strap until it is pressed down. A little above the core of H is held a strip of iron, on armature I. As soon as the circuit is closed at K, the current rushes through the circuit, and the core attracts I making a distinct _click_. As soon as K is raised, I springs away from the core, if it has been properly held. In regular instruments a click is also made when the armature springs back again. The time between the two clicks can be short or long, to represent _dots_ or _dashes_, which, together with _spaces_, represent letters. (For Telegraph Alphabet and complete directions for home-made keys, sounders, etc., see "Apparatus Book," Chapter XIV.) [Illustration: Fig. 109.] [Illustration: Fig. 110.] Fig. 109 shows a form of home-made sounder. Fig. 110 shows one form of telegraph sounder. Over the poles of the horseshoe electromagnet is an armature fixed to a metal bar that can rock up and down. The instant the current passes through the coils the armature comes down until a stop-screw strikes firmly upon the metal frame, making the down click. As soon as the distant key is raised, the armature is firmly pulled back and another click is made. The two clicks differ in sound, and can be readily recognized by the operator. =111. Connections for Simple Line.= Fig. 111 shows complete connections for a home-made telegraph line. The capital letters are used for the right side, R, and small letters for the left side, L. Gravity cells, B and b, are used. The _sounders_, S and s, and the _keys_, K and k, are shown by a top view. The broad black lines of S and s represent the armatures which are directly over the electromagnets. The keys have switches, E and e.