Coherer
Encyclopedia
The coherer was a primitive form of radio signal detector
used in the first radio receivers during the wireless telegraphy
era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly
, it consisted of a tube or capsule containing two electrode
s spaced a small distance apart, with metal filings in the space between them. When a radio frequency
signal was applied to the device, the initial high resistance of the filings reduced, allowing an electric current to flow through it. The coherer was a key enabling technology for radio
, and was the first device used to detect radio signals in practical spark gap transmitter wireless telegraphy. It became the basis for radio reception around 1900, and remained in widespread use for about ten years. It saw commercial use again briefly 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, called the Branly coherer, and a later imperfect junction type.
(sound), the first radio transmitters transmitted Morse code
, consisting of "dots" and "dashes", by on-off keying
, so early radio receiving apparatus merely had to detect the presence or absence of the radio signal, not convert it into audio. The device that did this was called a detector
. The coherer was the most successful of many detector devices that were tried in the early days of radio.
The basis for the operation of the coherer is that metal particles cohere (cling together) and conduct electricity much better after being subjected to radio frequency
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, 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
, or a mark on a paper tape, to record the signal. Unfortunately, the reduction in the coherer's electrical resistance
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
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
s across an imperfect junction between conductors.
, which was about half filled with sharply cut metal filings, often part silver
and part nickel
. Silver electrode
s made contact with the metal particles on both ends. 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
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
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 arm 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 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.
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 Hertz
ian, 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 British radio pioneer Oliver Lodge in his researches, and formed an important part of Guglielmo Marconi
's successful system of wireless telegraphy.
The coherer was replaced in receivers by the simpler and more sensitive crystal detector around 1907, and became obsolete.
in 1899. The device consisted of a small metallic cup containing a pool of mercury
covered by a very thin insulating
film of oil
; 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.
In 1899, Bose announced the development of an "iron-mercury-iron coherer with telephone
detector" in a paper presented at the Royal Society
, London. He also later received , "Detector for electrical disturbances" (1904), for a specific electromagnetic
receiver.
s, and other impulsive electrical noise:
As a simple switch registering the presence or absence of radio waves, the coherer could detect the on-off keying
of wireless telegraphy
transmitters, but it could not follow the complex waveforms of audio (AM
) broadcasting, which began to be experimented with in the first years of the 20th century. 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 around 1906, and then around 1920 by vacuum tube
technologies such as John Ambrose Fleming
's thermionic diode and the triode-based regenerative detector invented by Edwin Howard Armstrong.
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
used in the first radio receivers during the wireless telegraphy
Wireless telegraphy
Wireless telegraphy is a historical term used today to apply to early radio telegraph communications techniques and practices, particularly those used during the first three decades of radio before the term radio came into use....
era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly
Edouard Branly
Édouard Eugène Désiré Branly 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 Branly coherer around 1890.-Biography:The coherer was the first widely used detector for...
, it consisted of a tube or capsule containing two electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s spaced a small distance apart, with metal filings in the space between them. When a radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
signal was applied to the device, the initial high resistance of the filings reduced, allowing an electric current to flow through it. The coherer was a key enabling technology for radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
, and was the first device used to detect radio signals in practical spark gap transmitter wireless telegraphy. It became the basis for radio reception around 1900, and remained in widespread use for about ten years. It saw commercial use again briefly 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, called the Branly coherer, and a later imperfect junction type.
Operation
Unlike modern radio stations that transmit audioSound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
(sound), the first radio transmitters transmitted Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
, consisting of "dots" and "dashes", by on-off keying
On-off keying
On-off keying the simplest form of amplitude-shift keying modulation that represents digital data as the presence or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a...
, so early radio receiving apparatus merely had to detect the presence or absence of the radio signal, not convert it into audio. The device that did this was called a detector
Detector (radio)
A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
. The coherer was the most successful of many detector devices that were tried in the early days of radio.
The basis for the operation of the coherer is that metal particles cohere (cling together) and conduct electricity much better after being subjected to radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
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, the coherer would become conductive. The coherer's electrodes were also attached to a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
circuit powered by a battery that created a "click" sound in earphones or a telegraph sounder
Telegraph sounder
A telegraph sounder is a device which produces an audible sound when connected to an operating electrical telegraph.It is similar in form to a relay....
, or a mark on a paper tape, to record the signal. Unfortunately, the reduction in the coherer's electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
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
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
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
Quantum tunnelling
Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the sun, and has important...
of charge carrier
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
s across an imperfect junction between conductors.
Application
The filings coherer used in practical receivers was a glass tube, sometimes evacuatedVacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, which was about half filled with sharply cut metal filings, often part silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
and part nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
. Silver electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s made contact with the metal particles on both ends. 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
Choke (electronics)
A choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which...
