Telegraph

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.