The key is really a form of switch and is simply a contrivance for controlling the electric current. It consists of a steel lever, pivoted in the centre, and provided with a rubber knob, which the operator grasps lightly with the thumb and forefinger. When the lever is pressed downwards, a platinum point on the under side is brought into contact with another similar point set into a rubber bushing in the base of the key, so that there is no electrical connection between the two points unless the key is pressed down or "closed" as it is usually termed.
The receiving device, or "sounder," consists of two electro-magnets mounted on a base under a flat strip or iron, called the armature, which is attracted by the magnetism of the electro-magnets when a current flows through them, and is withdrawn by a spring when no magnetism is produced by the windings.
[Illustration: FIG. 84.—Sounder Frame.]
The armature is usually mounted on a strip of brass or aluminum called the "lever". The lever strikes against an anvil and produces the clicks which form the dots and dashes of the telegraph alphabet.
Every time the key is pressed, an electric current goes through the line, causing the magnets to draw the armature downward and produce a click. When the key lever is released, the current is shut off and the lever flies up and clicks against the top of the anvil.
The time between the first and second clicks may be easily varied at will, by changing the length of time that the key lever is held down. Two clicks very close together, made by pressing the key and immediately releasing it, are called a "dot". A "dash" is made by pressing the key down and holding it down for a fraction of a second so that the interval between the first and second clicks is about three times as long as that between the clicks of a dot.
[Illustration: FIG. 85.—The Electro Magnets.]
Any boy can make a simple telegraph set quite easily. The only drawback of such an apparatus, however, is that it is usually not practical for long lines but may only be used for ticking messages from one room to another.
It is equally possible for the young experimenter to make a perfectly practical telegraph outfit which will operate over a line a mile long, if he is willing to give the time and labor necessary to the work.
The telegraph set-described below is one which, because of its simplicity, particularly lends itself to construction by the young experimenter whose tools are rather limited.