Coherer
From Wikipedia, the free encyclopedia
The coherer is a primitive form of radio signal detector used in the late 1800s consisting of a capsule of metal filings in the space, sometimes evacuated, between two conductors. It was a key enabling technology that led to radio, and was the first device used to detect radio signals in practical wireless telegraphy. Its operation is based upon the large resistance offered to the passage of electric current by loose metal filings that decreases under the influence of radio frequency alternating current. The coherer became the basis for radio reception around 1900, and remained in widespread use for about ten years. The coherer saw commercial use again in the mid-1900 in a few primitive radio-controlled toys that used spark-gap transmitter controllers. There are two basic kinds of coherers, the original metal filings type and a later, imperfect junction type.
|
[edit] How it works
The metal particles in a particle coherer cohere and conduct electricity much better when subjected to high frequency electricity. Recent experiments[1] with particle coherers seem to have confirmed the hypothesis that the particles cohere by a micro-weld phenomenon caused by radio frequency electricity flowing across a small contact area. The effect is to cause a reduction in electrical resistance that is not persistent because the mechanically weak joints are easily broken by mechanical disturbance caused by the decoherer. The underlying principle of other kinds of so-called "imperfect contact" coherers may involve a kind of sub-atomic tunneling of charge carriers across an imperfect junction between conductors.
[edit] Application
The filings coherer used in practical receivers was a glass tube filled about half full with sharply cut metal filings, often part silver and part nickel. Silver electrodes make contact with the metal particles on both ends. The electrodes are slanted so their effective spacing filled by the filings can be varied by rotating the tube about its long axis, thus adjusting its sensitivity verses false coherance performance to the prevailaing 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 signal in the antenna-ground circuit enables 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 over to 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.
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. In practical implementations, the decoherer was the clapper of a door bell that was powered by the cohere current itself. This is referred to as 'decohering' the device and was subject to much innovation during the life of this component. Tesla, for example had the tube rotating continuously along its axis, following each successive activation.
[edit] History
In 1850 Guitard found that when dusty air was electrified, the particles of dust would tend to attach themselves together in the form of strings. Again, in 1879, it was observed[2] that drops of water from a small fountain, when exposed to the influence of a charged piece of sealing-wax, would not separate into small drops, but would cohere in large ones. It is probably due to the same principle that the drops of rain are so much larger in thunderstorms than in ordinary showers. Electric charge of the clouds probably cause the drops of water to cohere into large ones. Temistocle Calzecchi-Onesti is thought to have performed the first experiments with a predecessor of the coherer in 1884. These phenomina had been observed for many years, but it was not until 1890 that Edouard Branly made a practical application of the principle in the form of the filings coherer as it is now known. The invention of the device is usually credited to Branly.
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.
[edit] Imperfect junction coherer
The imperfect junction coherer is not clearly the same thing as the metal filings coherer. It was invented in 1899 by Jagdish Chandra Bose[3] and was viewed as an extension of the coherer principle. 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. Its principle of operation is not well understood. The action of detection occurs when the radio frequency signal somehow breaks down the insulating film of oil, allowing the device to conduct, operating the receiving sounder wired in series. This form of coherer is self-restoring and needs no decohering.
[edit] 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 problem was solved by the demodulation capability enabled by the crystal detectors.
[edit] References
- ^ E. Falcon, B. Castaing, and M. Creyssels: Nonlinear electrical conductivity in a 1D granular medium, Laboratoire de Physique de l’Ecole Normale Sup'erieure de Lyon UMR 5672 -46 all'ee d’Italie, 69007 Lyon, France
- ^ Louis Derr, A.M., S.B., Cyclopedia of Engineering, American School of Correspondence of Armour Institute of Technology, Chicago, 1904
- ^ Bose article by Varun Aggarwal
[edit] See also
[edit] 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)
