Electrolytic detector
Encyclopedia
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. It was considered very sensitive and reliable compared to other detectors available at the time such as the magnetic detector
and the coherer
.
realised that the radio detectors available would not be suitable for his purpose. Although they worked well for receiving the on-off signals
of spark gap transmitters, the coherer
and related types of detectors were not, in today's terms, fast enough for receiving audio signals
.
In 1902 Fessenden developed what he called a barretter detector that would work to receive AM signals
, but it was not very sensitive. The barretter used a fine platinum wire, called Wollaston wire
, manufactured as a platinum core in a silver sheath that had to be stripped off with acid. In the process of stripping some Wollaston wire, Fessenden left it immersed in acid too long, until only a tip remained in contact with the solution; he noted that it responded well to radio signals being generated nearby, and could be used as new type of detector.
This story was disputed at the time, with credit for the discovery also given to Michael I. Pupin, W. Schloemilch, Hugo Gernsback
and others. However, it is apparent that Fessenden was the first to put the device to practical use.
wire a few thousandths of an inch in diameter is immersed in an electrolyte
solution, and a small D.C.
voltage
bias is applied to the cell thus formed. Platinum is used because other metals are too quickly dissolved in the acid. The resulting bias current decomposes the solution by electrolysis
into tiny gas bubbles that cling to the metal point and tend to insulate the metal tip from the solution thus reducing the bias current. An incoming R.F.
current
can flow better in the direction across the point that makes the point more negative. That recombines the gases and increases point exposure to the liquid. RF current flow in the direction that makes the point more positive only reinforces gasious blocking of the point. Detection results from this asymmetrical flow.
In practical use a series circuit is made of the detector, headphones, and a battery
with a potentiometer
. The wire is made positive, and the signal to be demodulated is applied directly to it; a small (about 5 ml) platinum cup filled with either sulfuric
or nitric acid
completes the headphone circuit, and is also connected to ground
to complete the signal circuit. To adjust the cell, the point of the wire electrode is dipped into the electrolyte and the potentiometer adjusted until a hissing noise is heard in the headphones; the potentiometer is then moved back until the noise just ceases, at which point the detector is in its most sensitive state.
It was found that strong atmospheric noise
would render it insensitive, requiring that the device be rebiased
after each strong burst of static interference.
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...
was in extensive use between the years 1902 and 1913, after which the superior 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...
diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
became available. It was considered very sensitive and reliable compared to other detectors available at the time such as the magnetic detector
Magnetic detector
The magnetic detector was one of the first practical devices able to make radio signals audible through a pair of headphones. It was not as sensitive as some detectors but despite that, it was favored for early maritime use because of its reliability....
and the coherer
Coherer
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 electrodes spaced a...
.
History
While working to develop voice transmission, Reginald FessendenReginald 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...
realised that the radio detectors available would not be suitable for his purpose. Although they worked well for receiving the on-off signals
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 spark gap transmitters, the coherer
Coherer
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 electrodes spaced a...
and related types of detectors were not, in today's terms, fast enough for receiving audio signals
Audio frequency
An audio frequency or audible frequency is characterized as a periodic vibration whose frequency is audible to the average human...
.
In 1902 Fessenden developed what he called a barretter detector that would work to receive AM signals
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
, but it was not very sensitive. The barretter used a fine platinum wire, called Wollaston wire
Wollaston wire
Wollaston wire is a very fine platinum wire clad in silver and used in electrical instruments. For most uses, the silver cladding is etched away by acid to expose the platinum core.- History :...
, manufactured as a platinum core in a silver sheath that had to be stripped off with acid. In the process of stripping some Wollaston wire, Fessenden left it immersed in acid too long, until only a tip remained in contact with the solution; he noted that it responded well to radio signals being generated nearby, and could be used as new type of detector.
This story was disputed at the time, with credit for the discovery also given to Michael I. Pupin, W. Schloemilch, Hugo Gernsback
Hugo Gernsback
Hugo Gernsback , born Hugo Gernsbacher, was a Luxembourgian American inventor, writer, editor, and magazine publisher, best remembered for publications that included the first science fiction magazine. His contributions to the genre as publisher were so significant that, along with H. G...
and others. However, it is apparent that Fessenden was the first to put the device to practical use.
Description
The action of this detector is based upon the fact that if only the tip of a platinumPlatinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
wire a few thousandths of an inch in diameter is immersed in an electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
solution, and a small D.C.
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...
voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
bias is applied to the cell thus formed. Platinum is used because other metals are too quickly dissolved in the acid. The resulting bias current decomposes the solution by electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
into tiny gas bubbles that cling to the metal point and tend to insulate the metal tip from the solution thus reducing the bias current. An incoming R.F.
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...
current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
can flow better in the direction across the point that makes the point more negative. That recombines the gases and increases point exposure to the liquid. RF current flow in the direction that makes the point more positive only reinforces gasious blocking of the point. Detection results from this asymmetrical flow.
In practical use a series circuit is made of the detector, headphones, and 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...
with a potentiometer
Potentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
. The wire is made positive, and the signal to be demodulated is applied directly to it; a small (about 5 ml) platinum cup filled with either sulfuric
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
or nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
completes the headphone circuit, and is also connected to ground
Ground
Ground may refer to:* Earth's surface* Soil, a mixture of clay, sand and organic matter present on the surface of the Earth and serving as substrate for plant growth and micro-organisms development...
to complete the signal circuit. To adjust the cell, the point of the wire electrode is dipped into the electrolyte and the potentiometer adjusted until a hissing noise is heard in the headphones; the potentiometer is then moved back until the noise just ceases, at which point the detector is in its most sensitive state.
It was found that strong atmospheric noise
Atmospheric electricity
Atmospheric electricity is the regular diurnal variations of the Earth's atmospheric electromagnetic network . The Earth's surface, the ionosphere, and the atmosphere is known as the global atmospheric electrical circuit...
would render it insensitive, requiring that the device be rebiased
Bias
Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...
after each strong burst of static interference.
Sealed-point detector
Another form of electrolytic detector, the sealed-point electrolytic detector, which could stand considerable rough usage, was commercially known as the Radioson Detector; it had the cell sealed in a glass envelope. The operation was the same as in the bare-point electrolytic detector, the advantage being that the acid was sealed in, and consequently could not spill or evaporate.See also
- CohererCohererThe 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 electrodes spaced a...
- 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 :...