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magnetic detector
The magnetic detector or Marconi magnetic detector, sometimes called the "Maggie", was an early radio wave detector used in some of the first radio receivers to receive Morse code messages during the wireless telegraphy era around the turn of the 20th century. Developed in 1902 by radio pioneer Guglielmo Marconi〔〔 from a method invented in 1895 by New Zealand physicist Ernest Rutherford it was used in Marconi wireless stations until around 1912, when it was superseded by vacuum tubes. It was widely used on ships because of its reliability and insensitivity to vibration.
== History ==
The primitive spark gap radio transmitters used during the first three decades of radio (1886-1916) could not transmit audio (sound) and instead transmitted information by wireless telegraphy; the operator switched the transmitter on and off with a telegraph key, creating pulses of radio waves to spell out text messages in Morse code. So the radio receiving equipment of the time did not have to convert the radio waves into sound like modern receivers, but merely detect the presence or absence of the radio signal. The device that did this was called a detector. The first widely used detector was the coherer, invented in 1890.
Ernest Rutherford had first used the hysteresis of iron to detect Hertzian waves in 1896〔 by the demagnetization of an iron needle when a radio signal passed through a coil around the needle, however the needle had to be remagnetized so this was not suitable for a continuous detector.〔 Many other wireless researchers such as E. Wilson, C. Tissot, Reginald Fessendon, John Ambrose Fleming, Lee De Forest, J.C. Balsillie, and L. Tieri had subsequently devised magnetic hysteresis detectors, but none had become widely used due to various drawbacks.〔
During his transatlantic radio communication experiments in December 1902 Marconi found the coherer to be too unreliable and insensitive for detecting the very weak radio signals from long distance transmissions. It was this need that drove him to develop his magnetic detector. Most earlier versions of the detector had a rotating magnet above a stationary iron band with coils on it.〔( Phillips (1980) ''Early radio wave detectors'', p. 103-105 )〕 This type was only periodically sensitive, when the magnetic field was changing, which occurred as the magnetic poles passed the iron. Marconi devised a more effective configuration with a moving iron band driven by a clockwork motor passing by stationary magnets and coils, resulting in a continuous supply of iron that was changing magnetization, and thus continuous sensitivity (Rutherford claimed he had also invented this configuration).〔 The Marconi magnetic detector was the "official" detector used by the Marconi Company from 1902 through 1912, when the company began converting to the Fleming valve and Audion-type vacuum tubes. It was used through 1918.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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