Thermionic Emission - History

History

Because the electron was not identified as a separate physical particle until the 1897 work of J. J. Thomson, the word "electron" was not used when discussing experiments that took place before this date.

The phenomenon was initially reported in 1873 by Frederick Guthrie in Britain. While doing work on charged objects, Guthrie discovered that a red-hot iron sphere with a positive charge would lose its charge (by somehow discharging it into air). He also found that this did not happen if the sphere had a negative charge. Other early contributors included Johann Wilhelm Hittorf (1869–1883), Eugen Goldstein (1885), and Julius Elster and Hans Friedrich Geitel (1882–1889).

The effect was rediscovered by Thomas Edison on February 13, 1880, while trying to discover the reason for breakage of lamp filaments and uneven blackening (darkest near one terminal of the filament) of the bulbs in his incandescent lamps.

Edison built several experiment bulbs, some with an extra wire, a metal plate, or foil inside the bulb which was electrically separate from the filament, and thus could serve as an electrode. He connected a galvanometer, a device used to measure current, to the output of the extra metal electrode. When the foil was charged negatively relative to the filament, no charge flowed between the filament and the foil. We now know that this was because the filament was emitting electrons, and thus were not attracted to the negatively charged foil. In addition, charge did not flow from the foil to the filament because the foil was not heated enough to emit charge (later called thermionic emission). However, when the foil was given a more positive charge than the filament, negative charge (in the form of electrons) could flow from the filament through the vacuum to the foil. This one-way current was called the Edison effect (although the term is occasionally used to refer to thermionic emission itself). He found that the current emitted by the hot filament increased rapidly with increasing voltage, and filed a patent application for a voltage-regulating device using the effect on November 15, 1883 (U.S. patent 307,031, the first US patent for an electronic device). He found that sufficient current would pass through the device to operate a telegraph sounder. This was exhibited at the International Electrical Exposition in Philadelphia in September 1884. William Preece, a British scientist took back with him several of the Edison Effect bulbs, and presented a paper on them in 1885, where he referred to thermionic emission as the "Edison Effect." The British physicist John Ambrose Fleming, working for the British "Wireless Telegraphy" Company, discovered that the Edison Effect could be used to detect radio waves. Fleming went on to develop the two-element vacuum tube known as the diode, which he patented on November 16, 1904.

The thermionic diode can also be configured as a device that converts a heat difference to electric power directly without moving parts (a thermionic converter, a type of heat engine).

Following J. J. Thomson's identification of the electron, the British physicist Owen Willans Richardson began work on the topic that he later called "thermionic emission". He received a Nobel Prize in Physics in 1928 "for his work on the thermionic phenomenon and especially for the discovery of the law named after him".