Audion

The Audion was an electronic amplifying vacuum tube invented by American electrical engineer Lee De Forest in 1906. It was the first triode, consisting of a partially evacuated glass tube containing three electrodes; a heated filament, a grid, and a plate. It is important in the history of technology because it was the first widely used electrical device which could amplify; a small electrical signal applied to the grid could control a larger current flowing from the filament to plate.

Unlike later vacuum tubes, the primitive Audion had a small amount of gas in the tube, thought to be necessary by De Forest, which limited the dynamic range and gave it nonlinear characteristics and erratic performance. Originally developed as a radio receiver detector by adding a grid electrode to the Fleming valve, it found little use until its amplifying ability was recognized around 1912 by several researchers, who used it to build the first amplifying radio receivers and electronic oscillators. The many practical applications for amplification motivated its rapid development, and the original Audion was superseded within a few years by improved versions with higher vacuum, developed by Irving Langmuir at GE and others. These were the first modern "hard vacuum" triodes.

It had been known since the middle of the 19th century that gas flames were electrically conductive, and early wireless experimenters had noticed that this conductivity was affected by the presence of radio waves. De Forest found that gas in a partial vacuum heated by a conventional lamp filament behaved much the same way, and that if a wire were wrapped around the glass housing, the device could serve as a detector of radio signals. In his original design, a small metal plate was sealed into the lamp housing, and this was connected to the positive terminal of a 22 volt battery via a pair of headphones, the negative terminal being connected to one side of the lamp filament. When wireless signals were applied to the wire wrapped around the outside of the glass, they caused disturbances in the current which produced sounds in the headphones.

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