DescriptionFirst vacuum tube AM radio transmitter.jpg
English: Apparently the first commercial AM
Audionvacuum tuberadio transmitter, built in 1914 by
Lee De Forest who invented the Audion (
triode) in 1906, from a short announcement in Electrical World magazine. It was not the first AM (sound) transmitter; short-lived technologies like the
Poulsen arc and
Alexanderson alternator| had been transmitting sound since 1906. But the vacuum tube
feedback oscillator, invented in 1912 by Edwin Armstrong, replaced them, and has remained the key technology used in radio transmitters to the present day.
The device used a tickler-feedback
Armstrong oscillator circuit. The Audion is mounted outside, on the side of the enclosure, so the operator can check if the filament is glowing and adjust the filament voltage visually. Audions were always mounted hanging upside down, so the fragile
filament in the tube wouldn't sag and touch the grid. The telephone-type carbon microphone was connected directly in the transmitter's antenna wire, and its varying resistance modulated the current going to the antenna. The low power of the early Audion gave it a limited range of 1 to 3 miles.
The text of the article that accompanied the photo: Apparatus for generating in an incandescent lamp from direct-current energy continuous high-frequency oscillations to be used for wireless telephony and telegraphy, as well as for numerous laboratory applications, is being made by the Radio Telephone & Telegraph Company, 309 Broadway, New York. This equipment is called an oscillating "audion" transmitter and can be connected to either a 110-volt or a 250-volt lighting circuit. By means of the small 3.5-volt amplifier bulb used with this apparatus direct current can be transformed to alternating current at frequencies of from sixty cycles per second to 1,000,000 cycles per second. With this form of transmitter it is possible to telephone one to three miles, and the device is well adapted for use on small yachts, tugs, ferryboats, etc.
When a storage-battery supply is available this oscillating transmitter can serve as a receiver also, both the functions of transmitting and receiving being performed by means of the same bulb with the aid of a telephone receiver connected to the wing-filament circuit.
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