page_title: Radio receiver
section_title: Other early detectors
hierarchical_section_title: Radio receiver / History / Spark era / Other early detectors
caption_attribution_description: English: Magnetic detector in early radio receiver, used to detect Morse code radiotelegraphy signals broadcast by early spark radio transmitters between 1900 and 1920. Invented by Guglielmo Marconi, the magnetic detector was used in shipboard radio receivers because of its complexity. It was used for only a few years, until it was replaced by crystal and vacuum tube detectors. The magnetic detector was the "official" detector employee by Marconi Company. It consisted of a continuous loop of fine iron wires, which rotated at a rate of 7-8 cm per second around two pulleys turned by a motor or windup mechanism. The wire passed through two coils of copper wire, which were in the magnetic field of two horseshoe magnets. The radio signal from the receiver was applied to one coil, and the second was connected to an earphone. As it passed through the first magnet, the wire was magnetized longitudinally in one direction, then due to hysteresis the magnetization was "flipped" to the opposite direction as it passed through the second magnet. In the absence of a radio signal, the flipping of the magnetization always occured at the same place inside the coil, so the magnetic field through the coil was constant. However, when a radio signal was applied to the first coil, it created an oscillating magnetic field that demagnetized the iron wire, so the flux through the second coil changed, inducing a voltage that was heard as a click in the earphones. (information from G. W. Pierce (1910) Principles of Wireless Telegraphy, McGraw-Hill, p. 145-163)}It functions in the following way. A cable without end is consisted several bits of wire insulated with silk. This cable passes in the throats of two pulleys between which it is reasonably tended. The pulleys, pulled by a clockwork communicate to the cable a translatory movement of a few centimetres a second. The cable passes in the axis of a reel rolled up on a tube of glass. This reel is inserted in the circuit antenna-ground of reception. Around this first reel and according to the same axis, one lays out one second reel which is connected to a telephone earphone. A couple of magnets whose of the same poles name are touched is laid out with the top of the cable and the reels. Here what one observes with such a detector; if the cable is motionless, when a wave train arrives, the magnetic state present of the cable is modified and the telephone earphone makes hear a slapping. But if another wave train is presented, one understands nothing any more because the magnetic state of iron does not change any more. If the cable is moving, its passage in front of the magnet will determine a certain basic magnetization and, if a signal is presented, this magnetization will be modified and the consequence of this modification is a current armature in the circuit of the telephone with the corresponding noise.It will be thus for each high frequency signal. Français&#160;: station radiotélégraphique morse a détecteur magnétique était l'un des premiers dispositifs pratiques capables rendre les signaux radios audibles avec une paire d'écouteur. il a été pour l'usage maritime à court terme en raison de son de la complexcité de fabrication, il a été supplanté par les récepteurs à galène et les tubes à vide. Le détecteur magnétique était le détecteur "officiel" employé par Marconi Company. Il fonctionne de la façon suivante. Un câble sans fin est constitué par plusieurs brins de fil de fer isolé à la soie. Ce câble passe dans les gorges de deux poulies entre lesquelles il est raisonnablement tendu. Les poulies, entraînées par un mouvement d'horlogerie communiquent au câble un mouvement de translation de quelques centimètres par seconde. Le câble passe dans l'axe d'une bobine enroulée sur un tube de verre. Cette bobine est intercalée dans le circuit antenne-terre de réception. Autour de cette première bobine et selon le même axe, on dispose une seconde bobine qui est connectée à un écouteur téléphonique. Un couple d'aimants dont les pôles de même nom se touchent est disposé au dessus du câble et des bobines. Voici ce que l'on observe avec un tel détecteur; si le câble est immobile, lorsqu'un train d'ondes arrive, l'état magnétique présent du câble est modifié et l'écouteur téléphonique fait entendre un claquement. Mais si un autre train d'ondes se présente, on n'entend plus rien car l'état magnétique du fer ne change plus. Si le câble est en mouvement, son passage devant l'aimant va déterminer une certaine aimantation de base et, si un signal se présente, cette aimantation va être modifiée et la conséquence de cette modification est un courant induit dans le circuit du téléphone avec le bruit correspondant. Il en sera ainsi pour chaque signal de haute fréquence.
context_page_description: In radio communications, a radio receiver, also known as a receiver, a wireless or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.
