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The coherer is a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the 20th century. Its use in radio was based on the 1890 findings of French physicist Edouard Branly and adapted by other physicists and inventors over the next ten years. The device consists of a tube or capsule containing two electrodes spaced a small distance apart with metal filings in the space between. When a radio frequency signal is applied to the device, the metal particles would cling together or "cohere", reducing the initial high resistance of the device, thereby allowing an electric current to flow through it. In a receiver, the current would activate a bell, or a Morse paper tape recorder to make a record of the received signal. The metal filings in the coherer remained conductive after the signal (pulse) ended so that the coherer had to be "decohered" by tapping it with a clapper each time a signal was received, thereby restoring the coherer to its original state. Coherers remained in widespread use until about 1907, when they were replaced by more sensitive electrolytic and crystal detectors. ==History== The behavior of particles or metal filings in the presence electricity or electric sparks was noticed in many experiments well before Edouard Branly's 1890 paper and even before there was proof of the theory of electromagnetism.〔L. W. Turner, Electronics Engineer's Reference Book, Butterworth-Heinemann - 2013, pages 2-3, 2-4〕 In 1835 Swedish scientist Peter Samuel Munk〔Peter Samuel Munk af Rosenschold lecture assistant in Chemistry at the University of Lund was born at Lund in 1804 and died in 1860 (Michael Faraday, Christian Friedirich Schoenbein, The letters of Faraday and Schoenbein 1836-1862: With notes, comments and references to contemporary letters, Williams & Norgate - 1899, page 54)〕 noticed a change of resistance in a mixture of metal filings in the presence of spark discharge from a Leyden jar.〔(Eric Falcon and Bernard Castaing, Electrical conductivity in granular media and Branly’s coh erer: A simple experiment, page 1 )〕 In 1850 Pierre Guitard found that when dusty air was electrified, the particles would tend to collect in the form of strings. The idea that particles could react to electricity was used in English engineer Samuel Alfred Varley's 1866 lightning bridge, a lightning arrester attached to telegraph lines consisting of a piece of wood with two metal spikes extending into a chamber. The space was filled with powdered carbon that would not allow the low voltage telegraph signals to pass through but it would conduct and ground a high voltage lightning strike.〔T. K. Sarkar, Robert Mailloux, Arthur A. Oliner, M. Salazar-Palma, Dipak L. Sengupta, History of Wireless, John Wiley & Sons - 2006, pages 261-262〕 In 1879 the Welsh scientist David Edward Hughes found that the a loose contact between a carbon rod and two carbon blocks as well as the metallic granules in a microphone he was developing responded to sparks generated in a nearby apparatus.〔 Temistocle Calzecchi-Onesti in Italy began studying the anomalous change in the resistance of thin metallic films and metal particles at Fermo/Monterubbiano. He found that copper filings between two brass plates would cling together, becoming conductive, when he applied a voltage to them. He also found that other types of metal filings would have the same reaction to electric sparks occurring at a distance, a phenomenon that he thought could be used for detecting lightning strikes.〔 Calzecchi-Onesti's papers were published in il Nuovo Cimento in 1884, 1885 and 1886. In 1890 French physicist Edouard Branly published ''On the Changes in Resistance of Bodies under Different Electrical Conditions'' in a French Journal where he described his thorough investigation of the effect of minute electrical charges on metal and many types of metal filings. In one type of circuit, filings were placed in a tube of glass or ebonite, held between two metal plates. When an electric discharge was produced in the neighbourhood of the circuit, a large deviation was seen on the attached galvanometer needle. He noted the filings in the tube would react to the electric discharge even when the tube was placed in another room 20 yards away. Branly went on to devise many types of these devices based on "imperfect" metal contacts. Branly's filings tube came to light in 1892 in Great Britain when it was described by Dr. Dawson Turner at a meeting of the British Association in Edinburgh.〔Sungook Hong, Wireless: From Marconi's Black-box to the Audion, page 4〕〔(E C Green , The Development of the Coherer And Some Theories of Coherer Action, Scientific American: Supplement, Volume 84 - 1917, page 268 )〕 The Scottish electrical engineer and astronomer George Forbes suggested that Branly's filings tube might be reacting in the presence of Hertzian waves, a type of air-born electromagnetic radiation proven to exist by German physicist Heinrich Hertz (later called radio). In 1893 physicist W.B. Croft exhibited Branly's experiments at a meeting of the Physical Society in London. It was unclear to Croft and others whether the filings in the Branly tube were reacting to sparks or the light from the sparks. George Minchin noticed the Branly tube might be reacting to Hertzian waves the same way his solar cell did and wrote the paper "''The Action of Electromagnetic Radiation on Films containing Metallic Powders''".〔〔 These papers were read by English physicist Oliver Lodge who saw this as a way to build a much improved Herzian wave detector. On 1 June 1894, a few months after the death of Heinrich Hertz, Oliver Lodge delivered a memorial lecture on Hertz where he demonstrated the properties of "Hertzian waves" (radio), including transmitting them over a short distance, using an improved version of Branly's filings tube, which Lodge had named the "coherer", as a detector. In May 1895, after reading about Lodge's demonstrations, the Russian physicist Alexander Popov built a "Hertzian wave" (radio wave) based lightning detector using a coherer. That same year Italian inventor Guglielmo Marconi demonstrated a wireless telegraphy system using Hertzian waves (radio), based on a coherer. The coherer was replaced in receivers by the simpler and more sensitive electrolytic and crystal detectors around 1907, and became obsolete. One minor use of the coherer in modern times was by Japanese tin-plate toy manufacturer Matsudaya Toy Co. who beginning 1957 used a spark-gap transmitter and coherer-based receiver in a range of radio-controlled (RC) toys, called Radicon (abbreviation for Radio-Controlled) toys. Several different types using the same RC system were commercially sold, including a Radicon Boat (very rare), Radicon Oldsmobile Car (rare) and a Radicon Bus (the most popular). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「coherer」の詳細全文を読む スポンサード リンク
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