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A telegraph (Greek: τῆλε [ti:le] tele 'far away' and γραφειν [graphō] write) is a system for sending messages over long distances using symbols of light, electricity, or flag position. Most often associated with the electric telegraph system developed by Samuel Morse, the basic concept goes back to the ancient world.

Morse's telegraph used short and long pulses (i.e., pulse width modulation), which were eventually standardized as the International Morse Code. Radiotelegraphy systems, known in the early twentieth century as "wireless," largely replaced the wired electrical system, but telegraphy has been replaced in all major applications by packet-oriented transmission. 500 kHz radiotelegraphy, long the distress frequency monitored for SOS signals, is no longer part of the Global Maritime Distress and Safety System.

Ancient signals

The ability to send messages instantaneously over long distances has been sought after since the earliest times. Early peoples used sound (drums for example), flames or smoke signals to transmit simple messages over long distances.

The first telegraph

The first device to be called a telegraph was invented in 1792 by Claude Chappe in France. Working with his brothers, he developed a system of telegraph stations which used movable arms to communicate over long distances This kind of optical telegraph would later come to be known as a semaphore system.