Vacuum Tubes: Definition

“Vacuum Tube” is the term used to describe a class of devices using electron movement within a vacuum that originally brought us radio, TV and wireless communication.

It started with Edison’s discovery of an “effect” when a second metallic element was installed near the filament in one of his light bulbs. This “effect” was an invisible electron current that flowed from the incandescent filament to the “plate.” But he did not recognize its ability to detect radio waves; instead he patented it as a voltmeter for his nascent electric lighting system.

John Fleming, in England, learned of the Edison effect and through his own measurements in 1896 found that the effect produced a unidirectional current flow to the plate if a positive potential was applied to it. Later, in 1904, working for the Marconi Wireless Telegraph Co., Fleming remembered his earlier experiments as he worked to improve wireless detector sensitivity. He found to his amazement that the Edison effect could be used to rectify (detect) radio waves and could be used as a sensitive detector in Marconi’s wireless business. He is known for his “Fleming Valve.” A patent was issued in 1905.

Another of the well-known early wireless pioneers was the controversial Lee de Forest. A Yale PhD, he experimented with flame detectors, but apparently learned of Fleming’s “valve” detector in 1905. Duplicates were made for him by McCandless Co. and he patented a “Static Valve” showing a 2-element (diode) vacuum tube, similar to Fleming’s, in 1906. However, his greatest contribution was the result of inserting a serpentine “grid” wire between the filament and the plate. This 3-element (triode) was developed in late 1906. Tests were very successful and a patent was filed in January 1907. The first triode “Audion” was shown in March 1907 and the DeForest Radio Telephone Co. was formed immediately thereafter. The advent of the triode tube was a breakthrough due to its ability to not only detect radio waves, but also to amplify weak signals and, later, to become an oscillator, the forerunner of vacuum tubes used in radio transmitters which were superior to the “arc” transmitters of the day.

Many others including Wehnelt and Von Lieben contributed to the early development. As time passed, companies were formed and liquidated in the tumultuous efforts to capitalize on the rapidly expanding government and public demand for radio, TV and other forms of wireless communication. Now, vacuum tubes themselves have largely been replaced by transistors and integrated circuits, but are still used in high-power broadcasting and radar.

To see an example of a vacuum tube in action, watch as one of our volunteers, Bjorn Forsberg, attempts to amplify his 100-year-old von Lieben tube in early 2012: YouTube video of a von Lieben tube.