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The coherer was a primitive form of radio signal
detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
used in the first
radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...
s during the
wireless telegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
era at the beginning of the 20th century. Its use in radio was based on the 1890 findings of French physicist
Édouard Branly Édouard Eugène Désiré Branly (23 October 1844 – 24 March 1940) was a French inventor, physicist and professor at the Institut Catholique de Paris. He is primarily known for his early involvement in wireless telegraphy and his invention of the ...
and adapted by other physicists and inventors over the next ten years. The device consists of a tube or capsule containing two
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
s spaced a small distance apart with loose
metal filings Iron filings are very small pieces of iron that look like a light powder. As the name suggests, iron filings were traditionally obtained from metal working operations as the scrap material filed off larger iron and steel parts. They are very of ...
in the space between. When a
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
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 (pulse) 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. Coherers remained in widespread use until about 1907, when they were replaced by more sensitive
electrolytic An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
and
crystal detector A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (demod ...
s.


History

The behavior of particles or metal filings in the presence of electricity or electric sparks was noticed in many experiments well before Édouard Branly's 1890 paper and even before there was proof of the theory of
electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
. In 1835 Swedish scientist Peter Samuel Munk noticed a change of resistance in a mixture of metal filings in the presence of spark discharge from a Leyden jar. In 1850 Pierre Guitard found that when dusty air was electrified, the particles would tend to collect in the form of strings. The idea that particles could react to electricity was used in English engineer
Samuel Alfred Varley Samuel Alfred Varley (1832–1921) was an English electrical engineer. He was one of ten children born to Cornelius Varley and Elizabeth Livermore Straker. Career In 1852, Samuel started work for the Electric Telegraph Company in Manchester. ...
's 1866 lightning bridge, a
lightning arrester A lightning arrester (alternative spelling lightning arrestor) (also called lightning isolator) is a device, essentially an air gap between an electric wire and ground, used on electric power transmission and telecommunication systems to protect ...
attached to telegraph lines consisting of a piece of wood with two metal spikes extending into a chamber. The space was filled with powdered carbon that would not allow the low voltage telegraph signals to pass through but it would conduct and ground a high voltage lightning strike. In 1879 the Welsh scientist
David Edward Hughes David Edward Hughes (16 May 1830 – 22 January 1900), was a British-American inventor, practical experimenter, and professor of music known for his work on the printing telegraph and the microphone. He is generally considered to have bee ...
found that loose contacts between a carbon rod and two carbon blocks as well as the metallic granules in a microphone he was developing responded to sparks generated in a nearby apparatus. Temistocle Calzecchi-Onesti in Italy began studying the anomalous change in the resistance of thin metallic films and metal particles at
Fermo Fermo (ancient: Firmum Picenum) is a town and ''comune'' of the Marche, Italy, in the Province of Fermo. Fermo is on a hill, the Sabulo, elevation , on a branch from Porto San Giorgio on the Adriatic coast railway. History The oldest hum ...
/
Monterubbiano Monterubbiano is a town and ''comune'' in the Province of Fermo, in the Marche region of Italy. It is on a hill from the Adriatic Sea. History In pre-historic times the area was inhabited by the Piceni (9th-3rd centuries BC). After the Roman co ...
. He found that copper filings between two brass plates would cling together, becoming conductive, when he applied a voltage to them. He also found that other types of metal filings would have the same reaction to electric sparks occurring at a distance, a phenomenon that he thought could be used for detecting lightning strikes. Calzecchi-Onesti's papers were published in il Nuovo Cimento in 1884, 1885 and 1886. In 1890, French physicist Édouard Branly published ''On the Changes in Resistance of Bodies under Different Electrical Conditions'' in a French Journal where he described his thorough investigation of the effect of minute electrical charges on metal and many types of metal filings. In one type of circuit, filings were placed in a tube of glass or ebonite, held between two metal plates. When an electric discharge was produced in the neighbourhood of the circuit, a large deviation was seen on the attached
galvanometer A galvanometer is an electromechanical measuring instrument for electric current. Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely. A galvanom ...
needle. He noted the filings in the tube would react to the electric discharge even when the tube was placed in another room 20 yards away. Branly went on to devise many types of these devices based on "imperfect" metal contacts. Branly's filings tube came to light in 1892 in Great Britain when it was described by Dr. Dawson Turner at a meeting of the British Association in Edinburgh.Hong, Sungook (2010) ''Wireless: From Marconi's Black-box to the Audion''. MIT Press. p. 4. The Scottish electrical engineer and astronomer George Forbes suggested that Branly's filings tube might be reacting in the presence of Hertzian waves, a type of air-borne
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
proven to exist by German physicist
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's Maxwell's equations, equations of electrom ...
(later called
radio waves Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (short ...
). In 1893 physicist W.B. Croft exhibited Branly's experiments at a meeting of the Physical Society in London. It was unclear to Croft and others whether the filings in the Branly tube were reacting to sparks or the light from the sparks. George Minchin noticed the Branly tube might be reacting to Hertzian waves the same way his solar cell did and wrote the paper "''The Action of Electromagnetic Radiation on Films containing Metallic Powders''". These papers were read by English physicist
Oliver Lodge Sir Oliver Joseph Lodge, (12 June 1851 – 22 August 1940) was a British physicist and writer involved in the development of, and holder of key patents for, radio. He identified electromagnetic radiation independent of Heinrich Rudolf Hertz, H ...
who saw this as a way to build a much improved Hertzian wave detector. On 1 June 1894, a few months after the death of Heinrich Hertz, Oliver Lodge delivered a memorial lecture on Hertz where he demonstrated the properties of "Hertzian waves" (radio), including transmitting them over a short distance, using an improved version of Branly's filings tube, which Lodge had named the "coherer", as a detector. In May 1895, after reading about Lodge's demonstrations, the Russian physicist Alexander Popov built a "Hertzian wave" (radio wave) based lightning detector using a coherer. That same year, Italian inventor
Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italians, Italian inventor and electrical engineering, electrical engineer, known for his creation of a practical radio wave-based Wireless telegrap ...
demonstrated a wireless telegraphy system using Hertzian waves (radio), based on a coherer. The coherer was replaced in receivers by the simpler and more sensitive
electrolytic An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
and
crystal detector A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (demod ...
s around 1907, and became obsolete. One minor use of the coherer in modern times was by Japanese tin-plate toy manufacturer Matsudaya Toy Co. who beginning 1957 used a
spark-gap transmitter A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the main type use ...
and coherer-based receiver in a range of radio-controlled (RC) toys, called Radicon (abbreviation for Radio-Controlled) toys. Several different types using the same RC system were commercially sold, including a Radicon Boat (very rare), Radicon Oldsmobile Car (rare) and a Radicon Bus (the most popular).


