page_title: Coherer
section_title: Application
hierarchical_section_title: Coherer / Application
caption_attribution_description: Deutsch: Fritter
context_page_description: The coherer was 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 Édouard 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 loose 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 a much greater direct 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 ended so that the coherer had to be "decohered" by tapping it with a clapper actuated by an electromagnet, each time a signal was received, thereby restoring the coherer to its original state.
context_section_description: The coherer used in practical receivers was a glass tube, sometimes evacuated, which was about half filled with sharply cut metal filings, often part silver and part nickel. Silver electrodes made contact with the metal particles on both ends. In some coherers, the electrodes were slanted so the width of the gap occupied by the filings could be varied by rotating the tube about its long axis, thus adjusting its sensitivity to the prevailing conditions.
  In operation, the coherer is included in two separate electrical circuits. One is the antenna-ground circuit shown in the untuned receiver circuit diagram below.  The other is the battery-sounder relay circuit including battery B1 and relay R in the diagram.  A radio signal from the antenna-ground circuit "turns on" the coherer, enabling current flow in the battery-sounder circuit, activating the sounder, S.  The coils, L, act as RF chokes to prevent the RF signal power from leaking away through the relay circuit.
  One electrode, A, of the coherer, (C, in the left diagram) is connected to the antenna and the other electrode, B, to ground. A series combination of a battery, B1, and a relay, R, is also attached to the two electrodes. When the signal from a spark gap transmitter is received, the filings tend to cling to each other, reducing the resistance of the coherer. When the coherer conducts better, battery B1 supplies enough current through the coherer to activate relay R, which connects battery B2 to the telegraph sounder S, giving an audible click.  In some applications, a pair of headphones replaced the telegraph sounder, being much more sensitive to weak signals, or a Morse recorder which recorded the dots and dashes of the signal on paper tape.
  The problem of the filings continuing to cling together and conduct after the removal of the signal was solved by tapping or shaking the coherer  after the arrival of each signal, shaking the filings and raising the resistance of the coherer to the original value.  This apparatus was called a decoherer.    This process was referred to as 'decohering' the device and was subject to much innovation during the life of the popular use of this component. Tesla, for example, invented a coherer in which the tube rotated continually along its axis.
  In later practical receivers the decoherer was a clapper similar to an electric bell, operated by an electromagnet powered by the coherer current itself.  When the radio wave turned on the coherer, the DC current from the battery flowed through the electromagnet, pulling the arm over to give the coherer a tap.  This returned the coherer to the nonconductive state, turning off the electromagnet current, and the arm sprang back.  If the radio signal was still present, the coherer would immediately turn on again, pulling the clapper over to give it another tap, which would turn it off again.  The result was a constant "trembling" of the clapper during the period that the radio signal was on, during the "dots" and "dashes" of the Morse code signal.
  An automatic braking system for rail locomotives, patented in 1907, used  a coherer to detect electrical oscillations in a continuous aerial running along the track. If the block ahead of the train were occupied the oscillations were interrupted and the coherer, acting through a relay, showed a warning and applied the brakes.
caption: A radio receiver circuit using a coherer detector (C). The "tapper" (decoherer) is not shown.
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