  Radio receivers are essential components of all systems that use radio. The information produced by the receiver may be in the form of sound, moving images, or digital data. A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. The most familiar type of radio receiver for most people is a broadcast radio receiver, which reproduces sound transmitted by radio broadcasting stations, historically the first mass-market radio application.
context_section_description: The coherer's poor performance motivated a great deal of research to find better radio wave detectors, and many were invented.  Some strange devices were tried; researchers experimented with using frog legs and even a human brain from a cadaver as detectors.
  By the first years of the 20th century, experiments in using amplitude modulation (AM) to transmit sound by radio (radiotelephony) were being made.   So a second goal of detector research was to find detectors that could demodulate an AM signal, extracting the audio (sound) signal from the radio carrier wave.  It was found by trial and error that this could be done by a detector that  exhibited "asymmetrical conduction"; a device that conducted current in one direction but not in the other.  This rectified the alternating current radio signal, removing one side of the carrier cycles, leaving a pulsing DC current whose amplitude varied with the audio modulation signal.  When applied to an earphone this would reproduce the transmitted sound.
  Below are the detectors that saw wide use before vacuum tubes took over around 1920.  All except the magnetic detector could rectify and therefore receive AM signals:
  Magnetic detector - Developed by Guglielmo Marconi in 1902 from a method invented by Ernest Rutherford and used by the Marconi Co. until it adopted the Audion vacuum tube around 1912, this was a mechanical device consisting of an endless band of iron wires which passed between two pulleys turned by a windup mechanism.   The iron wires passed through a coil of fine wire attached to the antenna, in a magnetic field created by two magnets.  The hysteresis of the iron induced a pulse of current in a sensor coil each time a radio signal passed through the exciting coil.  The magnetic detector was used on shipboard receivers due to its insensitivity to vibration.  One was part of the wireless station of the RMS Titanic which was used to summon help during its famous 15 April 1912 sinking.
  Electrolytic detector  ("liquid barretter") - Invented in 1903 by Reginald Fessenden, this consisted of a thin silver-plated platinum wire enclosed in a glass rod, with the tip making contact with the surface of a cup of nitric acid.  The electrolytic action caused current to be conducted in only one direction.  The detector was used until about 1910. Electrolytic detectors that Fessenden had installed on US Navy ships received the first AM radio broadcast on Christmas Eve, 1906, an evening of Christmas music transmitted by Fessenden using his new alternator transmitter.
  Thermionic diode (Fleming valve) - The first vacuum tube, invented in 1904 by John Ambrose Fleming, consisted of an evacuated glass bulb containing two electrodes: a cathode consisting of a hot wire filament similar to that in an incandescent light bulb, and a metal plate anode.   Fleming, a consultant to Marconi, invented the valve as a more sensitive detector for transatlantic wireless reception.  The filament was heated by a separate current through it and emitted electrons into the tube by thermionic emission, an effect which had been discovered by Thomas Edison.  The radio signal was applied between the cathode and anode.  When the anode was positive, a current of electrons flowed from the cathode to the anode, but when the anode was negative the electrons were repelled and no current flowed.   The Fleming valve was used to a limited extent but was not popular because it was expensive, had limited filament life, and was not as sensitive as electrolytic or crystal detectors.
  Crystal detector  (cat's whisker detector) - invented around 1904-1906 by Henry H. C. Dunwoody and Greenleaf Whittier Pickard, based on Karl Ferdinand Braun's  1874 discovery of "asymmetrical conduction" in crystals, these were the most successful and widely used detectors before the vacuum tube era and gave their name to the crystal radio receiver (below).   One of the first semiconductor electronic devices, a crystal detector consisted of a pea-sized pebble of a crystalline semiconductor mineral such as galena (lead sulfide) whose surface was tou
caption: Magnetic detector
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