A "sounder" consists of two electromagnets mounted on a base under a movable flat piece of iron which is attracted by the magnetism of the electromagnets when a current flows through them and is withdrawn by a spring when no magnetism excites the windings. This piece of iron, which is called the armature, is mounted upon a strip of brass or aluminum called the lever. The lever strikes against a brass "anvil" and produces the "clicks," which form the dots and dashes of the telegraph alphabet. [Illustration: Fig. 131.—A Typical Telegraph Sounder, showing the Various Parts.] Every time that the key is pressed, an electric current is sent out into the line. The current flows through the magnets of the sounder and causes the armature to be drawn downward. The lever strikes the anvil and produces a "click." When the key lever is released, the current is shut up and the lever flies up and clicks against the top of the anvil. The period of time between the first click and the second click may be varied at will according to the length of time that the key is held down. A short period is called a _dot_ and a long period a _dash_. Combinations of dots, dashes, and spaces arranged according to the Morse Alphabet, make intelligible signals. How To Make a Simple Key and Sounder The little telegraph instruments shown in Figures 132 and 133 are not practical for long lines but may be used for ticking messages from one room to another, and can be made the source of much instruction and pleasure. [Illustration: Fig. 132.—A Simple Home-made Telegraph Key.] The key is a strip of brass fastened to a wooden base in the manner shown in Figure 132. It is fitted with a knob of some sort on the front end, so that it is conveniently gripped with the fingers. The little bridge is made from heavy sheet-brass and prevents the lever from moving too far away from the contact on the upward stroke. Connections are made to the key lever at the back end and the contact in front by the binding-posts, _A_ and _B_. The post, _C_, connects with the bridge. The sounder consists of two small electromagnets mounted in a vertical position on a wooden base. The magnets are connected at the bottom by a strip of heavy sheet-iron which acts as a yoke. [Illustration: Fig. 133.—A Simple Home-made Telegraph Sounder.] The armature is made out of sheet-iron, rolled up in the manner shown in the illustration. One end of the armature is fastened to a wooden block in such a position that the armature comes directly over the magnets and about one-eighth of an inch above them. The opposite end of the armature moves up and down for about an eighth of an inch between two screws, each fastened in a wooden block mounted on an upright board in the back of the magnets. The purpose of the screws is to make the "click" of the sounder louder and clearer than it would be if the armature only struck the wood.