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 20th century. Its use in radio was based on the 1890 findings of French physicist Edouard Branly and adapted by other physicists and inventors over the next ten years. The device consists of a tube or capsule containing two electrodes spaced a small distance apart with loose metal filings in the space between. When a radio frequency 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 and crystal detectors.
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 on an uninterrupted carrier wave, early radio stations transmitted information by radiotelegraphy. The transmitter was switched on and off to produce long or short periods of radio waves, spelling out text messages in Morse code. Therefore, early radio receivers had only to distinguish between the presence or absence of a radio signal. The device that performed this function in the receiver circuit was called a detector. A variety of different detector devices, such as the coherer, electrolytic detector, magnetic detector and the crystal detector, were used during the wireless telegraphy era until superseded by vacuum tube technology.
In radio communications, a radio receiver, also known as a receiver, wireless or simply 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. The antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.
Wireless telegraphy means transmission of telegraph signals by radio waves; a more specific term for this is radiotelegraphy. Before about 1910 when radio became dominant, the term wireless telegraphy was also used for various other experimental technologies for transmitting telegraph signals without wires, such as electromagnetic induction, and ground conduction telegraph systems.
The behavior of particles or metal filings in the presence of electricity or electric sparks was noticed in many experiments well before Edouard Branly's 1890 paper and even before there was proof of the theory of electromagnetism.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's 1866 lightning bridge, a lightning arrester 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 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/Monterubbiano. 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.
Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. The electromagnetic force usually exhibits electromagnetic fields such as electric fields, magnetic fields, and light, and is one of the four fundamental interactions in nature. The other three fundamental interactions are the strong interaction, the weak interaction, and gravitation. At high energy the weak force and electromagnetic force are unified as a single electroweak force.
Samuel Alfred Varley (1832–1921) was an English electrical engineer. He was one of ten children born to Cornelius Varley and Elizabeth Livermore Straker.
A lightning arrester is a device used on electric power systems and telecommunication systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge travels along the power line to the arrester, the current from the surge is diverted through the arrester, in most cases to earth.
In 1890, French physicist Edouard 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 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.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 proven to exist by German physicist Heinrich Hertz (later called radio waves).
A galvanometer is an electromechanical instrument used for detecting and indicating an electric current. A galvanometer works as an actuator, by producing a rotary deflection, in response to electric current flowing through a coil in a constant magnetic field. Early galvanometers were not calibrated, but their later developments were used as measuring instruments, called ammeters, to measure the current flowing through an electric circuit.
George Forbes (1849–1936) was a British electrical engineer, astronomer, explorer, author and inventor, some of whose inventions are still in use.
In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating (radiating) through space, carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
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 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 demonstrated a wireless telegraphy system using Hertzian waves (radio), based on a coherer.
Sir Oliver Joseph Lodge, was a British physicist and writer involved in the development of, and holder of key patents for, radio. He identified electromagnetic radiation independent of Hertz' proof and at his 1894 Royal Institution lectures, Lodge demonstrated an early radio wave detector he named the "coherer". In 1898 he was awarded the "syntonic" patent by the United States Patent Office. Lodge was Principal of the University of Birmingham from 1900 to 1920.
Guglielmo Marconi, 1st Marquis of Marconi was an Italian inventor and electrical engineer, known for his pioneering work on long-distance radio transmission, development of Marconi's law, and a radio telegraph system. He is credited as the inventor of radio, and he shared the 1909 Nobel Prize in Physics with Karl Ferdinand Braun "in recognition of their contributions to the development of wireless telegraphy".
The coherer was replaced in receivers by the simpler and more sensitive electrolytic and crystal detectors around 1907, and became obsolete.
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 by crystal detectors and vacuum tube detectors such as the Fleming valve and Audion (triode). It was considered very sensitive and reliable compared to other detectors available at the time such as the magnetic detector and the coherer. It was one of the first rectifying detectors, able to receive AM (sound) transmissions. On December 24, 1906, US Naval ships with radio receivers equipped with Fessendon's electrolytic detectors received the first AM radio broadcast from Fessenden's Brant Rock, Massachusetts transmitter, consisting of a program of Christmas music.
A crystal detector is an obsolete electronic component in some early 20th century radio receivers that used a piece of crystalline mineral as a detector (demodulator) to rectify the alternating current radio signal to extract the audio modulation which produced the sound in the earphones. It was the first type of semiconductor diode, and one of the first semiconductor electronic devices. The most common type was the so-called cat whisker detector, which consisted of a piece of crystalline mineral, usually galena, with a fine wire touching its surface. The "asymmetric conduction" of electric current across electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystals were first used as radio wave detectors in 1894 by Jagadish Chandra Bose in his microwave experiments. who first patented a crystal detector in 1901. The crystal detector was developed into a practical radio component mainly by G. W. Pickard, who began research on detector materials in 1902 and found hundreds of substances that could be used in forming rectifying junctions. The physical principles by which they worked were not understood at the time they were used, but subsequent research into these primitive point contact semiconductor junctions in the 1930s and 1940s led to the development of modern semiconductor electronics.
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 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).
A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark. Spark-gap transmitters were the first type of radio transmitter, and were the main type used during the wireless telegraphy or "spark" era, the first three decades of radio, from 1887 to the end of World War 1. German physicist Heinrich Hertz built the first experimental spark-gap transmitters in 1887, with which he discovered radio waves and studied their properties.