Operation

Unlike modern AM radio stations that transmit a continuous radio frequency, whose amplitude (power) is
modulated In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
by an
audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum *Digital audio, representation of sound ...
signal, the first
radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s transmitted information by
wireless telegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
(
radiotelegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
), the transmitter was turned on and off ( on-off keying) to produce different length pulses of unmodulated
carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...
signal, "dots" and "dashes", that spelled out text messages in
Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
. As a result, early radio receiving apparatus merely had to detect the presence or absence of the radio signal, not convert it to audio. The device that did this was called a
detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
. The coherer was the most successful of many detector devices that were tried in the early days of radio. The operation of the coherer is based on the phenomenon of
electrical contact resistance The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. This effect is describe ...
. Specifically as metal particles cohere (cling together), they conduct electricity much better after being subjected to
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
electricity. The radio signal from the antenna was applied directly across the coherer's electrodes. When the radio signal from a "dot" or "dash" came in, the coherer would become conductive. The coherer's electrodes were also attached to a DC circuit powered by a battery that created a "click" sound in earphones or a
telegraph sounder A telegraph sounder is an antique electromechanical device used as a receiver on electrical telegraph lines during the 19th century. It was invented by Alfred Vail after 1850 to replace the previous receiving device, the cumbersome Morse regist ...
, or a mark on a paper tape, to record the signal. Unfortunately, the reduction in the coherer's
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...
persisted after the radio signal was removed. This was a problem because the coherer had to be ready immediately to receive the next "dot" or "dash". Therefore, a decoherer mechanism was added to tap the coherer, mechanically disturbing the particles to reset it to the high resistance state. Coherence of particles by radio waves is an obscure phenomenon that is not well understood even today. Recent experiments with particle coherers seem to have confirmed the hypothesis that the particles cohere by a micro-weld phenomenon caused by
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
electricity flowing across the small contact area between particles. The underlying principle of so-called "imperfect contact" coherers is also not well understood, but may involve a kind of tunneling of
charge carrier In physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. The term is used ...
s across an imperfect junction between conductors.


Application

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 Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
and part
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
. Silver
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
s 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 Choking, also known as foreign body airway obstruction (FBAO), is a phenomenon that occurs when breathing is impeded by a blockage inside of the respiratory tract. An obstruction that prevents oxygen from entering the lungs results in oxygen de ...
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 Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...
and the other electrode, ''B'', to ground. A series combination of a
battery Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...
, ''B1'', and a relay, ''R'', is also attached to the two electrodes. When the signal from a
spark gap transmitter A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the main type use ...
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 A telegraph sounder is an antique electromechanical device used as a receiver on electrical telegraph lines during the 19th century. It was invented by Alfred Vail after 1850 to replace the previous receiving device, the cumbersome Morse regist ...
''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 An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the ...
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 Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
ahead of the train were occupied the oscillations were interrupted and the coherer, acting through a relay, showed a warning and applied the brakes.


