The practical possibility of transmitting sound via air, or “radio telephony”, is linked to the development of amplification techniques. It was the American inventor Lee De Forest who gave the decisive stimulus in 1906-7 by devising the triode, a vacuum tube capable of amplifying, in a controllable manner, the signal (voltage) detected: it was an improvement of the diode, introduced in 1905 by the English inventor Ambrose Fleming. De Forest quickly caught on that the principal application for his invention was in the audio field, so much so that he named his most evolved version “Audion”. However, it was the Canadian inventor Reginald Aubrey Fessenden who carried out, always in 1906, the first experiments in radiotelephony, not only with the objective of transmitting sound, as with the Morse code, but to send through the air complete audio signals. At the root of these developments lies another important invention, the microphone, introduced by Thomas Edison in the early 1880's as an evolution and improvement of communication via telephone. Radio telephony quickly found its place in the fields that were dominated by wireless telegraphy (communication between vehicles in motion, communication in the military) but in no more than a few years it developed into something that even Marconi had not foreseen, broadcasting, in other words the radio as a means of mass communication. Beginning in the Fifties the triode vacuum tube was progressively substituted with a new instrument for amplification, the transistor, which allowed for the miniaturization of the radio, and made it become portable.
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