The battery may be almost any form of battery. Gravity cells are preferred, however, for telegraph work.
Heavy galvanized iron wire is usually employed as the "line." It is necessary to use non-conductors wherever the wire is fastened. Glass insulators placed on a wooden pin or bracket, which is fastened to the pole or building on which the wire is to be supported, are used for outside work. Inside of buildings, rubber tubes are used where the wires pass through walls, etc.
The operation of a telegraph is not, as many people suppose, a complicated or difficult matter to understand, but is quite simple.
The key is a contrivance for controlling the passage of the electric current in much the same manner as an ordinary switch. It consists of a steel lever, swung on trunnion-screws mounted in a frame, and provided with a rubber knob which the operator grasps lightly with the thumb and forefinger. On pressing the lever downward, a platinum point fastened on the under side of the lever is brought into contact with another 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 often termed. The key is usually fastened to the operating bench by two rods called "legs." The lever is provided with screws which permit the stroke of the key to be very closely adjusted.
Fig. 130.—A Typical Telegraph Key, showing the Various Parts.
The line wire and battery are connected to the key, so that no current can flow until the key is pressed and the contacts brought together.
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flat piece of iron which is attracted by the magnetism of the electromagnets when a current flows through them and is withdrawn by a spring when no magnetism excites the windings.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.