Unlike modern AM radio stations that transmit a continuous radio frequency, whose amplitude (power) is modulated by an audio signal, the first radio transmitters transmitted information by wireless telegraphy (radiotelegraphy), the transmitter was turned on and off (on-off keying) to produce different length pulses of unmodulated carrier wave signal, "dots" and "dashes", that spelled out text messages in Morse code. 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. 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. Specifically as metal particles cohere (cling together), they 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 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, 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 carriers across an imperfect junction between conductors.
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 and part nickel. Silver electrodes 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 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, 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 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.
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 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 of the oil, 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. U.S. Patent 755,840 , "Detector for electrical disturbances" (1904), for a specific electromagnetic receiver.He also later received
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Coherers have difficulty discriminating between the impulsive signals of spark-gap transmitters, and other impulsive electrical noise:
This device [the coherer] was publicized as wonderful, and it was wonderfully erratic and bad. It would not work when it should, and it worked overtime when it should not have.— Robert Marriott
All was fish that came to the coherer net, and the recorder wrote down dot and dash combinations quite impartially for legitimate signals, static disturbances, a slipping trolley several blocks away, and even the turning on and off of lights in the building. Translation of the tape frequently required a brilliant imagination
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 transmitters, but it could not demodulate (rectify) the waveforms of AM radiotelephone signals, which began to be experimented with in the first years of the 20th century. This problem was solved by the rectification capability of Reginald Fessenden's hot wire barretter and electrolytic detector. These were replaced by the crystal detector around 1906, and then around 1912 by vacuum tube technologies such as John Ambrose Fleming's thermionic diode and Lee De Forest's Audion (triode) tube.
The early history of radio is the history of technology that produces and uses radio instruments that use radio waves. Within the timeline of radio, many people contributed theory and inventions in what became radio. Radio development began as "wireless telegraphy". Later radio history increasingly involves matters of broadcasting.
In electronics and telecommunications, a transmitter or radio 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 antenna. When excited by this alternating current, the antenna radiates radio waves.
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 important component, a crystal detector, originally made from a piece of crystalline mineral such as galena. This component is now called a diode.
The Audion was an electronic detecting or amplifying vacuum tube invented by American electrical engineer Lee de Forest in 1906. It was the first triode, consisting of an evacuated glass tube containing three electrodes: a heated filament, a grid, and a plate. It is important in the history of technology because it was the first widely used electronic device which could amplify; a small electrical signal applied to the grid could control a larger current flowing from the filament to plate.
Alexander Stepanovich Popov was a Russian physicist who is acclaimed in his homeland and some eastern European countries as the inventor of radio.
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.
Temistocle Calzecchi Onesti was an Italian physicist and inventor born in Lapedona, Italy, where his father, Icilio Calzecchi, a medical doctor from nearby Monterubbiano, was temporarily working at the time. His mother, Angela, was the last descendant of the ancient and noble Onesti family. His first name is the Italian version of the Athenian general Themistocles.
The invention of radio communication, although generally attributed to Guglielmo Marconi in the 1890s, spanned many decades, from theoretical underpinnings, through proof of the phenomenon's existence, development of technical means, to its final use in signalling.
The magnetic detector or Marconi magnetic detector, sometimes called the "Maggie", was an early radio wave detector used in some of the first radio receivers to receive Morse code messages during the wireless telegraphy era around the turn of the 20th century. Developed in 1902 by radio pioneer Guglielmo Marconi from a method invented in 1895 by New Zealand physicist Ernest Rutherford it was used in Marconi wireless stations until around 1912, when it was superseded by vacuum tubes. It was widely used on ships because of its reliability and insensitivity to vibration. A magnetic detector was part of the wireless apparatus in the radio room of the RMS Titanic which was used to summon help during its famous 15 April 1912 sinking.
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 which could recover amplitude modulated signals, something that the coherer could not do.
The timeline of radio lists within the history of radio, the technology and events that produced instruments that use radio waves and activities that people undertook. Later, the history is dominated by programming and contents, which is closer to general history.
The Fleming valve, also called the Fleming oscillation valve, was a thermionic valve or vacuum tube invented in 1904 by Englishman John Ambrose Fleming as a detector for early radio receivers used in electromagnetic wireless telegraphy. It was the first practical vacuum tube and the first thermionic diode, a vacuum tube whose purpose is to conduct current in one direction and block current flowing in the opposite direction. The thermionic diode was later widely used as a rectifier — a device which converts alternating current (AC) into direct current (DC) — in the power supplies of a wide range of electronic devices, until beginning to be replaced by the selenium rectifier in the early 1930s and almost completely replaced by the semiconductor diode in the 1960s. The Fleming valve was the forerunner of all vacuum tubes, which dominated electronics for 50 years. The IEEE has described it as "one of the most important developments in the history of electronics", and it is on the List of IEEE Milestones for electrical engineering.
The Musée Édouard Branly is a museum dedicated to the work of radio pioneer Édouard Branly (1844-1940). It is located in the 6th arrondissement at the Institut Catholique de Paris-ISEP, 21, rue d'Assas, Paris, France, and open by appointment only.
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