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The electric telegraph is based on a simple technology. Connect one end of a wire to a battery or other electric power source and the other end to something to show that current a flowing, like a light bulb, and then earth it. At the battery end, attach a simple switch, and use it to connect and then break the electrical circuit. Almost instantaneously the light bulb will go on and off.

By using Morse code to turn the light on and off watching the light bulb, you can easily send complex messages over long distances.