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 (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
and the other electrode, B, to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
. A series combination of a battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
, 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
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
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
Telegraph sounder
A telegraph sounder is a device which produces an audible sound when connected to an operating electrical telegraph.It is similar in form to a relay....
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 arm 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 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
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...
, for example, had the tube rotating continuously along its axis, following each successive activation.
History
In 1850 Pierre 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. Temistocle Calzecchi-OnestiTemistocle Calzecchi-Onesti
-References:* at radiomarconi.com* at browsebiography.com...
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
Edouard Branly
Édouard Eugène Désiré Branly 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 Branly coherer around 1890.-Biography:The coherer was the first widely used detector for...
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 Hertz
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
ian, 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 British radio pioneer Oliver Lodge in his researches, and formed an important part of Guglielmo Marconi
Guglielmo Marconi
Guglielmo Marconi was an Italian inventor, known as the father of long distance radio transmission and for his development of Marconi's law and a radio telegraph system. Marconi is often credited as the inventor of radio, and indeed he shared the 1909 Nobel Prize in Physics with Karl Ferdinand...
's successful system of wireless telegraphy.
The coherer was replaced in receivers by the simpler and more sensitive crystal detector around 1907, and became obsolete.
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. The iron and mercury variation is important because the Thomasina form was used by Marconi for the first transatlantic radio message. An earlier form was invented by Jagdish Chandra BoseJagdish Chandra Bose
Acharya Sir Jagadish Chandra Bose, CSI, CIE, FRS was a Bengali polymath: a physicist, biologist, botanist, archaeologist, as well as an early writer of science fiction...
in 1899. The device consisted of a small metallic cup containing a pool of mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
covered by a very thin insulating
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
film of oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
; above the surface a small iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
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
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
detector" in a paper presented at the Royal Society
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
, London. He also later received , "Detector for electrical disturbances" (1904), for a specific electromagnetic
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
receiver.
Limitations of coherers
Coherers had a hard time discriminating between the impulsive signals of spark-gap transmitterSpark-gap transmitter
A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...
s, and other impulsive electrical noise:
As a simple switch registering the presence or absence of radio waves, the coherer could detect the on-off keying
On-off keying
On-off keying the simplest form of amplitude-shift keying modulation that represents digital data as the presence or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a...
of wireless telegraphy
Wireless telegraphy
Wireless telegraphy is a historical term used today to apply to early radio telegraph communications techniques and practices, particularly those used during the first three decades of radio before the term radio came into use....
transmitters, but it could not follow the complex waveforms of audio (AM
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
) broadcasting, which began to be experimented with in the first years of the 20th century. This problem was solved by the demodulation
Demodulation
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.A demodulator is an electronic circuit that is used to recover the information content from the modulated carrier wave.These terms are traditionally used in connection with radio receivers,...
capability enabled by Reginald Fessenden
Reginald Fessenden
Reginald Aubrey Fessenden , a naturalized American citizen born in Canada, was an inventor who performed pioneering experiments in radio, including early—and possibly the first—radio transmissions of voice and music...
's hot wire barretter
Hot wire barretter
The hot wire barretter was a demodulating detector, invented in 1902 by Reginald Fessenden, that found limited use in early radio receivers. In effect it was a highly sensitive thermoresistor developed to permit the reception of amplitude modulated signals, something that the coherer could not...
and electrolytic detector
Electrolytic detector
The electrolytic detector, or the bare-point electrolytic detector as it was also called, was a type of wet demodulator used in early radio receivers. This form of detector was in extensive use between the years 1902 and 1913, after which the superior vacuum tube diode became available...
. These in turn were replaced by the crystal detector around 1906, and then around 1920 by vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
technologies such as John Ambrose Fleming
John Ambrose Fleming
Sir John Ambrose Fleming was an English electrical engineer and physicist. He is known for inventing the first thermionic valve or vacuum tube, the diode, then called the kenotron in 1904. He is also famous for the left hand rule...
's thermionic diode and the triode-based regenerative detector invented by Edwin Howard Armstrong.
See also
- Detector (radio)Detector (radio)A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
- Crystal Radio
- Spark-gap transmitterSpark-gap transmitterA spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...
- Radio receiverReceiver (radio)A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
- Antique radioAntique radioAn antique radio is a radio receiving set that is collectible because of its age and rarity. Although collectors may differ on the cutoff dates, most would use 50 years old, or the pre-World War II Era, for vacuum tube sets and the first five years of transistor sets.-Morse only sets:The first...
- Camille Papin TissotCamille Papin TissotCamille Papin Tissot , was a pioneer of wireless telegraphy and established the first French operational radio connections at sea.- Origins :...
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".
- "Tesla's US Patent: 613,809". ShareAPic.net.