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American Telephone & Telegraph: The Art of Telephony, 1916

The Audion

The Audion made it possible for telephone signals to reach from coast to coast. Inventor Lee de Forest first developed a working model of the audion in 1906. This device is made up of three elements: a filament, a plate, and a grid. The Audion is essentially a vacuum tube: an arrangement of the three elements listed above within an insulating, temperature-resistant envelope. De Forest's early model of the Audion had an "envelope" made of glass, causing it to resemble a light bulb.

The early Audion worked by creating a current of electrons that amplified the weak long-distance signals sent through AT&T's phone lines. When the filament mentioned above was heated, it released electrons into the vacuum created by the glass envelope enclosure. These negatively-charged electrons were drawn to the positively-charged plate. The current of electrons flowing from the filament to the plate were projected onto the grid, thus amplifying the telephone signal. The strengthening, or amplification, of telephone signals made it possible for these signals to reach from coast to coast.

De Forest's original patents stipulated that low-pressure gas present inside the Audion's glass envelope was essential to its function. The gas, however, was occasionally absorbed by the electrons, resulting in the production of a blue haze and the malfunction of the Audion. While in AT&T's employment, physicist Dr. Harold Arnold made improvements to the Audion by increasing the vacuum inside the glass envelope, thereby removing residual gases. By the summer of 1913, AT&T had tested high-vacuum tubes and determined their efficacy as signal amplifiers. The company completed its long-distance network in 1914, after the development, modification, and installation of loading coils and Audions.