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The coherer was the first device used to detect radio signals in wireless telegraphy. Its operation is based upon the large resistance offered to the passage of electric current by loose metal filings, which decreases under the influence of radio frequency alternating current. The coherer became the basis for radio reception, and remained in widespread use for about ten years.
History
Temistocle Calzecchi-Onesti is thought to have performed the first experiments with a predecessor of the coherer in 1884. The invention of the device is usually credited to Edouard Branly. Branly is best known for his researches concerning radio-conductors, and the coherer. He began his studies in this field in 1890, being led to undertake them by observing the anomalous change in the resistance of thin metallic films when exposed to electric sparks. Platinum deposited upon glass was first employed. The effect was at first attributed to the influence of the ultraviolet light of the spark. The variations in the resistance of metals in a finely divided state were even more striking, and they were shown by Branly to be due to the action of the electrical, or Hertzian, waves of which the spark was the source. The further developments from these experiments led to the coherer. Later this simple device was employed by Oliver Lodge in his researches and formed an important part of Guglielmo Marconi's successful system of wireless telegraphy.
Description
The coherer is based around the effect that had been known since the 1850s; that small particles of dust or even metal filings stick together or cohere when an electric field is present.
It is a glass tube filled with sharply cut silver and nickel shavings. Silver electrodes make contact with the shavings on both ends. One electrode is connected to the antenna and the other to ground. A series combination of a battery and a telegraph sounder is also attached to the two electrodes. When the signal from a spark gap transmitter is received, the shavings tend to cling to each other, reducing the resistance of the coherer. The battery supplies more current, and the telegraph sounder is actuated, giving an audible click. More often a pair of headphones replaced the telegraph sounder, being much more sensitive to weak signals. The problem of the filings continuing to cling together after the removal of the RF energy was solved by tapping the coherer with a small mallet attached to the sounder after the arrival of each signal shaking up the filings and raising the resistance of the coherer to the original value. This is referred to as 'decohering' the device and was subject to much innovation during the life of this component. Tesla's, for example had the tube rotating continuously along its axis, following each successive activation.
Development
One innovation was the development of the mercury coherer. This device consisted of a small metallic cup containing a pool of mercury on which has a very thin insulating film of oil over it; above the surface a small iron disc is suspended. By means of an adjusting screw the lower edge of the disc is made to touch the oil-covered mercury with a pressure small enough not to puncture the film of oil. The action of detection occurs when the signal breaks down the insulating film of oil, allowing the cell conduct operating the receiving instrument wired in parallel. This form of coherer is self-restoring and needs no decohering.
Limitations of coherers
Although the coherer was satisfactory for responding to the "on-off keying" characteristic of an early spark gap transmitter, they cannot follow the complex waveforms of audio broadcasting. This can only be accomplished by the crystal detectors.
See also
External links
- "The Coherer". World of Wireless, Virtual radiomuseum.
- "Coherer / Receiver". Marconi Calling Company.
- Slaby, Adolphus, "The New Telegraphy, Recent experiments in telegraphy with sparks.". The Century Magazine. April, 1898. (Earlyradiohistory.us)
"Coherer"