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.

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 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 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 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 coherers

Although 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

See also

• Detector (radio)
• Crystal Radio
• Spark-gap transmitter
• Radio receiver

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)
• Hirakawa Institute of Technology(Japan),"Coherer".