Imperfect junction coherer

There are several variations of what is known as the imperfect junction coherer. The principle of operation (microwelding) suggested above for the filings coherer may be less likely to apply to this type because there is no need for decohering. An iron and mercury variation on this device was used by Marconi for the first transatlantic radio message. An earlier form was invented by
Jagdish Chandra Bose Sir Jagadish Chandra Bose (;, ; 30 November 1858 – 23 November 1937) was a biologist, physicist, botanist and an early writer of science fiction. He was a pioneer in the investigation of radio microwave optics, made significant contribution ...
in 1899. The device consisted of a small metallic cup containing a pool of
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
covered by a very thin insulating film of
oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
; above the surface of the oil, a small
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
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. In 1899, Bose announced the development of an "''iron-mercury-iron coherer with
telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...
detector''" in a paper presented at the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
, London. He also later received , "''Detector for electrical disturbances''" (1904), for a specific
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
receiver.


Anticoherer


Limitations of coherers

Because they are threshold voltage detectors, coherers had difficulty discriminating between the impulsive signals of
spark-gap transmitter A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the main type use ...
s, and other impulsive electrical noise:quoted in o
Stay Tuned website
/ref> Coherers were also finicky to adjust and not very sensitive. Another problem was that, because of the cumbersome mechanical "decohering" mechanism, the coherer was limited to a receiving speed of 12 – 15 words per minute of Morse code, while telegraph operators could send at rates of 50 WPM, and paper tape machines at 100 WPM. More important for the future, the coherer could not detect AM (radio) transmissions. As a simple switch that registered the presence or absence of radio waves, the coherer could detect the on-off keying of
wireless telegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for ...
transmitters, but it could not
rectify ''Rectify'' is an American television drama series exploring the life of a man after he is released from prison after nearly 20 years on death row following a wrongful conviction. It was created by Ray McKinnon and is the first original series f ...
nor
demodulate Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated c ...
the waveforms of AM
radiotelephone A radiotelephone (or radiophone), abbreviated RT, is a radio communication system for conducting a conversation; radiotelephony means telephony by radio. It is in contrast to '' radiotelegraphy'', which is radio transmission of telegrams (mes ...
signals, which began to be experimented with in the first years of the 20th century. This problem was solved by the rectification capability of the hot wire barretter and
electrolytic detector The electrolytic detector, or liquid barretter, was a type of detector (demodulator) used in early radio receivers. First used by Canadian radio researcher Reginald Fessenden in 1903, it was used until about 1913, after which it was superseded ...
, developed by
Reginald Fessenden Reginald Aubrey Fessenden (October 6, 1866 – July 22, 1932) was a Canadian-born inventor, who did a majority of his work in the United States and also claimed U.S. citizenship through his American-born father. During his life he received hundre ...
around 1902. These were replaced by the
crystal detector A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (demod ...
around 1907, and then around 1912–1918 by
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
technologies such as Fleming's oscillation valve and Lee De Forest's
Audion The Audion was an electronic detecting or amplifying vacuum tube invented by American electrical engineer Lee de Forest in 1906.De Forest patented a number of variations of his detector tubes starting in 1906. The patent that most clearly covers ...
(
triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 19 ...
) tube.


See also

*
Detector (radio) In radio, a detector is a device or circuit that extracts information from a modulated radio frequency current or voltage. The term dates from the first three decades of radio (1888-1918). Unlike modern radio stations which transmit sound (an audio ...
* Electrical contact resistance (ECR) *
Crystal radio A crystal radio receiver, also called a crystal set, is a simple radio receiver, popular in the early days of radio. It uses only the power of the received radio signal to produce sound, needing no external power. It is named for its most impo ...
*
Spark-gap transmitter A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the main type use ...
*
Radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...
*
Antique radio An antique radio is a radio receiving set that is collectible because of its age and rarity. Types of antique radio Morse receivers The first radio receivers used a coherer and sounding board, and were only able to receive CW continuous wave (C ...
* Camille Papin Tissot *
Wetting current In electrical and electronics engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact. It is typically far below the contact's nominal maximum curr ...
*
Wetting voltage In electrical and electronics engineering, wetting current is the minimum electric current needing to flow through a contact to break through the surface film resistance at a contact. It is typically far below the contact's nominal maximum curre ...


Further reading

*. A comprehensive description of radio detectors up to the development of the vacuum tube, with many unusual types of coherer. *Cuff, Thomas Mark (1993)
Coherers, a review.
Philadelphia, PA, Temple University, Master's Thesis. A technical historical account of the discovery and development of coherers and coherer-like behaviors from the 1800s to 1993, including the investigations, in the 1950s, of using coherers in the, then, new field of digital computers. This thesis examined the similarities among coherers and electrolytic RF detectors, MOM (Metal-Oxide-Metal) 'diodes' used in laser heterodyning, and the STM (Scanning Tunneling Microscope).


References


External links

*Web archive backup: "

'". World of Wireless, Virtual radiomuseum. *"

'". Marconi Calling Company. *Slaby, Adolphus, "
The New Telegraphy
Recent experiments in telegraphy with sparks.''". The Century Magazine. April, 1898. Pages 867–874. * Hirakawa Institute of Technology(Japan),"

'". *"

'". ShareAPic.net.
Coherer: history & operation
{{Telecommunications History of radio Radio electronics Detectors Electronic amplifiers