page_title: Magnetic detector
section_title: History
hierarchical_section_title: Magnetic detector / History
caption_attribution_description: Français&#160;: exposition station radiotélégraphique type Marconi. English: Recreation of a Marconi Co. ship's wireless (radio) room from around 1910 at the Aalborg Maritime Museum, Aalborg, Denmark. It allowed the ship to keep in touch with shore using wireless telegraphy, sending messages in Morse code with a en:spark gap transmitter using the telegraph key visible on the desk. Signals were received using the receiver (center left) and magnetic detector (center right) on the desk. At the time, the Marconi Co. held a virtual monopoly on maritime radio equipment because it refused to allow its radio stations to communicate with radios made by any other manufacturer.
context_page_description: 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1895 by New Zealand physicist Ernest Rutherford it was used in Marconi wireless stations until around 1912, when it was superseded by vacuum tubes.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. A magnetic detector was part of the wireless apparatus in the radio room of the RMS Titanic which was used to summon help during its famous 15 April 1912 sinking.
context_section_description: 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. The coherer was a very poor detector, insensitive and prone to false triggering due to impulsive noise, which motivated much research to find better radio wave detectors.
  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 Fessenden, John Ambrose Fleming,  Lee De Forest, J.C. Balsillie, and L. Tieri  had subsequently devised detectors based on hysteresis, but none had become widely used due to various drawbacks.  Many earlier versions had a rotating magnet above a stationary iron band with coils on it. This type was only periodically sensitive, when the magnetic field was changing, which occurred as the magnetic poles passed the iron.
  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.  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.
caption: Recreation of a Marconi ship radio room at the Aalborg Maritime Museum, Aalborg, Denmark. A magnetic detector is on the desk to right of the Marconi tuner receiver, which provided the signal for the magnetic detector.
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