Coherer Projects and Experiments
See also Who Invented Radio? 
The coherer is a primitive form of radio signal detector, used in the late 19th century, consisting of a capsule of metal filings in the space, sometimes evacuated, between two electrodes. 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 being decreased 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 20th century 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.
How it worksFor some reason, the metal particles in a particle coherer cohere and conduct electricity much better after being 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. This reduction in electrical resistance persists after the high frequency excitation is removed. This persistence is a problem in most practical radio signal detection circuits. A decoherer, described below is introduced to tap the coherer, mechanically disturbing it to reset it to the high resistance state. Theory suggests that the mechanically weak joints are easily broken by mechanical disturbance. The underlying principle of so-called "imperfect contact" coherers is not well understood, but may involve a kind of sub-atomic tunneling of charge carriers across an imperfect junction between conductors. 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 made contact with the metal particles on both ends. The electrodes were slanted so their effective spacing filled by the filings could be varied by rotating the tube about its long axis, thus adjusting its sensitivity versus false-coherence performance 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 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 signal 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 was function and apparatus was called a decoherer. In practical implementations, the decoherer was the clapper of a door bell that was powered by the coherer 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. HistoryIn 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 into 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 on the clouds probably causes 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 phenomena 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. Imperfect junction cohererThe 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 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. Limitations of coherersAlthough the coherer is satisfactory for responding to the "on-off keying" characteristic of an early spark gap transmitter, it cannot follow the complex waveforms of audio broadcasting. This problem was solved by the demodulation capability enabled by Reginald Fessenden's hot wire barretter and electrolytic detector. These in turn were replaced by the crystal detector and Lee De Forest's vacuum tube or thermionic diode. References
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